Bathyisognomon, N. GEN

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 : 21

BATHYISOGNOMON N. GEN

Type species: Bathyisognomon smithwickensis . Upper Eocene, middle member, Keasey Formation , northwestern Oregon .

ZooBank LSID — urn:lsid:zoobank.org:act:A072890E-4645-4A24-AB91-1527878B3076

Diagnosis —Shell mytiliform, inflated and equivalve; edentulous; ligament area narrow, with few widelyspaced ligament pits and broad interspaces; ligament attached to interior nacreous shell layer; beaks small, prosogyrous, forming sharply acute angle at anterior end of hingeplate; lacking byssal gape, but with distinct byssal concavity immediately anterior to beaks; posterior margin of hinge line merging gradually with posterior shell margin.

Discussion —The shell shape, thick inflated valves, and details of the ligament area are in marked contrast to the thin, compressed, elongate shells of typical shallow-water isognomonids and indicate an endobyssate, semi-infaunal mode of life. This mode of life is consistent with the fine-grained tuffaceous siltstone in which it occurs, in marked contrast to the epibyssate mode of other isognomonids, which occur on or nestling in hard substrata in shallow water.

This contrast in mode of life and shell form also occurs in Mesozoic inoceramid bivalves, which include both endobyssate and epibyssate taxa (as well as some forms that are free-living), although the inoceramids are clearly distinguished by numerous closely-spaced ligament pits and ligament area that is attached to the outer prismatic calcitic shell layer. The outer shell layer is not preserved on any of the specimens of the new genus and species, which appears to be a result of exfoliation of outer shell as well as some of the interior nacreous layers. The exfoliation is attributed to post-exposure weathering. Available specimens were not collected in situ, although they are double valved and indicate lack of exposure and transport prior to burial.

Etymology —bathy (from the Greek bathos, deep water),+ Isognomon .

Stratigraphic range —upper Eocene, known only from massive siltstone beds of the upper part of the middle member of the Keasey Formation.