Pholadidea Turton, 1819

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 : 887-888

Genus Pholadidea Turton, 1819

Type species: Pholadidea loscombiana Turton, 1819 : Recent , eastern Atlantic .

Remarks.—The genus Pholadidea is characterized by single umbonal-ventral groove, no protoplax (replaced by a dorsal callum extension), no to incipient metaplax and hypoplax (resulting from calcification of periostracum extending beyond the valve), and by longitudinally divided mesoplax ( Turner 1955). The genus is subdivided into two subgenera: Pholadidea (Pholadidea) Turton, 1819 , and Pholadidea (Hatasia) Gray, 1851 . The main criteria of division are the shape and size of the mesoplax, the character of the umbonal reflection and of the siphonal tube. Pholadidea (Pholadidea) has a small, longitudinally divided mesoplax without basal extension and a raised umbonal reflection, and has no siphonal tube. Pholadidea (Hatasia) has longitudinally divided mesoplax with basal extension, an appressed umbonal reflection and has a siphonal tube ( Turner 1955).

Stratigraphic and geographic range.— Pholadidea ranges from Cretaceous to Recent according to Crampton (1990). Cretaceous occurences of Pholadidea are rare and most of them require re-examination according to the revised pholadoid systematics of Turner (1954, 1955, 1969). Crampton (1990) described Pholadidea (Hatasia) wiffenae from the Late Cretaceous (Campanian–Maastrichtian) sunken driftwood from the Maungataniwha Sandstone of North Island, New Zealand. The material is well preserved and shows longitudinally divided mesoplax and other features consistent with the definition of Pholadidea (Hatasia) , and to our knowledge is the oldest confirmed record of the genus. A similar wood-boring pholadoid species was described from the Maastrichtian sunken driftwood from Quiriquina Formation near Concepción, Chile, as Martesia leali by Stinnesbeck (1986). The species is slightly larger than P. (H.) wiffenae and has a very similar shape. The description states that meso-, meta-, and protoplax are unknown. These plates are absent on the holotype ( Stinnesbeck 1986: 185, pl. 5: 9b, c). As the meso- and metaplax in the genus Martesia Sowerby, 1824 , are large and robust (e.g., Turner 1955, 1969; Coan and Valentich-Scott 2012), and Quiriquina Formation material is found well preserved in life position within the wood, the absence of mesoplax and metaplax of M. leali are unlikely to be caused by taphonomic bias and are most likely a primary feature. The species could therefore potentially belong to Pholadidea , but requires re-examination using its type material before it could be formally included into this genus. Stephenson (1923) recorded three pholadoid species from the Upper Cretaceous (Campanian–Maastrichtian) Black Creek Group, North Carolina, which he attributed to Pholadidea . The species identified as Pholadidea ? subconica Stephenson, 1923, and Pholadidea ? cithara Morton, 1834, show characteristic radial ornament on the disc ( Stephenson 1923: pl. 87: 12–15), unknown in Pholadidea and most likely do not belong to this genus. The third species, small (ca. 7 mm long) and shell-boring Pholadidea fragilis Stephenson, 1923 , does not have such an ornament ( Stephenson 1923: pl. 87: 1–11). Stephenson (1923: 348) indicated the species has a double protoplax extending over the beaks. However, the reproduction ( Stephenson 1923: pl. 87: 3, 4) does not show protoplax or mesoplax but instead the uncovered dorsal extension of the callum and a calcified periostracum covering the dorsal margin. The species certainly requires revision; in our opinion, it should be rather classified as a member of the genus Diplothyra Tryon, 1862 . Another Late Cretaceous species in need of revision is Pholadidea ragsdalensis Stephenson, 1941 , from the Navarro Group, Campbell, Texas ( Stephenson 1941). It is a fairly small species (holotype ca. 7.5 mm long), boring within walls of large tubular fossils. Its callum partially seals the pedal gape, and the shell has two umbonal-ventral grooves ( Stephenson 1941: 246, pl. 44: 5), which are not typical of Pholadidea . The holotype and the only figured specimen is worn and lacks part of the ventral and posterior margins, so it is difficult to estimate the shell shape based on the figure only. Stephenson (1941) mentioned larger specimens he attributed to yet unidentified species of Pholadidea , occurring in a carbonate concretion from the Navarro Group close to Coolidge, Texas. These were poorly preserved and remained unfigured, and we do not comment on their generic affinity. Pholadidea has so far not been reported from Cretaceous xylic ( Kelly 1988a) and lithic substrates of Antarctica ( Wilckens 1910; Zinsmeister and Macellari 1988).

Worldwide Paleogene occurrences of Pholadidea are also relatively scarce. The Paleocene Pholadidea ? sp. nov. from the Red Bluff Tuff of Pitt Island, Chatham Islands, New Zealand ( Campbell et al. 1993: 84) is in fact a species of Jouannetia Des Moulins in Rang and Des Moulins, 1828, (KH personal observation), an opinion also expressed by Crampton (1990). Two Paleogene species of Pholadidea occur in lithic substrates of Patagonia, South America. The older Patagonian species is Pholadidea frenguellii from the Eocene shallow marine sediments of Río Turbio Formation in southwestern Patagonia ( Griffin 1991). The species is unknown outside of its type area. The second species was introduced by Philippi (1887) as Pholas patagonica from Oligocene–lower Miocene rocks cropping out near the mouth of Santa Cruz River, eastern Patagonia ( Del Río 2004). These identifications were later commented on by Ihering (1897, 1899), who attributed it to the genus Martesia , and by Ortmann (1902) who classified it in Pholadidea . The genus is unkown from Paleogene molluscan faunas of the Seymour Island in West Antarctica (cf. Wilckens 1911; Stilwell and Zinsmeister 1992). The elongate, slender borings from Eocene sunken driftwood from La Meseta Formation ( Pirrie et al. 1998) almost certainly have not been made by Pholadidea , which forms short, clavate borings similar to other martesiins ( Turner 1955; Turner and Johnson 1971; Crampton 1990). Pholadidea has also not been reported among the bivalves from the remaining part of the Polonez Cove Formation ( Gaździcki and Pugaczewska 1984; Quaglio et al. 2014). To our knowledge, Pholadidea has no confirmed fossil record in the Northern Pacific ( Kennedy 1974). A possible species of Pholadidea from the Eocene of Kamchatka ( Krishtofovich 1947) is not sufficiently well described and figured to confirm this generic assignment. A species reported from Oligocene of Hokkaido, Japan ( Uozumi and Fujie 1956) does not belong to Pholadidea as it has a single, undivided mesoplax.

Since the Neogene Pholadidea is broadly distributed in both hemispheres. Pholadidea patagonica is known from the lower Miocene of Eastern Patagonia ( Del Río 2004) and an undescribed pholadoid species forming mass-accumulations at the base of lower Miocene Chenque Formation ( Carmona et al. 2007) could also belong to Pholadidea . The latter species is medium sized, fairly short, oval, and has a single umbonal-ventral groove, which is similar to stratigraphically and geographically adjacent P. patagonica . The lack of callum reported by Carmona et al. (2007: 334, fig. 5b) needs to be confirmed. One of the oldest records of the genus in the Northern Hemisphere’s Neogene is Pholadidea loscombiana Turton, 1819 , from the middle Miocene sands of Ukraine ( Friedberg 1934) and from the Miocene of Hungary ( Csepreghy-Meznerics 1961). The population of Pholadidea loscombiana was well established in Europe in the younger Neogene and occurs frequently in the Pliocene and Pleistocene ( Malatesta and Zarlenga 1986). The genus is also known from the Plio-Pleistocene of New Zealand ( Laws 1936; Beu and Maxwell 1990; Beu 2006). Recent distribution of the genus comprises all of the world’s oceans apart from Western Atlantic ( Turner 1955) and Northern Pacific ( Coan et al. 2000). It bores in a variety of substrates like weakly compacted sediment, soft rock or peat in intertidal to subtidal environments down to ca. 120 m water depth ( Turner 1955; Coan and Valentich-Scott 2012).