GLYCYMERIDIDAE DALL, 1908

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 : 14-15

GLYCYMERIDIDAE DALL, 1908

The thick, subcircular shells of glycymeridids are generally considered to reflect life in shallow high-energy environments and poorly sorted coarse sediments. In contrast to arcids, they are free living burrowers, able to plough through sediment with a well-developed axeshaped foot and capable of reestablishing themselves beneath the sediment-water interface when disturbed (Stanley 1970, Thomas 1970). They are remarkably conservative (Thomas 1975), probably derived from cucullaeid arcoids (Nicol, 1950) and first appearing in the Early Cretaceous (Valenginian). Their evolutionary conservatism has been attributed to specialized environmental demands, leading Thomas (1975) to characterize the glycymeridid as “a much compromised organism.” In the absence of clear diagnoses of supraspecific groups, there is precedence for treating both living and fossil species under a single genus.

It should not be surprising that glycymeridids were well represented and locally abundant in the tropical Eocene fauna of the Pacific Northwest and that they disappeared with the onset of late Eocene global cooling. They are absent from the post-extinction transition fauna of the Keasey Formation in Oregon ( Hickman 1984, 2003) as well as in the recovery fauna (sensu Hickman 2003) of the overlying Pittsburg Bluff Formation (E.J. Moore 1976) and coeval Eugene Formation ( Hickman 1969). The two glycymeridid species treated here are part of an enigmatic transition faunal assemblage in the Gries Ranch Formation in southwestern Washington. The geographically isolated Gries Ranch beds are here considered coeval with the upper member of the Keasey Formation, based in part on the restricted occurrence of the parallelodontid arcoid Porterius gabbi . Although the Gries Ranch mollusks have been interpreted in the past as a free-living shallow-water fauna (e.g., Effinger 1938, E.J. Moore 1976), the assemblage also contains a deep-water giant limid and the thin-shelled P. gabbi , both taxa that form byssal attachments. The anomalous presence of the limid and the parallelodontid suggest a dramatic depositional mass movement that displaced shallow-water fauna into a near-shore deep slope setting to form an allochthonous jumbled shellbed (sensu Donovan et al. 2013). The glycymeridids are part of the displaced shallow-water component in the conglomeratic facies of the Gries Ranch Formation.

Stratigraphic range —Lower Cretaceous–Holocene.