LIMOPSIDAE DALL, 1895

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 : 17

LIMOPSIDAE DALL, 1895

Thin shells of small to minute limopsid bivalves are rare in Cenozoic faunas of the Northeastern Pacific, occurring primarily in deep-water assemblages. Limopsids appear in the Jurassic (Bathonian), with a probable deeper arcidan ancestry in either the Grammatodontidae or Cucullaeidae . There was a minor limopsid radiation in the Cretaceous followed by subsequent decline as siphonate burrowing bivalves replaced taxa constrained by a nestling, byssate, semi-infaunal life habit. Although the thin aragonitic shells do not preserve well, the typical obliquely ovate shape of the shell, crenulate interior margin, compressed valves, straight hingeline, taxodont dentition, and finely-tessellated sculpture aid in recognition. The alivincular ligament in a central triangular resilifer, when preserved, is an aid in recognizing fossil limopsids. The dense covering of flat-lying, thatched periostracal bristles may aid in the initial preservation of buried shells and is sometimes expressed in external molds in fine-grained siltstone and mudstone.

Although this is a relatively small family group with living diversity concentrated in the Antarctic, high southern latitudes, and deep sea, it has attracted considerable interest for subtle distinctions in shell shape, ligament, and anatomical features considered key to understanding arcidan evolution (Oliver 1981). The limopsoid ligament has figured in defining constraints on evolutionary diversification in arcidan bivalves as recorded in the fossil record (Thomas 1976). Considerable taxonomic confusion among living species has been resolved through detailed studies of anatomy, shell morphology, and life habits and the designation of thirteen morphological classes in three functional groups (Oliver 1981). Experimental studies demonstrate behavioral flexibility and life positions that vary with substrate type (Oliver and Allen 1980).

Lack of evolutionary diversification is popularly attributed to morphological constraints, and groups that fail to diversify are often viewed as unsuccessful or evolutionary dead ends. An alternative suggested here is that limopsids are highly successful in their geologically persistent ability to colonize and maintain semi-infaunal populations at the sediment-water interface, between the opportunistic epifaunal exploration of hard substrate diversity and the infaunal partitioning explored by siphonate burrowing bivalves.

Some of the disagreement over the use of available genus-group names and unresolved phylogenetic relationships has been attributed to lack of knowledge of early ontogenetic features (Malchus and Warén 2005). Early ontogeny in Jurassic arcidans suggests heterochronic evolutionary change (Malchus 2004). Deep ancestral relationships are fundamentally a paleontological problem awaiting more detailed study. As a sister group to Arcoidea, deeper Paleozoic origins for the Limopsoidea are possible.

The low-energy fine-grained sedimentary habitat of many limopsids at slope depths is in marked contrast to that of glycymeridids, a similarly conservative low-diversity family group occupying high-energy, poorlysorted sands and gravels at shelf depths. The deep-water Keasey limopsid described here is consistent with this ecological distinction.

Stratigraphic range —Middle Jurassic–Holocene.