taxonID	type	description	language	source
A2311D4D9F1FE32204E6F819FEDD2D17.taxon	type_taxon	Type species. Solemya mediterranea (Lamarck, 1818) = Tellina togata Poli, 1795 (by subsequent designation, Children, 1823).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1FE32204E6F819FEDD2D17.taxon	discussion	Remarks. Our species is included in Solemya Lamarck, 1818, because of its internal ligament (Taylor et al. 2008; Kamenev 2009; Oliver et al. 2011). It is not a member of the genus Acharax Dall, 1908, because this taxon has an external ligament positioned above the thickened shell margin, developed probably as a support for the ligament nymphs (Amano & Ando 2011; Oliver et al. 2011; Taviani et al. 2011). Solemya has an internal ligament with a distinct resilium supported on a chondrophore. Fossil solemyids are morphologically very conservative, having a cylindrical shape with inequilaterally positioned beaks and an elongated shell anterior (e. g. Logan 1967; Duff 1978; Liljedahl 1984; Cope 1996; Kiel et al. 2008 a; Bailey 2011). This conservatism (e. g. Taylor & Glover 2010) is maintained not only at the genus, but also at the species level, as testified by the presence of cryptic modern species of Acharax (Neulinger et al. 2006). Thus, the taxonomy of solemyids is difficult, and there are few characters useful for species discrimination. Subgenus Petrasma Dall, 1908	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1FE32204E6F819FEDD2D17.taxon	type_taxon	Type species. Solemya borealis Totten, 1834	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1FE32204E6F819FEDD2D17.taxon	discussion	Remarks. We have included our species in the subgenus Petrasma Dall, 1908, because of the presence of a chondrophore supporting an oblique buttress adjoining the anterior side of the posterior adductor muscle scar, and small ligament demipads elongated along the dorsal margin and developed in front of the chondrophore. A buttress in front of the posterior adductor muscle scar is also present in the subgenera Solemyarina Iredale, 1931, Zesolemya Iredale, 1939, and Austrosolemya Taylor, Glover & Williams 2008, all of which have large ligament demipads perpendicular to the dorsal margin in front of the chondrophore (e. g. Kamenev 2009, fig. 4 – 6). For more detailed discussion of subgeneric differences in the genus Solemya see Taylor et al. (2008), Kamenev (2009) and Oliver et al. (2011).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1EE32E04E7FB2DFCA3289A.taxon	description	(Figure 3) 1859 cf. Solemya woodwardiana sp. nov. — Leckenby, p. 14, pl. 3, fig. 7. 1978 cf. Solemya woodwardiana Leckenby — Duff, p. 31, pl. 1, figs. 34 – 44.? 1990 Solemya cf. woodwardiana Leckenby — Wignall, p. 8.? 1993 Solemya cf. woodwardiana Leckenby — Wignall & Pickering, p. 328. 2011 Solemyid — Hammer et al., fig. 7 h, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1EE32E04E7FB2DFCA3289A.taxon	materials_examined	Material examined. 26 specimens, articulated to partially articulated; mostly internal moulds with only small portions of shell preserved. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1EE32E04E7FB2DFCA3289A.taxon	description	Dimensions. 14 – 62 mm in length, 4.2 – 32 mm in width, 4 – 23.5 mm in height. See Figure 4 A – C and Appendix 2 A for details. Description. Shell medium to large, elongated and narrow. Anterior margin broadly rounded, with anteriormost extremity positioned roughly in middle of curvature. Dorsal and ventral margin parallel to subparallel. Valves of some partially articulated specimens rotated along ligament line, which gives a false impression of shell widening anteriorly. Posterior margin evenly rounded, with curvature more pronounced than that of anterior margin. Umbones weak, opisthogyrate. External ornament composed of radial ridges projecting from umbonal area towards shell margins and commarginal growth lines, preserved as weak and broad wrinkles. Radial ridges are two to three times as wide as interridge spaces. On internal moulds external radial ornament is weaker. Anterior adductor muscle scar weak and positioned close to anterior shell extremity, supported by dorsal margin. Anterior adductor muscle scar rarely preserved well enough to define shape, but when present forms a broad arch pointing anteriorly, with its posteroventral margin running dorsally, passing into oblique and elongated portion, probably representing a visceral mass integument attachment scar. Posterior adductor muscle scar well impressed, especially on larger specimens, egg-shaped, with acute end pointing towards umbo. Anterior margin of posterior adductor muscle scar supported by well-developed buttress, running obliquely in dorsal direction and connected with chondrophore. Dorsal margin of posterior adductor muscle scar supported on a chondrophore; posterodorsal margin of posterior adductor muscle scar projecting above chondrophore. Hinge edentulous. Ligament internal, supported on a chondrophore, with small ligament demipads parallel to dorsal margin anterior of chondrophore.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F1EE32E04E7FB2DFCA3289A.taxon	discussion	Remarks. We compare our species with Solemya woodwardiana Leckenby, 1859, from the Callovian of England, due to the overall similar shape and shell proportions. Duff (1978) noted the presence of a chondrophore and a buttress running along the anterior edge of posterior adductor muscle scar in S. woodwardiana. However, we leave our material in open nomenclature, because not all important solemyid characters are sufficiently defined in the Callovian S. woodwardiana. A similar species, identified as S. cf. woodwardiana, comes from the Kimmeridgian organic-rich facies of England (Wignall 1990) and Scotland (Wignall & Pickering 1993). Solemya species with shapes similar to S. woodwardiana are common in the Boreal Jurassic and Cretaceous (Figure 4 A). Unio togata from the Moscow area in Russia (Trautschold 1858, p. 550, pl. 4, fig. 3) is a solemyid, with a characteristic notch in the subumbonal area, possibly indicating a presence of a buttress in front of PAMS. Trautschold (1858), probably unaware of Lamarck’s (1818) designation of the genus Solemya, synonymized his finds with the recent solemyid Tellina togata Poli, 1795 = Solemya togata (Poli, 1795). Another solemyid with a shape comparable to the Svalbard seep specimens is Solemya strigata Lahusen, 1886, from the Bajocian around the Olenek and Lena River mouths (Lahusen 1886; Zakharov & Shurigin 1978). A solemyid with similar shape and proportions was also noted from the Volgian of East Greenland by Fürsich (1982). Solemya cf. woodwardiana differs from the Solemya voltzii (Roemer, 1839) from the Toarcian Posidonienschiefer of Germany by having beaks positioned closer to the posterior than that species. A species described by Lindström (1865) as Solenomya torelli from the Jurassic of Svalbard is more elongated than most of the specimens of S. cf. woodwardiana, and Lindström’s brief diagnosis lacks any information about the ligament morphology. There is no certainty about the stratigraphic position of this species, since there is no Jurassic sediments cropping out in the Adventfjorden [Advent Bay] area of Svalbard, where Lindström’s specimens are said to come from. The external ornament of modern solemyids, especially of the thin-shelled species, is likely to be subjected to early diagenetic modification due to shell dissolution. The external shell shape and proportions can also be subjected to some variation due to plasticity and deformation of the organic-rich shell during burrowing (e. g. Stanley 1970; Taylor & Glover 2010), and by later compaction. Therefore, ascribing a bivalve fossil to the solemyids based on elongated shape only can be problematic. For example, Solenomya (?) hoeli from the Upper Jurassic of Svalbard (Sokolov & Bodylevsky 1931; Birkenmajer et al. 1982) has a very similar shape and proportions to our Solemya cf. woodwardiana specimens, and also has a radial ornament. However, detailed investigation of Sokolov & Bodylevsky’s type specimen reveals that the radial pattern on the anterior part of the shell is in fact caused by cracking. Sokolov & Bodylevsky (1931) failed to notive very a fine radial ornament on the anterior and posterior margins of their type, which shows that Solenomya (?) hoeli is not a solemyid and instead belongs to the genus Musculus Roeding, 1798. Occurrence. Solemya woodwardiana: Callovian of England (Leckenby 1859; Duff 1978). Solemya cf. woodwardiana: Kimmeridgian of England (Wignall 1990) and Scotland (Wignall & Pickering 1993); seeps 1, 3, 5, 8, 9 and 12 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Palaeoecology. We assume that the Svalbard seep Solemya cf. woodwardiana was chemosymbiotic, as are all known modern members of the family. Since species similar to Solemya cf. woodwardiana are present in organicrich, offshore facies of the Middle and Upper Jurassic of Northern Europe and the Arctic (Trautschold 1858; Lahusen 1886; Duff 1978; Zakharov & Shurigin 1978; Wignall 1990; Wignall & Pickering 1993), we infer that Svalbard species did not have any particular affiliation with seep environments and had a broad ecological tolerance, populating high redox potential environments. Solemyids are widespread in modern oceans from intertidal to abyssal depths (Taylor & Glover 2010). The genus Solemya s. l. is a member of shelf and slope faunas (Coan et al. 2000; Coan & Scott 2012), where it is present in soft substrates enriched in organic matter, like reduced sediments (e. g. Conway et al. 1992), wood debris accumulations (Reid 1980), sea grass beds (e. g. Taylor et al. 2008) and carrion falls (Fujiwara et al. 2009). The deepest confirmed record of Solemya s. l. comes from 1510 m in Sagami Bay (Kamenev 2009), and it possibly occurs as deep as 1697 m in the Eastern Mediterranean (Rodrigues et al. 2011). Solemya s. l. is a burrower, especially favouring muddy, silty and sandy bottoms. Burrowing takes place anteriorfirst and leads to formation of U-to Y-shaped tubes, reinforced with mucus (Stanley 1970; Reid 1980; Stewart & Cavanaugh 2006). While resting in its burrow, the animal uses its foot in a pumping action, supplying water enriched in reduced compounds from the lower shafts of the burrow to their gills (Reid 1980). All investigated Solemya s. l. species live in symbiosis with thiotrophic Gammaproteobacteria (Stewart & Cavanaugh 2006; Taylor & Glover 2010). A gutless condition has been confirmed in some species (e. g. Reid 1980; Conway et al. 1992; Kamenev 2009), while others retain the gut (e. g. Taylor et al. 2008; Oliver et al. 2011) and have limited ability for particulate feeding (Krueger et al. 1992). Coan et al. (2000) noticed that gutless species of Solemya s. l. are more common in environments with extreme organic content.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32E04E6FD81FAC92D5E.taxon	type_taxon	Type species. Pleurodon ovalis Wood, 1840	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32E04E6FD81FAC92D5E.taxon	discussion	Remarks. Nucinella has sometimes been placed in the family Manzanellidae Chronic, 1952 (e. g. Amano et al. 2007; Kiel et al. 2008 a). However, the Permian genus Manzanella Girty, 1909, is longer than high, with roughly equidistantly positioned beaks. Furthermore, it is dimyarian, with anterior and posterior adductor muscle scars roughly equal in size (Chronic 1952). This contrasts with the genus Nucinella Wood, 1851, which is monomyarian and has a very short posterior shell margin (e. g. Allen & Sanders 1969). A dimyarian condition was previously postulated for the supposed sister genus Huxleyia Adams, 1860 (e. g. Habe 1958; pl. 9, fig. 16; La Perna 2004, p. 571). However, investigation of Huxleyia habooba Oliver & Taylor, 2012, and Huxleyia sulcata Adams, 1860, revealed no sign of a posterior adductor muscle, which supports separation of Nucinella and Huxleyia into a separate family Nucinellidae (Oliver & Taylor 2012), with Manzanellidae restricted to the genus Manzanella.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	description	(Figure 5 A – N) 2011 Nucinella sp. — Hammer et al., fig. 7 f – g, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	etymology	Etymology. After the archipelago of Svalbard.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	materials_examined	Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78 ° 18 ’ 49.9 ’’ N 16 ° 10 ’ 58.9 ’’ E. Type material. Holotype: PMO 217.171; a well preserved articulated internal mould showing external shape and anterior muscle scar arrangement. Paratypes: PMO 217.217; an articulated internal mould and silicone rubber casts showing lateral teeth morphology. PMO 224.978; an articulated internal mould with shell partially preserved, showing external ornament of commarginal growth lines. PMO 224.981; an articulated internal mould and silicone rubber cast showing shape of cardinal teeth, as well as lateral teeth length. PMO 225.020; an articulated internal mould and silicone rubber casts showing lateral teeth length. PMO 225.042; an articulated internal mould with shell partially preserved showing the shape of the posteriormost cardinal teeth. Material examined. 98 specimens; mostly articulated or semiarticulated shells or moulds. See Appendix 1 for list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	description	Dimensions. 3.5 – 23 mm in length, 1.2 – 9.1 mm in height, 1 – 8.2 mm in width. See Figure 6 A – C and Appendix 2 B for details.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	diagnosis	Diagnosis. A very large species of Nucinella with rounded posteroventral margin, and sabre-shaped lateral teeth, two in the right valve, one in the left.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	description	Description. Shell very large, moderately inflated, equivalve, inequilateral, covered with commarginal growth lines. Umbo opisthogyrate, weakly projecting above dorsal margin, situated around ¼ of shell length. Anterodorsal margin weakly and evenly convex, passing smoothly into broadly arched anterior margin. Curvature of anterior margin even and symmetrical, with extremity around half distance from dorsal to ventral sides. Ventral margin weakly convex, roughly parallel to dorsal margin. Posterior extremity asymmetrically rounded, with ventral part of arch smoother than dorsal. Posterodorsal margin weakly concave, hosting deep ligament pit. Anterior adductor muscle scar elongated, attached to, and slightly projecting over, pallial line. Ventral part of anterior adductor muscle scar convex. Dorsal part of anterior adductor muscle scar narrow and long, connected with anterior part of lateral tooth; elongation possibly representing anterior pedal retractor scar, merged with anterior adductor muscle scar. Entire surface of anterior adductor muscle scar covered with commarginal lines. Surface of internal moulds covered with fine radial striae. Pallial line entire. Cardinal teeth diverse in shape; specimen measuring ca. 27 mm in length has eight teeth in left valve and seven in the right valve. Innermost cardinal teeth of right valve strong and long. Teeth get progressively smaller towards anterior and posterior extremities of hinge plate. Anteriormost cardinal tooth small, button-shaped. Lateral teeth long, thickest close to their anterior extremities, giving them characteristic sabre-like shape. In all specimens with investigated lateral teeth two laterals of right valve form a socket hosting single lateral of left valve. The socket is deepest close to anteriormost margin. Ligament external, dorsal to cardinal teeth. Detailed shape unknown.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F12E32904E6FBC5FE5A2E56.taxon	discussion	Remarks. Nucinella svalbardensis sp. nov. can be distinguished from Nucinella gigantea Amano, Jenkins & Hikida, 2007, from the Cenomanian to Campanian hydrocarbon seeps of Hokkaido (Amano et al. 2007; Kiel et al. 2008 a) by its rounded posteroventral margin, less projecting beaks and greater shell inflation. The cardinal dentition of N. svalbardensis is similar to that of N. gigantea. The slightly larger number of teeth in N. svalbardensis compared to N. gigantea can be explained by the larger size of the former species, since the number of cardinal teeth is a function of specimen size, as shown by La Perna (2004) in other species of Nucinella. According to Amano et al. (2007) a single lateral is present in the both left and right valve of N. gigantea, unlike in N. svalbardensis. A possible fossil nucinellid from the Triassic (Norian) seeps in Oregon (Peckmann et al. 2011) has been only tentatively identified and comparison with N. svalbardensis is at the present not possible. Nucinella svalbardensis can be distinguished from non-seep Mesozoic species by its much larger size. The Hettangian Nuculina liasina Bistram, 1903, of Val Solda (Bistram 1903) is much smaller than N. svalbardensis, reaching only a couple of millimetres in length. Nucinella birkelundi Clausen & Wignall, 1990, from the Kimmeridgian of Southern England (Clausen & Wignall 1990), is much smaller, in addition to being much higher, being 1.5 times high as long. Occurrence. Seeps 1, 2, 3, 5, 9, 12, 13, 14 and 15 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Palaeoecology. There are a number of reasons to suggest that Nucinella svalbardensis was a chemosymbiotic shallow burrower. First, chemosymbiosis was suggested by McLeod et al. (2010) for the Recent species Nucinella maoriana (Hedley, 1904) because of the presence of light organic carbon in its tissues. Second, the presence of bacterial structures in the gills of Nucinella owenensis Oliver & Taylor, 2012, led Oliver & Taylor (2012) to infer that chemosymbiosis may be common within the family, although some species are known to be active deposit feeders (Oliver & Taylor 2012). Third, large species of Nucinella are common in fossil hydrocarbon seep deposits, suggesting a chemosymbiotic relationship (Amano et al. 2007; Kiel et al. 2008 a; Hammer et al. 2011; Peckmann et al. 2011); N. svalbardensis, being up to 32 mm in length, is the largest species of Nucinella currently known. However, some of the non-seep deep marine Nucinella species are also quite large, being up to 25 mm long (Thiele & Jaeckel 1931; Matsukuma et al. 1982; La Perna 2005). Hence, a relationship between size and seepassociation in Nucinella is not straightforward. In addition to seeps, Nucinella can be found in a range of other shallow to deep marine modern environments (Allen & Sanders 1969; Matsukuma et al. 1982; Okutani & Iwasaki 2003; La Perna 2004), including organic-rich sediments in fjords (McLeod et al. 2010), and at oxygen-minimum zones (Oliver & Taylor 2012). As a fossil it occurs in a range of post-Triassic marine deposits (i. e. Wood 1851; Bistram 1903; Vokes 1956; Clausen & Wignall 1990; Studencka et al. 1998; Harries & Little 1999; Schneider 2008). Order Nuculoida Dall, 1889	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F17E32B04E6FF3DFD6B286F.taxon	type_taxon	Type species. Nucula lacryma J. de C. Sowerby, 1824.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F17E32A04E6FE2FFEEB2830.taxon	description	(Figure 5 O – Q) 1976 Dacromya chetaensis nov. sp. — Sanin, p. 26, pl. 6, figs. 10 – 11; pl. 7, figs. 1 – 6.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F17E32A04E6FE2FFEEB2830.taxon	materials_examined	Material examined. Three specimens; articulated internal moulds with some preserved shell material. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F17E32A04E6FE2FFEEB2830.taxon	description	Dimensions. 7 – 11.5 mm in length, 5.1 – 7.25 mm in height, 3.1 – 5.25 mm in width. See Appendix 2 C for details. Description. Shell small, with tear-drop shaped outline. Preserved shell fragments thin, with fine, densely spaced commarginal growth lines. Moderately to strongly inflated, with greatest width medially. Shell height around two-thirds of length, reaching a maximum medially and gradually diminishing towards anterior and posterior extremities. Anterior part of shell longer than posterior, arcuate, gently tapering. Anterodorsal margin weakly convex, more inclined than anteroventral margin. Ventral margin very gently rounded. Posterior shell part shorter than anterior, rostrate, rounded, and curved dorsally. Posterodorsal margin concave, hosting deep escutcheon. Beaks opisthogyrate, lunule elongated, sunken close to beaks, emerging towards anterior. Dentition taxodont with more than 10 teeth in anterior row and 7 to 10 teeth in posterior row. Cardinal area and ligament not preserved. Anterior and posterior adductor muscle scars circular, small, adhering to pallial line and positioned close to shell extremities on inside of pallial line. Posterior adductor muscle scar connected to beak with grooves, most likely formed during descending muscle growth. Pallial line entire.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F17E32A04E6FE2FFEEB2830.taxon	discussion	Remarks. We include this species into Dacromya chetaensis Sanin, 1976, based on its shell proportions, gently arched anterior margin and external ornament. It differs from Dacromya venusta (Sauvage, 1871) from the Kimmeridgian of England (Clausen & Wignall 1990) in having a more rounded anterior margin, less sloping anterodorsal margin and an external ornament of commarginal growth lines. Dacromya chetaensis is less elongated, has more pointed umbones and shorter rostrum than D. gigantea Zakharov & Shurigin, 1974, from the Aalenian of the Taimyr Peninsula (Zakharov & Shurigin 1974; 1978). Occurrence. Seep 12 (Upper Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Known also from the Volgian – Valanginian of the Khatanga Depression, Northern Siberia (Sanin 1976). Palaeoecology. Dacromya chetaensis was probably a mobile shallow burrowing deposit feeder without a siphon, as indicated by the lack of a pallial sinus. The streamlined shell with a blunt, weakly pointed anterior and short, rostrate posterior shows that it was probably an efficient burrower. In Siberia it is associated with sandy mudstones that were deposited in a low energy, shallow water setting (Sanin 1976). This mode of life is typical for Recent nuculids, which are shallow burrowers using palps to collect sediment, which they process for organic material (Stanley 1970).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E32A04E6FE1DFD6D2DC2.taxon	type_taxon	Type species. Nucula foersteri Müller, 1847	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E32A04E6FE1DFD6D2DC2.taxon	discussion	Remarks. The genus Mesosaccella is probably a bucket taxon, as it contains species with a wide range of external morphological features. This diversity is encapsulated into three morphogroups. The first comprises roughly oval, variably elongated and non-carinate species with ornament composed only of growth lines, including M. elliptica (Goldfuss, 1837) from the Rhaetian – Pliensbachian of Europe (Palmer 1973; Hodges 2000), M. morrisi (Deshayes, 1853) from the Callovian of England and France (Duff 1978) and M. choroschovensis (Borissjak, 1904) from the Volgian of Russia and Greenland (Borissjak 1904; Fürsich 1982). The second Mesosaccella morphogroup comprises elongated species with slightly pointed posteriors, external ornament of growth lines, with or without weak commarginal ridges, and growth line deflections on the posterior parts of the shell. This group includes our M. rogovi sp. nov., M. grovei (Lartet, 1872) and M. larteti Chavan, 1947, from the Campanian of Palestine (Chavan 1947), as well as species currently classified within other genera, but also fitting the definition of Mesosaccella, such as Leda striatula Forbes, 1845, from the Cretaceous of India (Forbes 1845, p. 148, pl. XVII, fig. 14). The third Mesosaccella morphogroup comprises short and inflated forms with pointed posteriors, strong commarginal ornament fading on the anterior and posterior parts of the shell and carination of the posterior shell area. This group includes our M. toddi sp. nov., and species described as Nuculana speetoniensis Woods, 1899, and Nuculana lineata J. de C. Sowerby, 1836, by Woods (1899).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	description	(Figures 5 R – T, 7 A – F) 2011 Malletiid sp. 1 — Hammer et al., fig. 7 n – o, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	etymology	Etymology. After Mikhail A. Rogov, Russian palaeontologist specializing in Jurassic stratigraphy of the Arctic.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	materials_examined	Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78 ° 18 ’ 49.9 ” N 16 ° 10 ’ 58.9 ” E. Type material. Holotype: PMO 224.971; a well preserved left valve showing the shape and characteristic ornament of posterodorsal margin. Paratypes: PMO 217.229; a partially preserved articulated internal mould showing anterior adductor muscle scar, posterior adductor muscle scar and weak pallial line. PMO 217.371; a well preserved articulated internal mould with fragments of shell, which shows weak ridges formed by growth line deflections, a carina and lancet-shaped escutcheon; the mould shows the pallial line and weak imprints of the external ridges. PMO 217.539; a partial right valve with cardinal area showing continuous tooth row. Material examined. 89 specimens, mostly articulated valves or internal moulds with portions of the shell remaining. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	diagnosis	Diagnosis. Elongated, lancet-shaped species with posterior part more than four times the length of the anterior part. Up to three weak posterior ridges formed by growth line deflections on the posterior part of the shell. A carina subparallel to the posterodorsal margin runs from the umbo to the posterior shell extremity.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	description	Dimensions. 3.5 – 23 mm in length, 1.2 – 9.1 mm in height, 1 – 8.2 mm in width. See Figure 8 A – D and Appendix 2 D for details. Description. Medium to large, up to 23 mm long, 9.1 mm high and 8.2 mm wide, elongated (H / L ≈ 0.42), lancet-shaped, weakly inflated (W / L ≈ 0.32, W / H ≈ 0.77); greatest height and width around medial plane. Elongated shape especially well expressed in posterior part, which occupies around 80 % of total length (Pl / L ≈ 0.79). Shell shape parameters remain relatively constant throughout ontogeny. External ornament of narrowly-spaced, commarginal growth lines, strongest in median part of shell and weaker towards both anterior and posterior. Towards posterior, disappearance of concentric ridges marked by up to three closely spaced ridges composed of series of growth line deflections. A single carina runs subparallel to posterodorsal margin. Escutcheon shallow, pointed anteriorly, presumably lancet-shaped. Anterodorsal margin straight to weakly convex, without well demarcated lunule. Anterior margin rounded to blunt. Anteroventral margin rounded, passing into weakly rounded ventral margin. Ventral margin passes into weakly convex posteroventral margin. Posterior end pointed, dorsally sloping and passing into long, posterodorsal margin; parallel to dorsal carina. Beaks prosogyrate. Dentition taxodont, teeth orthomorphodont. Anterior and posterior teeth rows meet below umbo. Posterior teeth much smaller than anterior teeth. Specimen 20 mm long has up to 38 teeth in posterior row and ca. 11 teeth in anterior row. Anterior adductor muscle scar small, rounded. Posterior adductor muscle scar elongated, more impressed anteriorly, progressively fading towards posterior. In some specimens weak elongated muscle scars of uncertain origin visible along dorsal margin. Pallial line weakly impressed, parallel to commissure; pallial sinus very shallow to absent, and weakly impressed.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F16E33704E6FB78FC462BD6.taxon	discussion	Remarks. Mesosaccella rogovi differs from M. morrisi (Deshayes, 1853) in being more elongated, having stronger commarginal ornament and, in addition, showing a weak dorsal carina and growth line deflections in the posterior shell areas (cf. Duff 1978). Mesosaccella elliptica (Goldfuss, 1837) has a shape closer to M. rogovi, but it lacks the distinctive commarginal ornament deflections in the posterior part of the shell and the dorsal carina (Hodges 2000). Mesosaccella choroschovensis (Borissjak, 1904) is much shorter and more oval than M. rogovi, with a gently rounded posterior end and lacks the ridges of M. rogovi. Leda striatula Forbes, 1845, has a similar growth line deflection to M. rogovi in the posterior area. However, it is both less elongated and has a stronger carina than M. rogovi. Mesosaccella rogovi is as elongated as both M. grovei (Lartet, 1872) and M. larteti Chavan, 1947, but it is more inequilateral than the former and has much weaker commarginal ornament than the latter. Occurrence. Seeps 1, 2, 3, 5, 8, 9, 12, 13 and 15 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Palaeoecology. Mesosaccella rogovi was probably a shallow burrower, like other protobranch bivalves (Stanley 1970; Sanders & Allen 1985; Zardus 2002). The elongated and streamlined shell indicates it was an efficient burrower, and the very shallow pallial sinus implies the presence of a short siphon, developed either for respiration or for feeding from the sediment-water interface (e. g. Hodges 2000, text-fig. 32). Some protobranchs inhabiting modern vent and seep environments are believed to benefit from the concentration of chemosynthetically produced organic matter in the sediments, and possibly even feed on bacterial mats (e. g. Allen 1993; Sahling et al. 2002). We assume M. rogovi might have done the same.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	description	(Figure 7 G – P) 2011 Malletiid sp. 2 — Hammer et al., tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	etymology	Etymology. After Jonathan A. Todd, curator of the Mesozoic and Cenozoic mollusc collection in the Natural History Museum, London. Study of this collection helped the work on the fossils described in this paper.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	materials_examined	Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78 ° 18 ’ 49.9 ” N 16 ° 10 ’ 58.9 ” E. Type material. Holotype: PMO 224.861; a large, complete shell. Paratypes: PMO 225.031 Fragment of the cardinal area showing taxodont dentition. PMO 224.862; an internal mould showing weak anterior adductor muscle scar and posterior adductor muscle scar and pallial line with shallow pallial sinus. PMO 217.609; a small, complete shell. PMO 217.610; a small, complete shell. PMO 217.616; a large, well preserved shell showing external commarginal and rosette-shaped ornament. Material examined. 35 specimens, articulated valves and internal or external moulds. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	diagnosis	Diagnosis. Inflated and thick-shelled with posterior part approximately twice the length of the anterior part, and with three strong posterior carinae. External ornament of strong commarginal ridges fading towards posterior and anterior and crossed obliquely by commarginal growth lines. Interridge spaces occupied by rows of rosetteshaped ornamentation, fading anteriorly and posteriorly.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	description	Dimensions. 6 – 12.5 mm in length, 4 – 8.2 mm in height, 3 – 7.1 mm in width. See Figure 9 A – D and Appendix 2 E for details. Description. Shell small to medium in size, up to 12.5 mm long, 8.2 mm high, and 7.1 mm wide, elongated (H / L ≈ 0.72), and inflated (W / L ≈ 0.52, W / H ≈ 0.81), with more inflated specimens more frequent than less inflated. Moderately inequilateral (Pl / L ≈ 0.72) throughout observed ontogeny. Orthogyrate beaks, passing into broadly curved anterodorsal margin through shallow concavity directly in front of beaks. Anterodorsal margin bends down and gets more tightly rounded to form broadly arcuate anterior margin, which gets progressively less curved and passes smoothly into ventral margin. Ventral margin gently rounded, deepest approximately below beaks. Posterior to first carina ventral margin oblique, with shallow sinus developed on larger specimens, connected to shallow, but well defined sulcus. Posterior shell extremity pointed, supported on second carina. Posterodorsal shell margin straight, joining beak through very small and shallow concavity directly behind it. External ornament of weak commarginal growth lines, obscured in shell mid-flank by strong, commarginal ridges transecting growth lines obliquely. Interridge spaces occupied by rows of rosette-shaped ornament arranged in regular intervals. Posterior area ornamented only by growth lines and rosettes; delineated by two strong carinae. First carina forms angle of ca. 55 ° with anteroposterior axis and visible throughout observed ontogeny. Second carina forms approximate angle of 27 ° with anteroposterior axis and also visible throughout ontogeny. Escutcheon wide, shallow, bounded by third carina. Dentition taxodont with long and straight teeth arranged in rows along hinge line. Specimen approximately 9 mm in length has nine teeth in anterior row and 16 teeth in posterior row. No resilifer; teeth joining below beak to form single teeth row. Anterior adductor muscle scar circular and well impressed; rather small. In specimen ca. 9 mm long anterior adductor muscle scar is 1 mm in diameter. Anterior adductor muscle scar positioned slightly above anteroposterior axis. Posterior adductor muscle scar poorly visible, small, elongated, positioned above pallial sinus. Five pairs of elongated muscle scars present directly underneath hinge plate: two in anterior part of hinge plate, one underneath cardinal area and two pairs of longer muscle scars present underneath posterior part of hinge plate. Some of these may represent pedal muscle attachments. Pallial line well impressed; pallial sinus shallow.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0BE33004E6FD6FFD332BEB.taxon	discussion	Remarks. Protobranch bivalves with a shell shape similar to M. toddi are known from several other Cretaceous localities. Woods (1899) listed several carinated species from the Cretaceous of England which he assigned to the genus Nuculana Link, 1807. In our opinion these belong to both Nuculana and Mesosaccella. For example, no resilifer is visible on the drawings of N. lineata in Woods (1899, pl. 1, fig. 31 d), which excludes this species from Nuculana. Nuculana speetoniensis (Woods 1899, p. 3. pl. 1, figs. 6 – 7) from the Lower Cretaceous Speeton Clay has a very similar shell shape to M. toddi, the main difference between them being the shorter and less pointed posterior margin of N. speetoniensis. Nuculana lineata J. de Sowerby, 1836 (Woods 1899, p. 7, pl. 1, figs. 28 – 32), from the Lower Cretaceous of Southern England has a similar shape and ornament to M. toddi, having commarginal ridges fading on the anterior and posterior shell parts, but can be distinguished from M. toddi by being less inflated and having a rounded posterior with less pronounced carinae. In the Aptian of Spitsbergen Sokolov & Bodylevsky (1931, p. 66, pl. 12, figs. 4 – 6) recorded Leda angulatostriata, which has a very similar shell shape to M. toddi, the main difference being L. angulatostriata has a less pronounced posterior carination than M. toddi. In our opinion L. angulatostriata also belongs to Mesaccella. Occurrence. Seeps 1, 3, 9 and 12 (Upper Volgian – latest Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Palaeoecology. Mesosaccella toddi was probably a shallow burrowing deposit feeder, like M. rogovi. The inflated shell of M. toddi, its relatively short posterior and shallow pallial sinus suggests, however, that this species was a slow burrower living underneath the sediment-water interface. The strong commarginal ornament and relatively thick shell may have been adaptations for burrowing in coarser sediment, like silt and sand (e. g. Marshall 1978), or it might have been an adaptation for living in acidic environment (e. g. Allen 1993). As for M. rogovi, M. toddi might have fed on chemosynthetically produced organic matter. Subclass Autobranchia Grobben, 1894 Superorder Pteriomorphia Beurlen, 1944	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33004E6FC8BFE9E2F60.taxon	description	(Figure 10 A – B) 2011 Ostreoidean — Hammer et al., tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33004E6FC8BFE9E2F60.taxon	materials_examined	Material examined. Five specimens; fragmented and delaminated shells. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33004E6FC8BFE9E2F60.taxon	description	Description. Shell thin, roughly round in shape, only weakly inflated. Right valve thin, flat, visible only in cross-section. Left valve thin, weakly convex and larger than right valve; irregularly shaped, no sign of attachment found. Beak weakly incurved, accompanied by a shallow ventrally directed furrow. Weak muscle scar probably represent an adductor muscle scar. Shell ornamented by weak commarginal folds.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33004E6FC8BFE9E2F60.taxon	discussion	Remarks. The lack of a well demarcated external ligament, the thinness of the shells and the nature of cementation precludes our specimens belonging to the Ostreida Férrusac, 1822. Instead, the flat right valve facing the substrate and a larger covering left valve indicates that our specimens belong either to the Anomiidae Rafinesque, 1815, or the Dimyidae, Fischer, 1886 (e. g. Coan et al. 2000; Coan & Scott 2012). An anomiid origin is probably more likely because the dorsal furrow resembles the shell fusion scar characteristic for anomiids (Yonge 1977). However, the lack of well preserved right valves and adductor muscle scars means we cannot unequivocally include our specimens into either the Anomiidae and Dimyidae at present. Palaeoecology. Our species was most likely a filter-feeder attached to hard substrates. Modern anomiids are shallow water forms, attached by a byssus protruding through a foramen in the lower valve (e. g. Yonge 1977). Recent Dimyidae is a cementing group found in deep water and cryptic habitats (e. g. Waller 2012). In the Mesozoic both groups were fairly common in shallow water habitats (e. g. Fürsich & Werner 1989; Hodges 1991; Todd & Palmer 2002).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33304E6F968FF772B95.taxon	type_taxon	Type species. Avicula mosquensis von Buch, 1844	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33304E6F968FF772B95.taxon	discussion	Buchia spp.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33304E6F968FF772B95.taxon	materials_examined	Material examined. 156 specimens. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0CE33304E6F968FF772B95.taxon	discussion	Remarks. Species of the genus Buchia are considered valuable index fossils in Oxfordian to Hauterivian strata and are used for biostratigraphic subdivisions (e. g. Jeletzky 1966; Kauffman 1973; Zakharov 1981; Surlyk & Zakharov 1982). The Svalbard seep buchiids probably represent Buchia okensis (Pavlow, 1907), Buchia volgensis (Lahusen, 1888) and Buchia cf. inflata (Lahusen, 1888). A separate paper is planned to formally describe them, and their stratigraphic significance in relation to the ammonite stratigraphy of Wierzbowski et al. (2011). Palaeoecology. Buchia species were epifaunal byssally attached suspension-feeders (e. g. Wignall & Pickering 1993, fig. 7), often clustering around hard structures (e. g. shells, rocks) providing a substrate for byssal attachment (Fürsich 1982). The species of the genus had a broad ecological tolerance and were present in a variety of shallow to deep marine facies (e. g. Sokolov & Bodylevsky 1931; Håkansson et al. 1981; Fürsich 1982; 1984; Oschmann 1988; Kelly 1984; Wignall & Pickering 1993) and are suggested to have been opportunists (Fürsich 1984; Wignall 1990).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0FE33304E6FCA3FDCD2AC9.taxon	type_taxon	Type species. Avicula muensteri Bronn, 1830	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0FE33D04E6FC45FA352A9D.taxon	description	(Figure 10 C – G) 1850 Avicula Octavia sp. nov. — d’Orbigny, p. 61. 1984 Oxytoma (Oxytoma) octavia (d’Orbigny) — Kelly, p. 61, pl. 5, figs. 1 – 9 and references therein. 2011 Oxytoma sp. — Hammer et al. 2011, fig. 7 q, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0FE33D04E6FC45FA352A9D.taxon	materials_examined	Material. 37 specimens, all single valves with completely or partially preserved shells. Four right valves, 33 left valves. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0FE33D04E6FC45FA352A9D.taxon	description	Dimensions. 7 – 24.5 mm in length, 7 – 23.5 mm in height. See Figure 11 A – C and Appendix 2 F for details. Description. Adult shell more than 24 mm long, inequivalved with left valve significantly larger than right. Left valve oval and weakly posteroventrally elongated; thin, inequilateral, moderately inflated, ornament of 15 – 20 radial primary ribs, intercalated with secondary riblets and occasionally very weak tertiary riblets. Primary radial ribs dense, but thin, on early shell growth stages, progressively thicker during ontogeny. On internal moulds ribs visible only on later growth stages. Inter-rib spaces around five times wider than primary ribs. Secondary ribs appear on shell in late growth stages, but on internal moulds are present only on ventral margins of larger specimens. Ribs occasionally show disturbances and deflections. Shell growth lines weak; no nodes present at radial ribs intersections. Beak weakly prosogyrate, hinge line straight. Posterior auricle present; in investigated specimens not projecting beyond posterior valve margin, demarcated from the rest of valve by weak, oblique sulcus. Anterior auricle absent, groove in inner shell surface below anterior part of hinge line visible on outer shell surface as an oblique, elongated ridge. Right valve thin, circular to weakly oval in outline; inequilateral, flat to weakly inflated. Radial ribs present on external shell surface, absent on internal moulds. Beak weakly prosogyrate, hinge line straight, posterior auricle moderately long, projecting beyond posterior valve margin; indistinct sulcus demarcating auricle from rest of valve. Anterior auricle straight, triangular, acute, around a fifth to a sixth of hinge line length. Triangular and moderately deep byssal notch developed below anterior auricle, with main axis slightly oblique to the hinge line. Ctenolium not observed.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F0FE33D04E6FC45FA352A9D.taxon	discussion	Remarks. Our specimens are synonymized with Oxytoma octavia (d’Orbigny, 1850) due to their strong secondary ribs on the right valve, which is markedly smaller than the left valve. Oxytoma octavia is inequivalved and characterized by having 12 to 20 primary ribs on the left valve (12 to 19 in Kelly 1984, p. 62), separated by distinct secondary and tertiary riblets (e. g. Kelly 1984, text-fig. 40). The rather similar O. inequivalve (J. Sowerby, 1819), widespread from the Lower Jurassic up to the Kimmeridgian (e. g. Gerasimov 1955; Ichikawa 1958; Cox 1965; Duff 1978; Wierzbowski et al. 1981; Pugaczewska 1986; Clausen & Wignall 1990) has a left valve with a similar number of radial ribs to O. octavia and with inter-rib spaces containing variably strong secondaries and tertiaries (e. g. Duff 1978, p. 55). The difference between O. octavia and O. inequivalve lies in the presence of primary and secondary ribbing on the right valve in O. octavia contrasting with the smooth or weakly striated right valve of O. inequivalve (Duff 1978; Kelly 1984). The right valves of our specimens are small and circular, which suggests an inequivalve condition, separating our species from O. expansa (Phillips, 1829) which is approximately equivalved (e. g. Arkell 1931, p. 191). Our specimens are much less inequilateral and have a very different style of ribbing than O. (Boreioxytoma) aucta Zakharov, 1966, from the Volgian of Siberia (Zakharov 1966), which has seven to nine widely spaced primary ribs (Zakharov 1966, pl. I: 6, pl. II: 1). Occurrence. Seeps 3, 5, 8 and 9 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Occurs in the Volgian – Ryazanian of Europe, Greenland, Russia and Siberia (de Loriol & Pellat 1867; Lewinski 1922; Spath 1936; 1947; Gerasimov 1955; Zakharov 1966; Fürsich 1982; Birkenmajer et al. 1982; Kelly 1984). Palaeoecology. Oxytoma octavia was a byssally attached filter feeder. This mode of life is shared by many other pteriomorphs (Stanley 1970). The long posterior auricle likely sheltered the posterior exhalant current from being swept back into the shell by water currents, as in pteriids (Stanley 1970). Because of the relatively thin valves, O. octavia was more likely preserved in less agitated environments, where it was living attached to loose shells and local carbonate hardgrounds. A pseudoplanktonic mode of life and attachment to drifting algal fronds is also possible for O. octavia, an interpretation supported by the lack of articulated specimens in the studied material.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F00E33C04E6F8CCFC172E93.taxon	type_taxon	Type species. Camptonectes (Costicamptonectes) milnelandensis Fürsich, 1982	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F00E33C04E6F919FD942E4C.taxon	type_taxon	Type species. Pecten lens Sowerby, 1818	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33F04E6FF3DFC712C53.taxon	description	(Figure 10 H – K)	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33F04E6FF3DFC712C53.taxon	description	1982 aff. Camptonectes (Costicamptonectes) milnelandensis sp. nov. — Fürsich, p. 50, figs. 23 C – D, F, G.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33F04E6FF3DFC712C53.taxon	materials_examined	Material examined. Two poorly preserved right valves. In addition a single external mould with silicone rubber cast from the Dorsoplanites bed (PMO 226.604). See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33F04E6FF3DFC712C53.taxon	description	Dimensions. 15 – 30.5 mm in length, 15 – 28.2 mm in height. See Appendix 2 G for details. Description. Shell small to medium sized, very thin, appears to get shorter at later growth stages. Right valve suborbicular, moderately inflated, with small posterior and larger anterior auricle, ventrally delineated by a deep byssal notch. Ctenolium well developed. Antero- and posterodorsal margins subequal, anterodorsal margin slightly concave. Anterior margin slightly more convex than posterior margin, ventral margin evenly rounded. Preserved ornament consists of weak divaricating radial ribs, intersected by weak but dense growth lines. Anterodorsal margin ornamented by ca. 6 subparallel radial ribs. Left valve, musculature and dentition unknown.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33F04E6FF3DFC712C53.taxon	discussion	Remarks. We compare our specimens with C. (Co.) milnelandensis because of its ornament of commarginal growth lines accompanied by divaricate ribs and with additional radial ribs close to the anterodorsal margin. The Svalbard specimens have a slightly higher umbonal angle than comparatively sized specimens of C. (Co.) milnelandensis from Greenland (Fürsich 1982). However, no specimens larger than 18 mm were reported by Fürsich (1982) so we are unable to compare the ontogenetic changes of umbonal angle of specimens from both localities. In addition, the Svalbard material is composed of right valves only. Until more is known about C. (Co.) milnelandensis from both Greenland and Svalbard, we prefer to leave our species in open nomenclature. Occurrence. Camptonectes (Co.) milnelandensis: Upper Callovian – Upper Oxfordian of Milne Land, East Greenland (Fürsich 1982). Camptonectes (Co.) aff. milnelandensis: Seep 8 (Upper Volgian), Slottsmøya Member (Tab. 1); also Dorsoplanites bed (Middle Volgian), Svalbard. Palaeoecology. We assume that C. (Co.) aff. milnelandensis was a byssally attached suspension feeder, as indicated by the presence of a well-developed ctenolium. Byssal attachment is a characteristic feature for the genus Camptonectes, although larger species in the subgenus McLearnia Crickmay, 1930, probably lost a functional byssal attachment in maturity (e. g. Fürsich 1982). An expanded anterior auricle in pectinids is an adaptation for byssal attachment with the commissure non-perpendicular to the substrate (Stanley 1970). In such a position the auricle and anterior shell margin acts as a fulcrum and prevents the shell overturning from its set position. As in most of the pectinids, C. (Co.) aff. milnelandensis probably had the ability to swim to escape predation, although the umbonal angle is too acute to assume this was habitual (Stanley 1970).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33E04E6F9A3FB6A2A9D.taxon	description	(Figure 10 L – O) 1836 Pecten clathratus sp. nov. — Roemer, p. 212, pl. 13, fig. 9. 1984 Camptonectes (Camptochlamys) clathratus (Roemer) — Johnson, p. 143, pl. 4, figs. 23, 26, 27, pl. 5, figs. 1 – 3, 6, text-figs. 131 – 136 and references therein. 1984 Camptonectes (Camptochlamys) intertextus (Roemer) — Kelly, p. 75, pl. 6, figs. 16 – 18.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33E04E6F9A3FB6A2A9D.taxon	materials_examined	Material examined. Two poorly preserved specimens. In addition, a single disarticulated specimen from the Dorsoplanites bed (PMO 226.603). See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33E04E6F9A3FB6A2A9D.taxon	description	Dimensions. 15.8 – 23.8 mm in length, 15.8 – 23.2 mm in height. See Appendix 2 H for details. Description. Shell small to medium, thin. Both valves suborbicular, left valve moderately inflated, with anterior and posterior auricles subequal and not prominent. Umbonal angle ca. 100 °; hinge line straight, umbones orthogyrate and not projecting. Curvature of right valve unknown. Anterodorsal margin straight to very weakly convex, posterodorsal margin straight. Anterior and posterior margins subequal, with anterior margin slightly more rounded. Ventral margin evenly rounded. Ornament composed of series of dense commarginal lamellae intersected by fine primary ribs and secondary radial riblets. Intersection of radial and commarginal ornament forms a cancellate pattern. Specimen measuring 23.2 mm in height has three growth lines per millimeter. Musculature and dentition unknown.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F03E33E04E6F9A3FB6A2A9D.taxon	discussion	Remarks. Our specimens have an external ornament and shell morphology within the range of Camptonectes (Camptochlamys) clathratus (Roemer, 1836). Similarly shaped and ornamented specimens have been recorded by Kelly (1984) as C. (Cc.) intertextus (Roemer, 1839), which is the younger synonym of C. (Cc.) clathratus according to Johnson (1984). Occurrence. The species is known from Bajocian to Kimmeridgian of Western and Central Europe (Johnson 1984), and the Middle Volgian of Greenland (Fürsich 1982). It has been recorded previously from the Jurassic of Svalbard as Pecten (Chlamys) pertextus var. densiradiatus var. nov. by Sokolov & Bodylevsky (1931, p. 54, pl. 3, figs. 6 a – b). Seep 8 (Upper Volgian), Slottsmøya Member (Tab. 1); also Dorsoplanites bed (Middle Volgian), Svalbard. Palaeoecology. Camptonectes (Camptochlamys) clathratus was a byssally attached filter feeder (Johnson 1984) and in the Svalbard seeps it probably attached onto exposed carbonate substrates. The umbonal angle increase during ontogenesis (Johnson 1984) suggests that juveniles may have been attached to a firm substrate with a byssus, which was then lost in adults so they became recliners (e. g. Stanley 1970). In the Mid- to early Late Jurassic C. (Cc.) clathratus is often associated with shallow water corals, upon which juveniles were supposed to attach byssally, and oolitic facies, in which the adult forms reclined (Johnson 1984). The Svalbard specimens (Sokolov & Bodylevsky 1931; this study) represent both the youngest occurrence and the most offshore occurrence of the species, which suggests that it either had a broader ecological tolerance than previously suspected, or C. (Cc.) clathratus shifted its preference to more offshore environments around the latest Jurassic.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33E04E6F9D1FD7C2F85.taxon	type_taxon	Type species. Plagiostoma duplicata (J. Sowerby, 1814)	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	description	(Figure 10 P – R) 1966 Lima (Pseudolimea) arctica sp. — Zakharov, p. 62, pl. 16, figs. 6 – 11. 1966 Pseudolimea aff. parallela Orbigny [sic] — Zakharov, p. 64, pl. 16, fig. 12. 1982 Pseudolimea cf. arctica (Zakharov) — Fürsich, p. 61, fig. 25 A. 2011 Pseudolimea sp. — Hammer et al., fig. 7 k, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	materials_examined	Material examined. 18 specimens, all single valves; shells and internal moulds. See Appendix 1 for the list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	description	Dimensions. 18.1 – 34 mm in length, 16 – 31.5 mm in height. See Figure 12 A – C and Appendix 2 I for details.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	diagnosis	Diagnosis. A Pseudolimea with up to 35 radial, v-shaped primary ribs with weaker ribs on anterior and posterior parts of the shell. Inter-rib spaces with single smaller riblet.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	description	Description. Shell small to medium, up to 34 mm long and 31.5 mm high. Shell oval, inequilateral, posteroventrally elongated, triangular, with umbonal angle around 90 °. Anterodorsal margin weakly convex, passing abruptly into broadly arcuate ventral margin. Posteroventral margin tightly arcuate, passing gently into straight and long posterodorsal margin. Posterodorsal margin about twice length of anterodorsal margin. Hinge line straight, around quarter shell length, with smaller anterior and larger posterior auricles not projecting beyond shell outline. Umbo small, weakly prosogyrate beak projecting beyond hinge line. Shell ornament of 24 – 35 v-shaped primary ribs with intercalated weaker secondary riblets in interspaces. Radial ornament dissected by weak, commarginal growth lines visible on outer shell surface only. Internal moulds show only rounded primary ribs. Ribs start on umbo, getting progressively stronger towards ventral margin; strongest on shell mid-flank, becoming weaker close to anterior and posterior shell margins. Ribs fade and then disappear on shell anterior where ventral curvature bends up towards anterodorsal margin; on posterior margin ribs disappear where posteroventral curvature passes into straight posterodorsal shell margin. Dorsal margins and auricles ornamented by concentric growth lines only. Dentition not observed.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F02E33904E6F884FC952EDB.taxon	discussion	Remarks. We include this species in the Siberian Kimmeridgian – Volgian taxon Pseudolimea arctica (Zakharov, 1966), based on its external shell proportions and the number of ribs. Comparison of shell measurments (Fig. 12 A – C) shows that the Svalbard Pseudolimea specimens and P. arctica have an almost identical shell shape. The Svalbard specimens are more densely ribbed than P. arctica (Zakharov, 1966). However, more sparsely ribbed Svalbard specimens form a morphological continuity with the densely ribbed specimens of P. arctica, so there is no reason to separate them on rib number. Coarsely ribbed P. arctica specimens from Siberia overlap in rib number with P. multicostata Fürsich, 1982, from England, Greenland and Siberia (Tab. 10, Zakharov 1966; Fürsich 1982; Kelly 1984) and with Lima (Mantellum) parallela (J. Sowerby, 1812) from the Aptian – Albian of England (Woods 1904), which has no more than 23 ribs. However, both P. multicostata and P. parallela are more elongated than P. arctica from Siberia and Svalbard (Fig. 12 B, C). Interestingly, Lima (Pseudolimea) aff. parallela (J. Sowerby, 1812) non (d’Orbigny), from the upper Ryazanian of Siberia (Zakharov 1966) has 27 ribs and similar shell proportions to P. arctica, but not to P. parallela as shown by the data of Woods (1904), and should therefore also be included into P. arctica. Occurrence. Seep 9 (uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). It also occurs in the Kimmeridgian, Volgian and possibly also the Upper Ryazanian of Siberia (Zakharov 1966) and the Volgian of Greenland (Fürsich 1982). Palaeoecology. Pseudolimea arctica probably took advantage of hard substrates present in the seeps, such as exposed carbonate and other shells, for byssal attachment. Modern limids can be found both in shallow and deep marine environments (Allen 2004). They are suspension feeders, byssally attached to rocks, shells and marine plants. They can also form ‘ nests’ composed of mucus, byssal threads and various incorporated hard elements if the substrate proves unsuitable for byssal attachment (Merrill & Turner 1963). Some species possess the ability to swim by clapping their valves, and others are able to relocate using their foot and byssus (Mikkelsen & Bieler 2008). Unlike other monomyarian bivalve families, limids are not pleurothetic, i. e. they do not rest on one of the valves, but orient their commissure perpendicular to the substrate (Mikkelsen & Bieler 2008). The extinct genus Pseudolimea probably behaved in the same way. No association between limids and symbiotic bacteria has been recorded to date, but Acesta oophaga Järnegren, Schander & Young, 2007 in Järnegren et al. 2007, from hydrocarbon seep environments in the Gulf of Mexico has morphological adaptations for feeding on the eggs of the tubeworm Lamellibrachia luymesi van der Land & Nørrevang, 1975. It has a folded posterodorsal shell margin that surrounds the tubeworm plume, where eggs are being released from the tube (Järnegren et al. 2005). No such adaptation has been recorded in any species of Pseudolimea.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F04E33B04E6F8F5FBBC2D60.taxon	type_taxon	Type species. Lucina ovalis Stanton, 1895	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F04E33B04E6F8F5FBBC2D60.taxon	discussion	Remarks. We assign our lucinid specimens to the genus Tehamatea Kiel, 2013, based on their external ornament composed of commarginal growth lines only, oval external shape of the shell, reduced 3 a cardinal, and the length and divergence angle of the anterior adductor muscle scar (Kiel 2013). Tehamatea is a Late Jurassic – Early Cretaceous seep-restricted genus known from California (Stanton 1895), the Basque-Cantabrian Basin (Agirrezabala et al. 2013), the Vocontian Basin and Planerskoje in Crimea (Kiel & Peckmann 2008; Kiel 2013). The similar lucinid genus Beauvoisina Kiel, Campbell & Gaillard, 2010, has much weaker muscle scars than the Svalbard specimens, and has a ridge developed within the lunule, a feature not seen in our material (Kiel et al. 2010; Kiel 2013). Beauvoisina also has beaks located further towards the posterior than the Svalbard specimens. Another seep-restricted lucinid genus, Cubatea Kiel, Campbell & Gaillard, 2010, differs from our specimens by having a much stronger anterior lateral teeth and lacking a 3 b cardinal. The Late Cretaceous – Paleocene seep lucinid genus Nymphalucina Speden, 1970, has an external ornament composed of sparse commarginal ridges, stronger sulcation, and strong cardinal and lateral dentition (Speden 1970; Kiel 2013), all features lacking in our material. Comparable Mesozoic non-seep lucinid genera include Jagonoma Chavan, 1946, from the Jurassic of France (Chavan 1946; 1947; 1952), but our specimens do not belong to this genus because it has stout cardinal dentition, with thick and well-formed 3 a and 3 b, and an anterior adductor muscle scar that only weakly diverges from the pallial line. Another European Jurassic genus, Discomiltha Chavan, 1952, is on average less inflated than the Svalbard specimens, has an external ornament composed of regularly spaced, commarginal ridges and cardinal dentition of two weak denticles (e. g. Duff 1978). The Jurassic genus Mesomiltha Chavan, 1938, differs from our specimens with its regular commarginal ornament and more angular shape (e. g. Kelly 1992). Discoloripes Wellnhofer, 1964, has a similar shape to the Svalbard material, but has a much longer and club-shaped anterior adductor muscle scar (e. g. Wellnhofer 1964; Kelly 1992). The taxonomy of Mesozoic lucinids is currently problematic, and in need of revision, because many Mesozoic and Recent species are homeomorphic and difficult to distinguish without in-depth study (e. g. Gerasimov 1955; Wellnhofer 1964; Kelly 1992). Further, many Mesozoic lucinids have been classified tentatively as “ Lucina ” (e. g. Woods 1907, p. 152 – 153, fig. 2 – 6, 10 – 19; Lewinski 1922, p. 78, pl. 4, fig. 4). Recent molecular phylogenetic studies show that most of the modern lucinid lineages can be traced back through the Cenozoic, but are much more difficult to recognize in the Mesozoic (Williams et al. 2004; Taylor et al. 2011).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	description	(Figure 13 A – J) 2011 Lucinid sp. — Hammer et al., fig. 7 d – e, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	etymology	Etymology. After Jan Audun Rasmussen, curator of the Greenland collection in the Natural History Museum of Denmark, Copenhagen, study of which helped with taxonomic determinations in this paper.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	materials_examined	Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78 ° 18 ’ 49.9 ” N 16 ° 10 ’ 58.9 ” E. Type material. Holotype: PMO 217.234; a partially articulated shell the vertically displaced valves; right valve shows cardinal and anterior lateral dentition of the right valve; left valve is an internal mould, with silicone rubber cast. Paratypes: PMO 217.169; an articulated, partially preserved shell showing outline and details of external ornament. PMO 217.173; an internal mould and silicone rubber cast showing part of the right valve cardinal dentition. PMO 217.227; an articulated specimen showing the ligament and posterior sulcation. PMO 217.243; an articulated, partially preserved internal mould and silicone rubber cast showing the right valve cardinal dentition. PMO 217.247; an articulated, partially preserved internal mould showing bioimmuration trace. PMO 225.101; an internal mould with silicone rubber cast showing anterior lateral dentition. PMO 225.104; an internal mould showing anterior adductor muscle scar and posterior adductor muscle scar and possible mantle gill scars. PMO 225.111; an internal mould with silicone rubber cast showing anterior and posterior hinge plate areas. Material examined. 60 specimens, mostly articulated internal moulds, some with adhering shell, and a few single valves. See Appendix 1 for list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	description	Dimensions. 17 – 100 mm in length, 13 – 76 mm in height, 7.3 – 54 mm in width. See Figure 14 A – F and Appendix 2 J for details.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	diagnosis	Diagnosis. Shell oval to weakly hexagonal in shape, covered with dense commarginal growth lines. Posterodorsal margin gently convex, posterior area slightly flattened. Posterior margin slightly truncated. Anterior and posterior laterals short and weak. Cardinal tooth 3 a very small, 3 b large, not bifid. Anterior adductor muscle scar elongated, weakly incurved. Anterior pedal retractor scar well impressed, separated from anterior adductor muscle scar. Posterior adductor muscle scar large, deeply impressed, merged with posterior pedal retractor scar.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	description	Description. Shell large, up to 100 mm long, 76 mm high, and 54 mm wide. Average H / L ≈ 0.78, relatively constant throughout ontogeny. W / H ratio ≈ 0.67, also relatively constant throughout ontogeny. Shell oval to weakly hexagonal in outline, inequilateral with umbones positioned closer towards anterior; average Pl / L ratio of ≈ 0.64. Umbones prosogyrate, not very prominent. Anterodorsal margin short and relatively straight. Lunule asymmetric, larger in left valve than in right; large, lancet-shaped and deep, occupying on average ≈ 0.46 of anterior shell length. Anterior margin arcuate, with dorsally tightening curvature. Ventral margin broadly rounded, smoothly passing into moderately tight posteroventral margin. Posterior margin inclined, truncated, posterodorsal margin long, weakly and evenly convex; some specimens develop weakly flattened posterior area. Ligament opisthodethic, external, long. Hinge plate thick. AI short, thick but not prominent, positioned at anterior end of hinge plate. PI short, with socket corresponding to right valve anterior lateral tooth. Posterior laterals weak. PII unknown, presumably shallow. PIII very weak and short, oval. Cardinal dentition present. Right valve: 3 a very small, opisthocline; 3 b prosocline, strong, not bifid, weakly curved; at base 3 b supported by a thickening of hinge plate. Left valve: 2 triangular, deepest at base, becoming weaker dorsally; 4 b arcuate, prosocline. Anterior adductor muscle scar deep, striated and detached from pallial line around two thirds to three quarters of length. Ventral margin of anterior adductor muscle scar sharp, dorsal margin irregular and jagged. Anterior pedal retractor scar circular, positioned below anterior lateral teeth, weakly separated from anterior adductor muscle scar. Deep groove connecting anterior adductor muscle scar with umbonal cavity represents trace of descent during growth. Posterior adductor muscle scar deep, striated, pointed ventrally, heart-shaped, projecting beyond the pallial line for about half of diameter. Trace of descending posterior adductor muscle visible as deep grooves connecting it with umbonal cavity. Posterior pedal retractor scar not seen, presumably merged with posterior adductor muscle scar. Pallial line strong, entire; wavy below anterior adductor muscle scar. Some specimens develop dispersed pustules on internal shell surface.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F07E30704E6FBD8FD792FCE.taxon	discussion	Remarks. Tehamatea rasmusseni sp. nov. differs from T. ovalis (Stanton, 1895), from Tithonian to Albian seeps from California by the smaller size of the 3 a anterior cardinal in the former (Stanton 1895; Kiel 2013). Otherwise, the two species are very similar. Tehamatea rasmusseni differs from another Californian seep lucinid, the Tithonian – Hauterivian T. colusaensis (Stanton, 1895), by the more rounded posterior margin and more equilaterally positioned beaks in T. rasmusseni. Tehamatea rasmusseni has similar dentition to T. agirrezabalai Kiel, 2013, from Albian seeps of northern Spain, but in comparison with that species T. rasmusseni has a longer and more detached anterior adductor muscle scar and less projecting beaks. Tehamatea vocontiana from Hauterivian seep carbonates of southern France (Lemoine et al. 1982) and Crimea (Kiel & Peckmann 2008) has a cardinal dentition with only an orthocline 3 b developed, unlike T. rasmusseni, which has both 3 a and 3 b developed, with the 3 b weakly curved and prosocline. Occurrence. Seep 9 (uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Known only from the type locality. Palaeoecology. The seep-restricted distribution of the genus Tehamatea (Kiel 2013), together with the clustering of T. rasmusseni at seep 9 and the presumed antiquity of chemosymbiosis among lucinids (Taylor & Glover 2010) strongly suggests that T. rasmusseni was chemosymbiotic and took advantage from the reduced compounds available in the seep environment. Lucinids are a diverse group of burrowing bivalves having obligate chemosymbiotic relationships with sulfide-oxidizing bacteria (e. g. Dando et al. 1986; Reid & Brand 1986; Herry et al. 1989; Taylor & Glover 2000; Glover et al. 2004). They inhabit a variety of environments, being especially diverse in tropical and temperate shallow water environments with high redox potential, like seagrass beds (Mikkelsen & Bieler 2008), mangrove swamps (Frenkiel et al. 1996), coral sands (Glover & Taylor 1997), reducing sediments (Dando et al. 1986) and sewage outfalls (Herry et al. 1989). Lucinids are also common at hydrocarbon seeps, from both shallow and deep water (e. g. Salas & Woodside 2002; Holmes et al. 2005; Taylor & Glover 2009; Oliver et al. 2011). An increasing number of modern lucinid genera are being reported from deep water sites, and have been recovered from organic-rich sediments in water as deep as 2570 m (Cosel 2006). To reach sulfide-rich pore waters necessary to feed their symbionts, most lucinids burrow down to the oxic / dysoxic interface, where they remain stationary with their umbones facing upwards. Their muscular foot is then used to construct ventral tunnels to supply sulfide-rich water to the symbionts in the gills (Taylor & Glover 2010). Oxygenated seawater is supplied via a subvertical mucus-lined tube entering the body parallel to the anterior adductor muscle (Stanley 1970). The elongated and detached anterior adductor muscle acts as a partition separating symbiont-bearing gills from the respiratory surface of the mantle around the anterior opening (Taylor & Glover 2000). Similar anatomical features as shown by internal shell features are found in the lucinid fossil record back to the Silurian (Boyd & Newell 1979; Fürsich 1982; Kelly 1992; Liljedahl 1992). The wavy pallial line below the anterior adductor muscle scar suggests that T. rasmusseni developed mantle lobes with possible respiratory function (e. g. Taylor & Glover 2000). One specimen shows traces in both valves of tubular structures that represent either shell boring activity or an organism living between the mantle and the shell (bioimmuration traces). Very similar structures have been found in other fossil seep bivalves (Kiel & Peckmann 2008; Jenkins et al. 2013), and have been attributed to possible polychaete worms. All specimens found are articulated or semi-articulated shells filled with carbonate micrite and were enclosed in in matrix composed of cracked and worn Buchia shells with some rare and disarticulated Pseudolimea arctica, Oxytoma octavia and Camptonectes spp. As Svalbard seeps developed in low-depositional rate environment and seabed omission was frequently the case (Hryniewicz et al. 2012), Tehamatea rasmusseni must have been a relatively deep burrower. After death, specimens remained buried for some time and were not exposed by the bottom currents until early carbonate cementation in the seep environment kept the valves in an articulated state.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3BE30104E6F890FC592848.taxon	type_taxon	Type species. Cretaxinus hurumi sp. nov.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3BE30104E6F890FC592848.taxon	etymology	Etymology. Refers to the Cretaceous occurrence of the type species, and to the genus Axinus.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3BE30104E6F890FC592848.taxon	diagnosis	Diagnosis. Shell inequilateral, triangular in outline, thin. Ornament of commarginal growth lines. Beaks weakly prosogyrate, not very prominent. Cardinal area edentulous, with small ligament groove. No lateral teeth. Lunule long and deep. Posterior sulcus shallow, posterior fold not very prominent. Submarginal sulcus shallow; no auricle. Ligament short, thick, external, with possible small internal portion. Anterior adductor muscle scar elongated, very weak; size difficult to estimate, but relatively small. Posterior adductor muscle scar larger than anterior one, circular, well impressed. Pallial line entire.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3BE30104E6F890FC592848.taxon	discussion	Remarks. Comparison of Cretaxinus gen. nov. with other large, chemosymbiotic thyasirids (Dufour 2005) suggest it can be clearly differentiated from all of them (Tab. 2). The most noticeable feature is its subtriangular shape, which is very distinct from the shapes of the genera Axinus J. Sowerby, 1821, Thyasira Lamarck, 1818, Parathyasira Iredale, 1930, and Conchocele Gabb, 1866 (Tab. 2). Another feature distinguishing Cretaxinus from other thyasirids is its short, thick and external ligament set in a deep escutcheon, accompanied by a possible small internal portion. A similar feature is seen only in Axinus. However, Axinus has a totally different shape, sulci, adductor muscle proportions and external ornament (Tab. 2). The ligament of Cretaxinus is very distinct from that of Thyasira, Parathyasira and Conchocele, all of which have variably long, sunken ligaments (Tab. 2). Another major difference between Cretaxinus and Axinus, Thyasira, Parathyasira and Conchocele is the proportions of adductor muscle scars. In Cretaxinus the anterior adductor muscle scar is small and weak, and the posterior adductor muscle scar is large and well impressed; in the other genera the proportions of the muscle scars are reversed. The sulci are less distinct and somehow anteroposteriorly flattened in comparison to the sulci of the other discussed genera. The escutcheon is wider and deeper than that of Thyasira and Parathyasira. Knowledge of small thyasirid species is far less complete than of their large relatives (Payne & Allen 1991; Oliver & Killeen 2002) and therefore their direct comparison with Cretaxinus gen. nov. is more problematic. Mendicula Iredale, 1924, has a very small, oval shell (<3 mm in length) with a pointed posterior margin (Payne & Allen 1991; Oliver & Killeen 2002), unlike Cretaxinus gen. nov. Both sulci of Mendicula are shallow and the ligament is entirely internal (Zelaya 2010), also unlike in Cretaxinus gen. nov., which has weak sulci but largely external ligament. Adontorhina Berry, 1947, apart from small, oval shells with an internal ligament, differs from Cretaxinus by characteristic granules on the hinge plate of the former (Scott 1986; Barry & McCormick 2007). Axinulus Verrill & Bush, 1898, has a small oval shell which is higher than long (Payne & Allen 1991) and a largely internal ligament, also unlike Cretaxinus gen. nov. The current lack of taxonomically robust characters in the shells of the Thyasiridae leads to problems of generic separation within the family (e. g. Payne & Allen 1991; Oliver & Killeen 2002; Oliver & Sellanes 2005; Oliver & Holmes 2006; Rodrigues et al. 2008). Molecular data suggest that current thyasirid generic definitions might be inadequate and additional morphological studies may result in redefinition of the genera (Taylor et al. 2007). Further, the same data show only a weak separation between larger, chemosymbiotic thyasirids and small (<10 mm) thyasirid genera without symbionts, like Mendicula Iredale, 1924, Leptaxinus Verrill & Bush, 1898, Adontorhina Berry, 1947, and Axinulus Verrill & Bush, 1898 (Dufour 2005).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	description	(Figures 13 K – N, 15, 16 A – G) 2011 Thyasira sp. — Hammer et al., fig. 7 a – c, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	etymology	Etymology. After Jørn H. Hurum, leader of 2004 – 2012 Svalbard expeditions of the Natural History Museum, University of Oslo.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	materials_examined	Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78 ° 18 ’ 49.9 ” N 16 ° 10 ’ 58.9 ” E. Type mterial. Holotype: PMO 217.277; an internal mould with shell partially preserved, showing a triangular outline, external ornament, a sulcated posterior margin and a thick, short, external ligament. Paratypes: PMO 217.172; an almost complete internal mould showing outline and anterior adductor muscle scar. PMO 217.175; a fragment of an internal mould and silicone rubber cast showing posterior adductor muscle scar and posterior pedal retractor. The silicone rubber cast shows the cardinal area with an elongated resilifer. PMO 217.540; complete internal mould showing the triangular outline, well impressed rounded posterior adductor muscle scar and deep escutcheon. PMO 225.128; an almost complete internal mould showing anterior adductor muscle scar and crosssection through the external ligament and ligament nymphs. PMO 225.136; an internal mould showing very weak anterior adductor muscle scar. Material examined. 56 specimens, all articulated or semi-articulated internal moulds with variable amounts of shell preserved. See Appendix 1 for list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	description	Dimensions. 23 – 56.5 mm in length, 12.6 – 50 mm in height, 19 – 39 mm in width. See Figure 17 A – D and Appendix 2 K for details.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	diagnosis	Diagnosis. As for the genus.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	description	Description. Shell large, subtriangular in outline, up to 56.5 mm long, 50 mm high, and 39 mm wide. Average H / L ratio ≈ 0.85, W / L ≈ 0.75, and W / H ≈ 0.88. Shell less than 0.5 mm thick, covered with commarginal growth lines. Umbonal angle usually larger among smaller specimens, which have concave anterodorsal and posterodorsal margins. Beaks incurved, prosogyrate, not prominent, positioned closer towards the anterior, with average Pl / L ≈ 0.67. Umbonal angle slightly acute. Lunule large, deep, heart-shaped. Anterior margin tightly rounded, more angular in larger specimens. Ventral margin curved, with curvature deepest close to mid-line in smaller specimens, progressively displaced towards posterior during growth. Curvature shows some intraspecific variation, from deep and prominent, to shallow and gentle. Posterior extremity weakly pointed, posterodorsal margin sloping, with very weak sulcus. Smaller specimens usually have slightly concave posterodorsal margin, which is more straight in larger specimens. Escutcheon large and deep. Ligament external, thick, short, occupying 1 / 3 of escutcheon. Hinge plate narrow. Cardinal area with single, elongate groove, probably representing a ligament groove. Lateral dentition not observed. Anterior adductor muscle scar very weak and small, elongated along pallial line, with straight ventral margin and irregular dorsal margin; well impressed in anterior part, fading towards posterior so length of anterior adductor muscle scar cannot be fully ascertained. Anterior pedal retractor scar small and weak, circular, separated from the anterior adductor muscle scar by narrow margin, visible in one specimen only. Posterior adductor muscle scar larger than anterior adductor muscle scar, circular, deeply impressed, displaced towards hinge plate. Posterior pedal retractor small, circular, approximately same size as anterior pedal retractor scar; positioned close to hinge plate and separated from posterior adductor muscle scar by narrow distance. Pallial line entire, weak, marked by pallial muscle scars in some specimens. Internal shell surface covered with fine radial ornament, probably representing traces of descending pallial muscles.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F3DE30D04E6FEC7FE46286E.taxon	discussion	Remarks. Cretaxinus hurumi gen. et sp. nov. is the oldest thyasirid species known to date. The slightly younger Valanginian to Hauterivian thyasirid is Lucina? rouyana d’Orbigny, 1844, from the shelf deposits of Europe and possible seep sites of Grodziszcze beds in the Carpathians (Ascher 1906, p. 164, pl. XIV, fig. 9 a – c; Kiel et al. 2008 a; Kaim et al. 2013), which has more of a typical Thyasira shape. The Albian Lucina? sculpta Phillips, 1829, from Southern England (Woods 1907, p. 153, pl. 24, fig. 7 – 9) has a very distinct shape reminiscent of the genus Axinus Sowerby, 1821 (Taylor et al. 2007). Thyasira tanabei Kiel, Amano & Jenkins, 2008 (a) is known from Albian to Campanian hydrocarbon seeps of Hokkaido, Japan. Thyasira tanabei has a large anterior adductor muscle scar, unlike C. hurumi, but the lack of information about the ligament of T. tanabei makes more detailed comparison difficult. Thyasira sp. from the Cenomanian Kanajirasawa seep of Hokkaido (Kiel et al. 2008 a) is known from a single, partially preserved specimen only and, therefore, is also difficult to compare to C. hurumi. Various Campanian thyasirid species from the Western Interior Seaway (Kauffman 1967) differ in shape from C. hurumi, having flattened posterior areas, narrow escutcheons, deeper sulci and more ovate shapes, reminiscent of Recent Thyasira species. Occurrence. Seep 9 (uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Known only from the type locality. Palaeoecology. We infer that Cretaxinus hurumi was a chemosymbiotic and possibly seep-restricted Mesozoic thyasirid. This is supported by the large shell size of C. hurumi. Among modern thyasirids chemosymbiosis is present mainly in species with two gill demibranchs (Dufour 2005) and these usually have large shells, up to around 10 mm for the genera Axinus, Thyasira and Parathyasira, but can reach up to 110 mm in Conchocele (e. g. Payne & Allen 1991; Kamenev et al. 2001; Oliver & Killeen 2002). Large shells provide enough space in the mantle cavity for large symbiont-bearing gills (Taylor & Glover 2010). In contrast, thyasirids with a single gill demibranch are usually asymbiotic (Dufour 2005; Taylor & Glover 2010). These asymbiotic species are much smaller, with sizes of only a few millimeters (Payne & Allen 1991; Oliver & Levin 2006). Cretaxinus hurumi shells are up to 56.5 mm long, and this is very large for the family, which strongly suggests the species had hypertrophied gills suitable for symbiosis with chemoautotrophic bacteria. Smaller chemosymbiotic thyasirids (≈ 10 mm in length) are known from seep environments (e. g. Dando et al. 2004), but also occur in non-seep settings with high redox potential, such as organic-rich sediments in fjords (Dando & Spiro 1993), pulpmill effluents (Dando & Southward 1986) and in the vicinity of offshore hydrocarbon production sites (Oliver & Killeen 2002). However, the only Recent thyasirid genus which attains sizes comparable to C. hurumi is Conchocele, and this is closely associated with seeps (Kamenev et. al 2001; Okutani 2002; cf. Weaver 1942; Coan et al. 2000). Another line of evidence indicating that C. hurumi was both chemosymbiotic and seep-restricted comes from absence of the species in contemporary ‘ normal’ marine sediments on Svalbard (e. g. Sokolov & Bodylevsky 1931; Weir 1933; Birkenmajer et al. 1982), despite being relatively abundant in the hydrocarbon seeps (Hammer et al. 2011). Thyasirids are burrowers (e. g. Dando & Southward 1986; Oliver & Killeen 2002). Chemosymbiotic species dig into fine grained sediment to a depth a few times the length of the shell (Pearson 1972). In these burrows they use a vermiform foot to construct a three dimensional network of downward directed tunnels reaching up to 30 times the length of the shell, which serve as conduits for sulfide-rich pore waters from deeper interstitial levels (Dando & Southward 1986; Zuschin et al. 2001; Oliver & Killeen 2002; Dando et al. 2004; Taylor & Glover 2010). However, they are usually unable to tolerate high sulfide concentrations. Chemosymbiotic thyasirids are also capable of feeding on photosynthetic organic matter if conditions for chemosynthesis become unfavourable (Dando & Spiro 1993). Cretaxinus hurumi was probably a relatively deep burrower, based on similar lines of evidence to the lucinid Tehamatea rasmusseni. Order Venerida Gray, 1854	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F31E30D04E6FEC4FD5228A2.taxon	type_taxon	Type species. Cypricardia bathonica Morris & Lycett, 1853	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F31E30F04E6FD9FFDFD2F57.taxon	description	(Figure 16 H – P) 1936 aff. Pseudotrapezium groenlandicum sp. nov. — Spath, p. 125, pl. 49, fig. 7 a – c.? 1982 aff. Hartwellia (Hartwellia) groenlandica (Spath) — Fürsich, p. 89. 2011 Arcticid — Hammer et al., fig. 7 m, tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F31E30F04E6FD9FFDFD2F57.taxon	materials_examined	Material examined. 820 specimens, mostly articulated or semi-articulated shells and internal moulds. See Appendix 1 for list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F31E30F04E6FD9FFDFD2F57.taxon	description	Dimensions. 12 – 15 mm in length, 9.8 – 11 mm in height, 8 – 10 mm in width. See Figure 18 A – D and Appendix 2 L for details. Description. Small, moderately inflated with very thin shell. Beaks prosogyrate, very strongly incurved. Lunule moderately deep. Anterior margin projecting, arcuate, passing into broadly arched ventral margin. Posterior area with two carinae, posteroventral extremity slightly rostrate where intersected by first carina, passing into truncated oblique posterior margin. Second carina weak, parallel to the posterodorsal margin. Posterodorsal margin straight. External ornament composed of very weak commarginal growth lines. Hinge plate large and thick; lateral tooth AI developed, separated from 1; 3 a absent. 1 and 3 b separated, diverging from umbo; 1 strong and vertical; 3 b thick, anterodorsally inclined. Angle between 1 and 3 b ca. 70 °. Left valve dentition unknown. Anterior adductor muscle scar weak, elongated along pallial line, around twice as long as wide, not detached. Posterior adductor muscle scar weak, rounded. Pallial line weak, entire.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F31E30F04E6FD9FFDFD2F57.taxon	discussion	Remarks. We compare our specimens with Pseudotrapezium groenlandicum Spath, 1936, from Milne Land, East Greenland (Spath 1936) due to their thick hinge plate, similar cardinal dentition and external shape. However, the Svalbard specimens are much smaller and more thin shelled than the Greenland material, so we are not entirely sure they represent the same species and leave them in open nomenclature. Fürsich (1982) moved P. groenlandicum into the genus Hartwellia Kitchin, 1926, and redescribed it as Hartwellia (H.) groenlandica. Hartwellia was considered by Cox (1944) to be a synonym of Pronoella Fischer, 1887. Including P. groenlandicum into Pronoella is not justified by the hinge of our Svalbard specimens. Both Pronoella and Pseudotrapezium have relatively thick hinge plates and a similar external shape (e. g. Casey 1952), but AI and 1 are connected in Pronoella into a single elongate denticle and separated in Pseudotrapezium (Benecke 1905), showing the genera are not the same. Occurrence. P s eudotrapezium groenlandicum: Lower Volgian – Lower Ryazanian of Milne Land, East Greenland (Spath 1936; Fürsich 1982). Pseudotrapezium aff. groenlandicum: seeps 2, 3, 5, 8, 9 and 12 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Palaeoecology. We assume that P. aff. groenlandicum was a burrower, feeding on organic-rich sediment layer while resting in the shallow subsurface. The lack of a pallial sinus suggests that P. aff. groenlandicum possessed very short siphons, similar to its extant relative Arctica islandica (Linnaeus, 1767). Arctica islandica has very short siphons and positions its posterior extremity at the sediment-water interface for feeding on organic matter from the sediment surface (Morton 2011). As it does so it remains relatively motionless, circulating water only by means of ciliary movement (Brand & Taylor 1974). It can rebury itself into deeper sediment layers, where it remains isolated from seawater for up to seven days when it is not feeding, and respires anaerobically (Taylor 1976). The clustering of very large numbers of P. aff. groenlandicum specimens in seep 9 is difficult to explain. No association between arcticids and chemosymbiotic bacteria has been noted to date, so we do not envisage any specific trophic link between P. aff. groenlandicum and the seep environment. Subclass Anomalodesmata Dall, 1889 Order Pholadomyoida Newell, 1965	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F33E30F04E6F988FBE42F9A.taxon	type_taxon	Type species. Mya angulifera J. Sowerby, 1819. Subsequent designation Herrmannsen, 1846.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F33E30E04E6F885FB4A2FC5.taxon	description	(Figure 19 A – B) 1819 Mya? literata — J. Sowerby, p. 45, pl. 224, fig. 1. 1931 Goniomya arctica nov. sp. — Sokolov & Bodylevsky, p. 76, pl. 4, fig. 8. 1934 Goniomya literata (J. Sowerby) — Arkell, p. 344, pl. 47, figs. 1 – 7 and references therein.? 1982 Goniomya cf. dubois Agassiz — Birkenmajer et al., pl. 43, fig. 5. 1982 Goniomya literata (Sowerby) — Fürsich, p. 98, figs. 35 e, 36 c – d. 1985 Goniomya literata (Sowerby) — Bäckström & Nagy, p. 35, pl. 5, figs. 1 – 2 and references therein. 2011 Goniomya sp. — Hammer et al., tab. 2.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F33E30E04E6F885FB4A2FC5.taxon	materials_examined	Material examined. Three internal moulds with small pieces of shell. See Appendix 1 for list of specimens.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F33E30E04E6F885FB4A2FC5.taxon	description	Description. Shell small to medium, elongated, outline ovoid; no carina. Umbonal area positioned submedialy. Umbones not preserved. Ornament of up to 13 ventrally directed chevrons, with ridges and depressions between them roughly equal in size. Chevron ridges gently rounded with both ventral and dorsal slopes inclined at an equal angle. At younger growth stages tips of chevrons often blunt so whole ornament is trapezoidal or w-shaped. Line transecting tips of chevrons slightly inclined towards shell posterior. Angle between chevron limbs between 85 and 95 degrees. Chevron ornament transected by weak, concentric growth lines. Pallial line unknown.	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
A2311D4D9F33E30E04E6F885FB4A2FC5.taxon	discussion	Remarks. The oval shape of our specimens with almost equilaterally positioned beaks puts them into Goniomya literata (J. Sowerby, 1819) (e. g. Arkell 1934; Fürsich 1982). The Svalbard specimens differs from G. bicarinata Fürsich, 1982, from Greenland, which has a much longer posterior and two distinct carinae running from the umbo towards the anteroventral and posteroventral margins, respectively. Occurrence. Seeps 3 and 9 (Upper Volgian – uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Goniomya literata is relatively wide-spread (Hallam 1976) and is known from the Middle Jurassic of England (e. g. Arkell 1934), Lithuania (Krenkel 1915), Spitsbergen (Bäckström & Nagy 1985) and the Upper Jurassic of England (Arkell 1934; Clausen & Wignall 1990), Greenland (e. g. Fürsich 1982) and Spitsbergen (Sokolov & Bodylevsky 1931). The genus Goniomya had much wider distribution and is common in many Jurassic fine-grained rocks (e. g. Trautschold 1865; Gerasimov 1955; Koshelkina 1962; Zakharov & Mesezhnikov 1974; Pugaczewska 1986). Palaeoecology. We assume that G. literata was a deep-burrowing suspension feeder, by comparison with its extant relative Pholadomya candida J. de C. Sowerby, 1823 (Morton 1980). This species is a sluggish deep burrower, which remains stationary in the sediment with its siphons protruding towards the surface. The oblique shell ornament of Goniomya, comprising symmetrical ribs, is present among a variety of Late Palaeozoic and Mesozoic bivalves and was probably a shell reinforcement (e. g. Checa & Jiménez-Jiménez 2003).	en	Hryniewicz, Krzysztof, Little, Crispin T. S., Nakrem, Hans Arne (2014): Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard. Zootaxa 3859 (1): 1-66, DOI: http://dx.doi.org/10.11646/zootaxa.3859.1.1
