Xylophagella littlei Hryniewicz, 2019

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 : 120-123

Xylophagella littlei Hryniewicz sp. nov.

Figs. 16 , 17 .

1925 Xylophaga spec. n. sp.; Hägg 1925: 48, pl. 3: 13a–c. 2016 Pholadoidea indet.; Hryniewicz et al. 2016: fig. 12c.

ZooBank LSID: urn:lsid:zoobank.org:act:E8836D7A-EAA0-4522-AA79-639B1E5E3629

Etymology: In honour of the British palaeontologist Crispin T.S. Little, in recognition of his studies on fossil chemosynthesis-based faunas.

Type material: Holotype: ZPAL V.48/5, partial shell ( Fig. 17A ) . Paratypes: ZPAL V.48/6–8, partial shells ( ZPAL V.48/6–7), internal mold ( ZPAL V.48/8) ( Fig. 17B–D ) from the upper Paleocene , Fossildalen, Spitsbergen, Svalbard .

Type locality: Fossildalen , Spitsbergen, Svalbard .

Type horizon: Sunken driftwood associated with cold seep carbonates from the Basilika Formation, upper Paleocene .

Material.— Five specimens, mostly partial shells, including four ( ZPAL V.48/5–8) from the 2015 field season and one ( NRM-PZ Mo 149147); Fossildalen , Spitsbergen, Svalbard .

Measurements. —The holotype ( ZPAL V.48/5): W, 11.8 mm; H, 9.7 mm; W, 10.7 mm. Dimensions range: L, 8.1–20.2 mm; H, 7.4–16.9 mm; W, 8.5–23.5 mm; H/L, 0.84–0.98; W/L, 0.88–1.16; n = 5. See Table 2 for full list of measured specimens.

Diagnosis. —A species of Xylophagella with inner posterior ridge strong dorsally but diminishing ventrally.

Description. —Shell of normal size for genus (L ≤ 20.2 mm), thin, globular. Ventral margin convex, deepest around umbonal-ventral groove, rising posteriorly. Faint posteroventral angulation present; posterior margin rounded. Anterior gape wide, posterior gape narrower than anterior one. Prora triangular, occupying ca. 40% of height of anterior slope. Anterior slope short, well-demarcated from prora but poorly demarcated from disc; its dimensions difficult to ascertain. Rasp consists of oblique ridges covered with transverse lamellae; ridges sub-parallel to lower margin of prora, and to anterior margin of anterior slope. Obtuse angle of ridges between prora and anterior slope section of rasp forms well-defined line separating the two shell regions. This angle causes shell thickening, forming internal ridge. Rasp diminishes posteriorly but remains present until umbonal-ventral groove. Umbonal-ventral groove relatively wide, bounded anteriorly and posteriorly by shell thickening, forming two outer and inner ridges. Faint flat-topped outer ridge amidst umbonal-ventral groove. Section of shell posterior to umbonal-ventral groove ornamented with low commarginal folds superimposed on fine, commarginal growth lines. Internal ridge from umbo to posteroventral shell margin prominent dorsally, diminishing ventrally, disappearing completely from later growth stages. Faint muscle scars visible perpendicular to anterior margin; posterior adductor muscle scar faint, formed by multiple oblique attachment scars.

Remarks.— Xylophagella littlei Hryniewicz sp. nov. differs from X. dubius ( Stanton, 1901) by its smaller prora and convex ventral margin (as opposed to straight to concave in X. dubius ). Xylophagella littlei also has a more posteriorly elongate shell compared to the roughly circular shell of X. dubius . However, we have seen only drawings of the latter species ( Stanton 1901: pl. 6: 5–8), and a more detailed comparison of the two species is not possible. Xylophagella littlei has a convex ventral margin with the deepest point around the umbonal-ventral groove, as opposed to a straight ventral margin in X. zonata Casey, 1961 (Woods 1909: pl. 38: 17). Xylophagella elegantula ( Meek and Hayden, 1858) has not been figured to our knowledge; therefore, our comparison has to rely on the description ( Meek and Hayden 1858: 141), which indicates that the anterior gape of X. elegantula covers ~33% of the shell length. The gape is larger in X. elegantula than in X. littlei , where it occupies 20–25% of the shell length. The description of X. elegantula also indicates that the umbo placed nearly in the middle of the shell length, whereas it is located anteriorly in X. littlei . Xylophagella littlei is more elongate posteriorly as opposed to X. truncata Kelly, 1988 , and has a rounded posterior margin as opposed to truncated in X. truncata . The inner umbonal ridge of X. littlei diminishes towards the ventral margin, whereas it is continuous in X. truncata .

Stratigraphic and geographic range.—Upper Paleocene sunken driftwood associated with cold seep carbonates from the Basilika Formation, Fossildalen, Spitsbergen, Svalbard.

Figures

Fig. 16. Schematic illustration of a xylophagain bivalve Xylophagella littlei Hryniewicz sp. nov., from the upper Paleocene, Basilika Formation, Spitsbergen, Svalbard, showing the main morphological features discussed. Outer (A1) and inner (A2) views of right valve. Area above dashed line represents the morphological features we were unable to illustrate due to poor preservation.

Fig. 17. Xylophagain bivalve Xylophagella littlei Hryniewicz sp. nov. from the upper Paleocene, Basilika Formation, Fossildalen, Spitsbergen, Svalbard. A. Holotype, ZPAL V.48/5, partial shell, left valve (A1), right valve with fragment of posterior adductor muscle scar (PAMS) (A2), dorsal view of both valves (A3), oblique ventral view showing partially overlapping shells (A4). B. Paratype, ZPAL V.48/6, partial shell, left valve, with trace of a ridge on the inner mold (B1), right valve (B2), dorsal view of both valves (B3), oblique posterior view showing internal surface of left valve internal mold with trace of a ridge on inner surface of shell (B4). C. Paratype, ZPAL V.48/7, partial shell, left valve (C1), right valve with fragment of posterior adductor muscle scar (PAMS) (C2), oblique anterior view (C3), enlarged fragment of prora with imprints of pallial muscles perpendicular to the shell edge, rasp composed of raised ridges covered with perpendicular lamellae (C4), enlarged fragment of prora, showing ornament of raised ridges covered with perpendicular lamellae (C5). D. Paratype, ZPAL V.48/8, internal mold, right valve showing trace of a ridge on the inner shell surface.