PARALLELODONTIDAE DALL, 1898

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 : 9-10

PARALLELODONTIDAE DALL, 1898

GRAMMATODONTINAE STEPHENSON, 1941

Triassic and Jurassic arcoid taxa assigned to the presumed extinct Parallelodontidae Dall (1898) and Grammatodontinae Stephenson (1941) have received considerable attention from paleontologists although taxonomic distinctions vary among authors. Despite treatment by Arkell (1930a, b) of the genus Parallelodon Meek and Worthen (1866) , as a subgenus of Grammatodon Meek and Haydn (1858) , followed by a nomenclaturally misguided attempt to change Dall’s family group name to Grammatodontidae ( Branson, 1942), the name Parallelodontidae persists. Stephenson’s Grammatodontinae stands as a morphologically justified family group name ( ICZN Article 24.1) with priority over subsequently-proposed family group names for extinct Mesozoic and Cenozoic taxa that share differences in the form and orientation of anterior and posterior hinge teeth.

On the Pacific Coast of North America most fossil arcoids were described under Arca Linnaeus (1758) or Barbatia J.E. Gray (1842) in the Arcidae Lamarck (1809) . Detailed comparative studies and revision of Cretaceous and Tertiary Arcidae (Schenck and Reinhart 1938, Reinhart 1943) identified very few taxa that required re- moval and reallocation to the more basal arcoid families. Basal arcoids underwent a dramatic Cetaceous and Cenozoic decline and regional extinction in western North America. Especially notable is the Cenozoic disappearance of the cucullaeid bivalves that thrived during the Cretaceous in the interior seaway. Following a different pattern from that of the parallelodontids, cucullaeid history is marked by progressive biogeographic contraction into a present-day Indo-Pacific species complex ( Buick 2009). The pattern of decline in parallelodontids is more complicated, with relict taxa surviving only in deep water in the Paleogene on the active margin of North America while persisting on shallow carbonate platforms of the epicontinental seas of eastern Europe ( Hickman 2021).

The occurrence of a relict parallelodontid in the Keasey Formation is remarkable not only in its narrowly restricted temporal stratigraphic and geographic distribution but also in its occurrence in coeval bathyal assemblages of the Lincoln Creek and Gries Ranch formations of southwestern Washington. These unusual assemblages are part of a transitional “recovery fauna” following local extinction of the “tropical Eocene fauna” and prior to establishment of the “temperate modern fauna” ( Hickman 2003). The local extinction and recovery coincide with major tectonic realignment of plates on the Cascadia Margin. The transitional recovery fauna also marks the close of the doubthouse interval (sensu Hickman 2021) of global cooling prior to establishment of permanent ice sheets at high latitudes.

Although parallelodontids continue to be treated as extinct ( Bouchet and Rocroi 2010), there is at least one confirmed living species and new genus Kamenevus Valentich-Scott, Coan, and Zelaya (2020) . Other deep-water arcoids, including species assigned to Bathyarca Kobelt (1891) and Bentharca Verrill and Bush (1898) bear close reexamination as relict parallelodontids or potentially close allies.

Additional genus group names based on small (mostly <20 mm length), thin-shelled species with posterior teeth subparallel with dorsal shell margin and a central edentulous gap are documented with excellent figures and descriptions by Kamenev (2007a, b). These include Asperarca Sacco (1898) ; Deltaodon Barnard (1962) ; Pseudoporterius Kamenev (2007a) , and Samacar Iredale (1936) . These taxa also live at offshore depths, frequently> 200 m.

Oliver and Holmes (2006) recognized that the evolutionary radiation of modern arcoids mimics that of Paleozoic and Mesozoic parallelodontids. It is therefore possible that some living deep-water forms with subparallel hinge teeth may be parallelodontids while others “may simply display a secondary appearance

of this character associated with the thin nature of the hinge plate” (p. 240–241). It is also a possible result of heterochronic change in miniaturized arcoids.

Stratigraphic range —Lower Ordovician–Holocene.