Porterius, CLARK, 1925

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 10

PORTERIUS CLARK, 1925

Type species —By original designation, Barbatia andersoni Van Winkle (1918) . Upper Eocene Lincoln Creek Formation, Washington (= Barbatia gabbi Dickerson (1917) , Gries Ranch Formation).

Pseudogrammatodon Arkell (1930a) . p. 307. Type species: Arca adversidenta Deshayes (1860) , Eocene, Paris Basin.

?Notogrammatodon Maxwell (1966). p. 439–441. Type species: Pseudogrammatodon (Notogrammatodon) inexpectatus . Eocene , New Zealand .

? Siptionella Berezovsky (2014) . p. 451–452. Type species: Siptionella prompta ( Berezovsky, 2002, as

Porterius promptus ), upper Eocene, Ukraine.

Differential Diagnosis — Porterius is uniquely characterized by a combination of derived arcid shell shape and sculpture typical of species of Barbatia and parallelodontid hinge features that include elongate posterior hinge teeth parallel to the hinge line and separated by an edentulous gap from a series of short, diagonal anterior teeth. The edentulous gap widens during ontogeny as the ligament overgrows the hingeline. The posterior-most teeth in the series are short and chevron shaped and the anteriormost tooth is longer and subparallel to the hingeline. Anterior and posterior teeth are finely striate as in Barbatia .

Discussion —When Clark (1925) originally proposed Porterius as a subgenus of Parallelodon Meek and Worthen (1866) , he had limited material and believed that the hinge was “almost if not quite identical” with that of species of the Ordovician to Jurassic parallelodontids, but distict in having the shell form of younger arcids of the genus Barbatia . Clark’s original description and illustrations were an inadequate basis for evaluating the name, and its subsequently contorted history reflects a combination of factors. Most importantly, more than 18 authors who have speculated on the allocation and relationships of Porterius have done so based on an inadequate description and without examining fossil specimens. Full review of this confused literature on arcoid relationships is beyond the scope of this treatment, but a few examples follow.

Stewart (1930, p. 68) stated that Porterius “is related to Cucullaria ” Deshayes (1829). Reinhart (1935, p. 7) suggested that “careful comparison may show that Pseudogrammatodon is to be considered a synonym of Porterius .” Glibert and van de Poel (1965, p. 48) questioned the validity of Porterius because it seemed closely similar to the type of the Paris Basin Cucullaria Conrad (1869) . Heinberg (1979, p. 116) expressed similar doubt about validity. Maxwell (1966) proposed the name Notogrammatodon as a subgenus of Psuedogrammatodon, allocating both to Arcidae (?). He stated that “the position of Porterius and Pseudogrammatadon is in doubt”, but he noted that his knowledge of Porterius was based exclusively on Clark’s description and figures. Notogrammatodon is treated here as a questionable synonym of Porterius .

Berezovsky (2014) described the new genus Siptionella to accommodate a species previously described under Porterius . However alleged differences in the hinge based exclusively on Clark’s original description are difficult to evaluate and it is here questionably treated as a synonym. Although Berezovsky’s (2015) generic assignments of Paris Basin Eocene arcoid species were based on examination of “well-preserved valves” provided by a colleague in the Netherlands, the possibility for error is always present in material that has not been compared directly with primary types. This same potential for error exists in the Cloez Collection in the Museum of Paleontology at Berkeley, although misidentifications are minimized by indications that “identifications of many or all of the species in the collection were either made by or checked by Cossmann” (Peck 1957). For differences between the hinge plates and teeth of Porterius and Cucullaria see treatments and illustrations in Hickman (2021).

The possibility of living deep-sea parallelodontids persisting in disguise as arcoids has been expressed by Morton (1982, p. 280), who noted the retention of three elongate posterior teeth parallel to the hinge in the living genus Bathyarca Kobelt (1891) . As noted above, Bathyarca and Bentharca Verrill and Bush (1898) are candidates for scrutiny as living parallelodontid derivatives.

Stratigraphic Range —lower–upper Eocene; Holo- cene?

Geographic occurrences —Washington, Oregon, California, Paris Basin,? Ukraine,? New Zealand.