Conchocele taylori Hickman, 2015

Hryniewicz, Krzysztof, Amano, Kazutaka, Jenkins, Robert G. & Kiel, Steffen, 2017, Thyasirid bivalves from Cretaceous and Paleogene cold seeps, Acta Palaeontologica Polonica 62 (4), pp. 705-728 : 711-714

Conchocele taylori Hickman, 2015

Fig. 6 , 7.

1979 Conchocele cf. nipponica ( Yabe and Nomura, 1925) ; Katto and Masuda 1979: pl. 2: 12a, b, 13a, b.

2003 Conchocele bisecta ( Conrad, 1849) ; Goedert et al. 2003: pl. 42: 9.

2007 Conchocele bisecta ( Conrad, 1849) ; Amano and Jenkins 2007: fig. 2A, D–F.

2013 Conchocele bisecta ( Conrad, 1849) ; Amano et al. 2013: fig. 6E, I.

2015 Conchocele taylori sp. nov.; Hickman 2015: 12, fig. 4E–G.

2016 Conchocele bisecta ( Conrad, 1849) ; Nobuhara et al. 2016: figs. 2–15.

Type material: Holotype UCMP 110688 , paratype UCMP 110689

( Hickman 2015).

Type locality.— Rock Creek seep carbonate (for details, see Hickman

2015: 14).

Type horizon.— Keasey Formation , upper Eocene .

Material.— The material examined comprises at least 13 specimens from Canyon River (uppermost Eocene–lower Oligocene , western Washington State , USA), including two figured specimens ( ZPAL L.16/3–4; Fig. 7C, E); nine specimens from Bullman Creek ( lower Oligocene , northwestern Washington State, USA), including one figured specimen ( ZPAL L.16/5; Fig. 7F); three specimens from Murdock Creek ( upper lower Oligocene , northwestern Washington State, USA); and three specimens from Whiskey Creek ( upper Eocene , northwestern Washington State, USA), and three specimens from LACMIP loc. 16504 ( upper Oligocene , western Washington State, USA). The collection from Japan comprises 66 specimens from Tanami ( upper Eocene –early Oligocene, Honshu); 199 specimens from Tappu ( upper Eocene , Hokkaido); and 82 specimens from Yayoi (upper Eocene, Hokkaido), all housed in JUE; 14 specimens from Kami-Atsunai (lower Oligocene, Hokkaido) housed at ZPAL, including two figured ( ZPAL L.16/2 and ZPAL L.16/6; Fig. 7B, G); and 226 specimens from Kami-Atsunai housed at JUE. The original description of Hickman (2015) was based on small specimens <25 mm long, in this study we figure and describe specimens up to 69 mm long .

Emended diagnosis.—Shell up to 69 mm long, equivalve. Smaller specimens ovate, larger specimens ovate-triangular; posterior sulcus on smaller specimens weak, progressively stronger on larger specimens; umbones prominent; anterior dorsal margin protruding and concave on specimens smaller than approximately 30 mm; anterior margin of larger specimens with protruding anteroventral angle. Shell surface with numerous, fine prominent growth lines; ligament sunken in deep groove; lunule broad and shallow; dentition unknown.

Description.—Umbo incurved, moderately prominent. Umbonal angle close to 90°. Lunule lanceolate, moderately deep. Anteroventral angle broad and convex, bound by two indistinct ridges. On largest specimens lunule absent, anterior margin weakly convex in dorsalmost part, straight in the remaining part. Ventral margin convex, broadly arcuate, with deepest point posterior to mid-line. Posterior of the shell moderately pointed in specimens, rounded in larger specimens. Posterior shell margin indented by a moderately deep sinus, posterior sulcus rather shallow and weakly convex. Posterior fold prominent and well visible, some specimens have an additional, weak ridge mid-fold. Dorsal margin arcuate. Ligament sunken, occupying approximately 2/3 of the dorsal margin. Anterior adductor muscle scar relatively well impressed, elongated but shorter than that of other Conchocele species, occupying ca. 1/4 of the shell length. Anterior pedal retractor muscle scar small, separated from anterior adductor muscle scar. Posterior adductor muscle scar small, elongated, located inside posterior fold.

Remarks.— Conchocele taylori differs from the late Eocene– early Oligocene C. bathyaulax Hickman, 2015 , from several non-seep localities from the Keasey Formation in Oregon, USA, by its greater umbonal angle, a less prominent umbo, a more prominent posterior fold, and by having a concave anterior margin ( Hickman 2015). The Miocene Conchocele bisecta ( Conrad, 1849) from the Astoria Formation in Oregon differs from Conchocele taylori by its more pronounced umbo in smaller specimens, and more quadran-

Fig. 7. Thyasirid bivalve Conchocele taylori Hickman, 2015 . A. JUE 16038 from a upper Eocene seep carbonate of the Poronai Formation, Yayoi Town , → Hokkaido, Japan; a partially preserved smaller specimen in right (A 1) and left (A 2) lateral views, showing a relatively rounded shell outline; dorsal aspect with relatively weak posterior sulcus (A 3); an anterior view (A 4), showing relatively flat anterior area with weak ridges running towards the anteroventral angle. B. ZPAL L16 /2 from a lower Oligocene seep carbonate of the Nuibetsu Formation, Kami-Atsunai, Hokkaido, Japan; an internal mold of a smaller specimen in left lateral view, showing the anterior adductor muscle scar, and its more angular outline compared to the specimen illustrated in A. C. ZPAL L16 /3 from an upper Eocene–lower Oligocene seep carbonate from Canyon River , Lincoln Creek Formation, western Washington State, USA; a medium-sized, well preserved shell in right (C 1) and left (C 2) lateral views, showing external ornament of fine commarginal growth lines and shell outline; dorsal view (C 3) shows shallow posterior sulcus similar to that of JUE 16038 ; anterior view (C 4) shows lunule. D. JUE 16039 from a lower Oligocene seep carbonate from Nuibetsu Formation , Kami-Atsunai , Hokkaido, Japan; a partially preserved shell of a medium-sized individual in right lateral view, showing the well-developed anteroventral angle similar to the specimen shown in C. E. ZPAL L.16/4 from an upper Eocene–lower Oligocene seep carbonate from Canyon River , Lincoln Creek Formation , western Washington State, USA; a large specimen in left lateral view, with poorly incurved umbo (the anteroventral angle was covered with matrix during the preparation of this figure). F. ZPAL L.16/5 from a lower Oligocene seep carbonate from Makah Formation , Bullman Creek , northwestern Washington State, USA; an internal mold with slightly damaged posterior showing right (F 1) and left (F 2) lateral views and left anterior adductor muscle scar; anterior view (F 3) shows incurved umbones and shallow lunule; dorsal view (F 4) shows shallow posterior sulcus. G. ZPAL L.16/6 from a lower Oligocene seep carbonate of the Nuibetsu Formation , Kami-Atsunai , Hokkaido, Japan; a partially preserved internal mold in left lateral view of showing anterior adductor muscle scar; note that the anteroventral angle, the curvature of the umbones, and the shape of the preserved dorsal and ventral margins is very similar to that of F .

gular shell shape in larger specimens. The anteroventral margin of smaller specimens of Conchocele taylori is less pointed than that of C. bisecta . The anterior adductor muscle scar of Conchocele taylori is shorter than that of C. bisecta . Conchocele townsendi has much more elongated shell, and deeper posterior sulcus as compared to C. taylori .

Stratigraphic and geographic range. —Late Eocene–Oligocene, North Pacific margin from Oregon to southern Honshu.

Figures

Fig. 6. Schematic illustration of the thyasirid bivalve Conchocele taylori Hickman, 2015, highlighting its main morphological features. A. Based on JUE 16038, a larger, presumably adult shell, exterior of the right valve. B. Based on ZPAL L.16/6, a small, presumably juvenile shell, exterior of the left valve. C. Based on ZPAL L.16/6, an internal mold of a larger presumably adult specimen, left valve. Not to scale.