Glycymeris, DA COSTA, 1778

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 : 15

GLYCYMERIS DA COSTA, 1778

Type species —By tautonymy, Arca orbicularis Da Costa (1778) (= Arca glycymeris Linnaeus, 1758 ). Living, Mediterranean.

The relatively thick glycymeridid shell is easily recognized by its nearly equilateral, subcircular shape; typically, with broad flat-topped radial ribs separated by narrow interspaces. The strongly-crenulate interior valve margins are interlocking. The triangular ligament area and alternating ridges and grooves serve as attachment for the duplivincular ligament. The curved hinge plate is typically robust, with interlocking taxodont teeth of nearly equal number anterior and posterior to the beak. Worn shells are frequently chalky, and shell alteration dramatically accentuates radial sculpture that may be only faintly expressed on pristine specimens.

The two Keasey-equivalent glycymeridids treated below, combine with the Keasey parallelodontid and limopsid to provide a snapshot of persistence of the ancient Arcida at a time of marine climate change and geologic change coinciding with tectonic reorganization on the Cascadia margin. These species are restricted to the transition fauna and are absent from the late Eocene tropical faunas of the underlying Cowlitz Formation. Glycymeridids subsequently disappear and are not represented in the recovery faunas of the Pittsburg Bluff and Eugene formations.

Stratigraphic Range —Lower Cretaceous–Holocene.