Pholadidea gradzinskii, Hryniewicz & Gaździcki, 2016

Hryniewicz, Krzysztof & Gaździcki, Andrzej, 2016, A new sediment-dwelling pholadid bivalve from Oligocene glaciomarine sediments of King George Island, West Antarctica, Acta Palaeontologica Polonica 61 (4), pp. 885-896 : 889-892

Pholadidea gradzinskii sp. nov.

Figs. 5–7 .

1982 Penitella sp.; Gaździcki et al. 1982: 729, figs. 6, 7.

ZooBank LSID: urn:lsid:zoobank.org:act:2C072B3E-3357-4417-B825-8D2FEFC3B2AE

Etymology: In honour of the late Professor Ryszard Gradziński, Kraków 1929 – 2014), in recognition of his understanding of the sedimentology of the Polonez Cove Formation.

Type material: Holotype: ZPAL Mo. XVIII/3 : a well preserved articulated shell with some details of accessory plates visible . Paratypes: ZPAL Mo XVIII/1 , an internal mould with traces of pallial line preserved ; ZPAL Mo XVIII/4 , an articulated shell with rasp visible ; ZPAL Mo XVIII/11 , a fragment of the prora and anterior slope showing rasp : ZPAL MoXVIII/12/A – W, a set of acetate peels with transverse sections.

Type locality: Cliff at Mazurek Point , King George Island , South Shetland Islands, West Antarctica .

Type horizon: Four mudstone intervals bored by P. gradzinskii ; capped by diamictite beds; Oligocene , Siklawa Member , Polonez Cove Formation , King George Island , South Shetland Islands , West Antarctica. Three pholadid-bearing beds crop out approximately 20 m above sea level, about a meter above the base of the Siklawa Member. A fourth bed crops out approximately 4.5 m higher in the section .

Other material.—Other borings containing fragmentary preserved specimens used for this work were already figured and referred to by Gaździcki et al. (1982). The material stored in ZPAL comprises 210 boring hosting specimens. These are separate weathered boring traces or accumulations of several in loose blocks.

Diagnosis.—A large, elongate species of Pholadidea with commarginal ornament. Prora and anterior slope covered with rasp formed by coarse commarginal ridges with few indentations, the disc and posterior slope with fine commarginal growth lines superimposed on broad commarginal folds. Accessory plates highly simplified and formed by partial calcification of periostracum extending beyond the valve (metaplax, hypoplax, siphonoplax) or artophied (mesoplax, protoplax).

Description. —Shell up to 62 mm in length and 30 mm in height, with virtually all of the available specimens close to the maximum size. Shell elongated, large for the genus, with moderately thick shell anteriorly and thin posteriorly. Umbo located at 1/6 th of the length from the anterior. Most of the shell features (prora, disc, posterior slope) have no clear boundaries, only clear boundary is the umbonal-ventral groove separating the anterior slope from the disc. Prora short and reduced; anterior slope inflated, representing the widest and highest portion of the shell. Pedal gape large, in adults completely sealed by callum; in juveniles partially opened, as suggested by shape of the growth lines. Callum thin and weakly calcified, often deformed; ornamented with growth lines superimposed on radial striae. Dorsal extension of the callum reaches the beaks and covers the umbonal reflection of the shell. The umbonal reflection raised anteriorly, more apressed near the beaks. No protoplax and mesoplax observed in any of the investigated specimens. As all investigated specimens are found articulated in life position, we conclude that lack of protoplax and mesoplax is a primary feature. Metaplax is not a true plate, instead being formed by partial calcification of the periostracum connecting the dorsal margin of both valves; fragments close to the shell margins calcified while median part remained uncalcified. Hypoplax similarly is not a true plate but weakly calcified periostracum extending beyond the ventral margin of both valves. Siphonoplax short, pointed, separate in each valve, composed of calcified periostracum connected with that from the ventral and dorsal margin. Ventral margin gapes, with no ventral condyle observed. The ornament of prora and anterior slope composed of 17 – 20 coarse, densely spaced coarse commarginal ridges with few weak indentations forming a rasp. On the disc and posterior slope the ornament is finer and formed of low commarginal growth lines superimposed over coarser commarginal folds. Umbonal-ventral groove straight, narrow and moderately deep, impressed on the mould. Posterior margin narrow, truncated but pointed when siphonoplax is taken into account. Pallial line weak and only partially visible. Muscle scars unknown.

Remarks.— Pholadidea gradzinskii does not have a mesoplax, but other characters typical for Pholadidea are present and we conclude that a lack of mesoplax is caused by a secondary reduction and include this species into Pholadidea . However, due to the lack of a mesoplax, which is the most straightforward feature used to separate both subgenera, we restrain from including current species to any subgenus of Pholadidea .

Pholadidea gradzinskii differs from P. (Hatasia) wiffenae Crampton, 1990 , from the Upper Cretaceous (Campanian – Maastrichtian) of New Zealand by its larger size (<20 mm in lenght for P. (H.) wiffenae ), longer and more slender disc and posterior slope, and lack of mesoplax ( Crampton 1990). It is much larger and has finer commarginal ornament than P. frenguellii Griffin, 1991 , from the Eocene of southwestern Patagonia, which in addition has wider and deeper umbonal-ventral groove ( Griffin 1991). Pholadidea gradzinskii sp. nov. differs from P. patagonica ( Philippi, 1887) from Oligocene – lower Miocene of Eastern Patagonia ( Del Río 2004) by its more elongate shell and weaker commarginal ornament on anterior slope. Also, P. gradzinskii has tapering and truncated posterior of the shell, contrasting with rounded posterior of the shell of P. patagonica . The growth lines on the anterior slope of P. gradzinskii reach the umbonal-ventral groove at a weakly acute angle (ca. 80°), which is

Fig. 6. Pholadid bivalve Pholadidea gradzinskii sp. nov. from the Oligocene of King George Island. A. Paratype, ZPAL XVIII/1 ; right-lateral view of → internal mould showing fragments of the pallial line and the umbonal-ventral groove. B. Paratype, ZPAL Mo XVIII/4 complete shell; anterior (B 1) and dorsal (B 3) views, showing umbonal reflection covered by dorsal extension of the callum and prora and anterior slope covered with rasp formed by raised commarginal lamellae; left lateral view (B 2), showing a strongly elongated shape and pointed siphonoplax; enlarged photo of the anterior of the right valve B 4), showing rasp formed by raised commarginal lamellae. Arrows mark radial striae on the callum, engraved by substrate particles probably during the boring action. C. Paratype, ZPAL Mo XVIII/11 ; SEM photomicrograph of the prora (C 1), showing detailes of rasp formed by coarse commarginal lamellae; detail of coarse commarginal lamellae (C 2), showing signs of wearing .

much more acute (ca. 30°) in P. patagonica ( Del Río 2002) . Pholadidea gradzinskii is much more elongate than P. finlayi Laws, 1936 , from the Pliocene of New Zealand ( Laws 1936: pl. 11: 41, 42), and has a somewhat less truncated posterior part of the shell. It has also growth lines transecting the umbonal-ventral groove at less acute angle than P. finlayi . Pholadidea gradzinskii is very similar to P. suteri Lamy, 1926 , from the Pleistocene of New Zealand with respect of shape, size and no mesoplax sensu Turner (1969) (see Beu and Maxwell 1990). The rasp of P. suteri is, however, coarser and has well defined radial ribs, unlike that of P. gradzinskii and we prefer to keep both species separate. Pholadidea tridens (Gray, 1843) from the Pleistocene of New Zealand is much shorter than the current species, and has wider umbonal-ventral groove ( Beu 2006: fig. 38F).

Recent species of Pholadidea were discussed by Turner 1955), Beu and Climo (1974) and Coan and Valentich-Scott 2012). Pholadidea gradzinskii is more elongated than P. loscombiana Turton, 1819 , which in addition has anterior slope with growth lines forming an sharp acute angle with the umbonal-ventral groove, and has broad, rectangular posterior margin, unlike the current species. Pholadidea gradzinskii is more elongate than P. acherontea Beu and Climo, 1974 , which has siphonoplax composed of two separate plates, missing in the current species. Pholadidea Hatasia) melanura ( Sowerby, 1834) has much more inflated and broader callum than P. gradzinskii , and stronger radial threads on the anterior slope. Pholadidea (Hatasia) quadra ( Sowerby, 1834) has more rectangular shell and more apressed umbonal reflection than the current species. Pholadidea (Hatasia) tubifera ( Sowerby, 1834) has similar shape, but has stronger radial threads on the anterior slope and more appressed umbonal reflection. All the above species of Hatasia contain a mesoplax, a feature entirely missing in P. gradzinskii .

Stratigraphic and geographic range. — Type locality and horizon only.

Figures

Fig. 5. Pholadid bivalve Pholadidea gradzinskii sp. nov. (ZPAL Mo. XVIII/3, holotype) from the Oligocene of King George Island. A. Left-lateral view of the complete specimen (A1), arrow marks deformation of the callum; anterior of the left valve (A2), showing callum covered with growth lines. B. Rightlateral view of the complete specimen (B1), arrow marks deformation of the callum; anterior of the right valve (B2), showing umbonal reflection raised anteriorly and appressed on the umbo. C. Dorsal view of the complete specimen showing metaplax formed by partially calcified periostracal mantle connecting the dorsal margin of the valves. D. Ventral view of the complete specimen showing a hypoplax formed by partially calcified periostracal mantle extending beyond the ventral margin of the shells and connected with the callum; arrows mark deformation of the callum and hypoplax. E. Inclined dorsal view of the anterior showing umbonal reflection covered with the dorsal extension of the callum. F. Anterior view showing the callum and umbonal reflection covered by the dorsal extension of the callum.

Fig. 7. Vertical transverse sections through pholadid bivalve Pholadidea gradzinskii sp. nov. (ZPAL Mo XVIII/12/A–W), preserved in life position within a boring from the Oligocene of King George Island. The numbers refer to distance from the anterior of the shell in milimeters. Abbreviations: ap, apophysis; b, beak; ch, chondrophore; cl, callum; dcle, dorsal extension of the callum; hp, hypoplax; mt, metaplax; r, rasp; sp, siphonoplax; ur, umbonal reflection.