Rhacothyas spitzbergensis ( Anderson, 1970 )

Hryniewicz, Krzysztof, Amano, Kazutaka, Bitner, Maria Aleksandra, Hagström, Jonas, Kiel, Steffen, Klompmaker, Adiël A., Mörs, Thomas, Robins, Cristina M. & Kaim, Andrzej, 2019, A late Paleocene fauna from shallow-water chemosynthesis-based ecosystems, Spitsbergen, Svalbard, Acta Palaeontologica Polonica 64 (1), pp. 101-141 : 115-118

Rhacothyas spitzbergensis ( Anderson, 1970)

Figs. 11 , 12 .

1925 Lucina spec. ; Hägg 1925: 46, pl. 3: 10a, 11

1927 Thyasira spec. 2 : Gripp 1927: 32, pl. 7: 18a.

1970 Anodontia spitzbergensis n. sp.; Anderson 1970: 95, pl. 9: 8a–c. 2016 Myrteinae sp.; Hryniewicz et al. 2016: table 2, fig. 12B 1 –B 3.

Material.— Holotype ( GPIBo 157 ), relatively well preserved shell, from the upper Paleocene , locality 500 m west of Trigonometric point 25, Hollendarbukta , Spitsbergen , Svalbard; 28 specimens, mostly internal molds (including NRM-PZ Mo 149144, 149145, 182205, 182205a, 183970) from the upper Paleocene of Fossildalen , Spitsbergen , Svalbard; and 21 specimens, mostly shells (including NRM-PZ Mo 183958, 183968, 183969, 183970, 183971, 183974, 186243, 186244) from the upper Paleocene of Zachariassendalen , Spitsbergen, Svalbard.

Measurements.—L, 4.7–16.9 mm; H, 4.2–15.3 mm; W, 2.1– 10.4 mm; n = 27.

Description.—Shell thin, luciniform, longer than high, of normal inflation, with umbones located centrally or slightly towards anterior. Beaks prosogyrate, anterodorsal margin concave with small, sunken lunule. Anterodorsal margin of some specimens with weak angulation. Anterior shell margin rounded, passing smoothly into convex ventral margin. Posterior margin truncated, with no sinus visible. Posterodorsal margin slightly convex, accompanied by weak sulcus located very close to posterodorsal margin. External ornament of raised, narrowly spaced commarginal lamellae. Hinge edentulous. Anterior adductor muscle scar elongated along pallial line, transversally striated, posterior adductor muscle scar positioned close to lower part of posterodorsal shell margin, transverally striated. Anterior pedal retractor muscle scars small, rounded, separated from anterior adductor muscle scars. Ligament sunken, relatively long, occupying approx. half of length of posterodorsal shell margin.

Remarks.—The species discussed was initially attributed to Lucina by Hägg (1925). In the paper following shortly thereafter, Gripp (1927) included the same species in Thyasira , although Gripp (1927) did not comment on the previous identification by Hägg (1925). The contents of the work of Gripp (1927) show that he was aware of the preceeding paper of Hägg (1925); therefore, it is unclear why he did not comment on the previous identification. Anderson (1970), on the other hand, identified this Paleocene form from Spitsbergen as a new species of the lucinid genus Anodontia Link, 1807 , based on the outer shell shape and information on the edentulous hinge provided by Hägg (1925: 46). The species was subsequently classified as an unnamed myrtein lucinid by Hryniewicz et al. (2016), based on overall luciniform shape and the fine, commarginal ornament.

Here we argue that this species, with its luciniform shell, small and sunken lunule, edentulouse hinge, long and sunken ligament, and weak posterior sulcus, belongs to the luciniform thyasirid genus Rhacothyas Åström and Oliver in Åström et al., 2017. The sole modern species of the genus is Rhacothyas kolgae Åström and Oliver in Åström et al., 2017, from the cold seeps off Prins Karls Forland in the North Atlantic (ca. 350 m water depth). Rhacothyas kolgae is somewhat larger (Lmax 27.7 mm), and has a more sloping posterodorsal margin, with a deeper sulcus than the current species, but its general morphology is very similar, with a somewhat stronger posterior sulcus and more incurved umbones than R. spitzbergensis . Other luciniform thyasirid bivalves from chemosynthesis-based ecosystems comprise “ Maorithyas ” hadalis Okutani, Fujikura, and Kojima, 1999 , from a methane seep in the trench off northern Honshu, Japan (7326 m water depth; Fujikura et al. 1999; see Valentich-Scott et al. 2014; Åström et al. 2017, for discussion of the systematic placement of this species), has a weakly defined lunule and a much shorter ligament than the species from Spitsbergen discussed here. Ochetoctena tomasi Oliver, 2014 , from the Regab Pockmark seeps in the Gulf of Guinea, equatorial Atlantic (3167 m water depth), the sole species of Ochetoctena Oliver, 2014 , is very similar in shape to the Paleocene species from Spitsbergen. However, it has periostracal spines on the outer

right (C 1) and left (C 2) valve views, dorsal view, showing partially preserved internal ligament (C 3). D. NRM-PZ Mo 182205, internal mold. E. NRM-PZ Mo 182205a, internal mold. Left valves (D 1, E 1), same valves with outlined positions of anterior ( AAMS) and posterior ( PAMS) adductor muscle scars, anterior ( APRMS) and posterior ( PPRMS) pedal retractor muscle scars, and pallial line (D 2, E 2). F. NRM-PZ Mo 186243, silicified shell, inner view of left valve, showing length of sunken ligament, and no cardinal dentition. G. NRM-PZ Mo 186244, silicified shell, inner view of right valve, showing length of sunken ligament, and no cardinal or anterior lateral dentition.

shell surface not seen in the Spitsbergen material examined, and also less pronounced beaks. Spinaxinus emicatus Oliver in Oliver et al. 2013, from artificial sulfide biogenerators planted on the Louisiana slope (584–643 m water depth) has periostracal spines, less pronounced beaks and a resilifer that broadens towards the posterior. Spinaxinus phrixicus Oliver in Oliver et al. 2013, from hydrothermal vents in the Fiji and Lau Basins, equatorial Pacific (1847–1977 m water depth), in addition to periostracal spines has less pronounced beaks, a resilifer broadening towards the posterior, and commarginal ornament composed of broadly spaced commarginal lamellae, unlike the narrowly-spaced, raised lamellae of R. spitzbergensis . Wallerconcha sarae Valentich-Scott and Powell in Valentich-Scott et al., 2014, from a probable seep in the Beaufort Sea, Arctic Ocean (2358 m water depth), the sole species of Wallerconcha Valentich-Scott and Powell in Valentich-Scott et al., 2014, has a less pronounced umbo and a straight to weakly concave ventral margin, as opposed to the evenly rounded ventral margin of R. spitzbergensis . Wallerconcha sarae also lacks a lunule and has a broad hinge plate with a deeper nymph, whereas the current species possesses a lunule and has a relatively narrow hinge plate. Small, luciniform specimens interpreted as juvenile Conchocele bisecta ( Conrad, 1849) from a cold seep off Paramushir Island, Sea of Okhotsk (~ 800 m water depth; Kharlamenko et al. 2016; see Hryniewicz et al. 2017, for a discussion of nomenclatoral problems of extant species of Conchocele ) are very similar in shape to the current species. However, the luciniform thyasirids from the Sea of Okhotsk are much smaller L 5.5–8.4 mm) than the current species (L 4.7–16.9 mm); at the shell length achieved by R. spitzbergensis , the shell of the thyasirid species from Sea of Okhotsk has a shape more similar to that of typical Conchocele .

Rhacothyas spitzbergensis clearly does not belong in Anodontia , which has a globular shell with short, weakly sloping anterodorsal and posterodorsal margins, and a curved ventral margin forming a deep arch continuous with anterior and posterior margins. In contrast, the Spitsbergen species has relatively long, sloping anterodorsal and posterodorsal margins, and a ventral margin forming a relatively shallow arch unlike that known from extant Anodontia species ( Taylor and Glover 2005). There is no reported occurrence of Anodontia older than the Miocene (e.g., Ludbrook 1959, 1978; Olsson 1964; Kiel et al. 2018). Paleogene lucinid bivalves similar to Anodontia have usually been assigned to different genera, such as Eophysema Stewart, 1930 . Eophysema encompasses species from the Eocene of the Gulf Coast, USA, and France (see Bretsky 1976; Taylor et al. 2011). It is a genus with relatively small species (L ≤ 32 mm) with a thin hinge plate; however, it has a globular shell and cardinal dentition, unlike the Paleocene specimen from Spitsbergen herein. Afranodontia marginidentana Adegoke, 1977 , from the Paleocene Ewekoro Formation in Nigeria Adegoke 1977), exhibits cardinal dentition and marginal shell crenulations not known from the Paleocene species of Spitsbergen. Edentulous fossil seep lucinids identified

so far comprise Early Cretaceous–Oligocene Amanocina Kiel, 2013 , Eocene–Miocene Elongatolucina Gill and Little, 2013 , Oligocene–Miocene Nipponothracia Kanie and Sakai, 1997 , Oligocene–Recent Elliptiolucina Cosel and Bouchet, 2008 , and Miocene–Recent Meganodontia Bouchet and Cosel, 2004 . Representatives of all five genera are much larger than the Spitsbergen species, exceeding 130 mm in length ( Kelly et al. 2000; Kiel 2013). Among those genera, Elliptiolucina and Elongatolucina have characteristic, elongate shells, very different from the oval shell of Rhacothyas spitzbergensis , whereas the shell of Meganodontia is very large and globular (e.g., Kiel and Taviani 2017; Amano et al. 2018b), also unlike that of the current species. Seep lucinids with a similarly shaped shell with dentition comprise the Late Jurassic genus Beauvoisina Kiel, Campbell, and Gaillard, 2010 ; the Late Jurassic–Late Cretaceous genus Tehamatea Kiel, 2013 , the Early Cretaceous–Oligocene genus Cubatea Kiel, Campbell, and Gaillard, 2010 , the Late Cretaceous–Paleocene genus Nymphalucina Speden, 1970 , and the Oligocene–Recent genus Lucinoma Dall, 1901 . All these genera have cardinal dentition and moderately thick ( Beauvoisina , Tehamatea , Cubatea , and Lucinoma ) or thick ( Nymphalucina ) hinge plates ( Speden 1970; Kiel 2013; Kiel and Taviani 2017; Amano et al. 2018b), features that are absent from the material studied.

Lucina planistria Koenen, 1885 , and Lucina lepis Koenen, 1885 , from Paleocene deposits in Copenhagen, Denmark ( Koenen 1885) are very similar to Rhacothyas spitzbergensis in shell size and shape, as well as the shape of the resilifer and the apparent lack of dentition, and could potentially represent another fossil occurrence of Rhacothyas . However, the details of muscle scars of both species are unknown, and we refrain from any further comparison with R. spitzbergensis until more data on both L. planistria and L. lepis are available.

Stratigraphic and geographic range.—Upper Paleocene cold seep carbonate and associated sunken driftwood from the Basilika Formation, Fossildalen, Zachariassendalen and locality 500 m west from Trigonometric point 25, Hollendarbukta, Spitsbergen, Svalbard.

Figures

Fig. 11. Schematic illustration of Rhacothyas spitzbergensis (Anderson, 1970) from the upper Paleocene, Basilika Formation, Spitsbergen, Svalbard, showing morphological features discussed. Right valve, outer (A1) and inner (A2) views.

Fig. 12. Thyasirid bivalve Rhacothyas spitzbergensis (Anderson, 1970) from the upper Paleocene, Basilika Formation, Zachariassendalen (A, F, G) and Fossildalen (B–E), Spitsbergen, Svalbard. A. NRM-PZ Mo183968, shell, right valve (A1), dorsal view of a right valve showing posterior sulcus (A2), oblique anterior view showing small lunule (A3). B. NRM-PZ Mo 183970, shell, left valve view. C. NRM-PZ Mo 149144, partial shell, →