Abstract

Middle Devonian bivalve-dominated assemblages occurring in shallow shelf deposits of the Weatherall Formation include species referrable to Nuculites Conrad, Nuculopsis Girty, Paracyclas Hall, Eoschizodus? Cox, Ptychopteria Hall, Limoptera? Hall and Whitfield and three new genera of Cycloconchidae. Additionally, a single species representing a new Tanaodon-like genus of cycloconchids is noted from interdistributary/interdelta lobe bay deposits of the overlying Hecla Bay Formation. Exceptional preservation of bivalves from both formations includes details of the hinge structure, muscle insertions, shell microstructure and, in some cycloconchids, remnants of ligament tissue.

The new genera of Cycloconchidae more than double the known generic diversity of that family in the Devonian. All lack ligament nymphae. One genus shows an elongate anterior adductor scar and a submarginal, posteriorly expanded opisthodetic ligament as in the Devonian lucinacean, Paracyclas. These comparisons indicate the Lucinacea may have evolved directly from the Cycloconchidae rather than from their alleged Ordovician progenitor, Babinka Barrande.

Biogeographic comparisons reveal that of the ten genera of bivalves represented, five are widespread in Middle Devonian rocks, Nuculopsis is of Michigan Basin aspect, one cycloconchid genus is probably endemic and three cycloconchid genera are possibly endemic. Of the latter three, the new Tanaodon-like genus suggests faunal affinities with Kazakhstan, south China and eastern Australia.

New taxa are not named, pending more detailed descriptions and comparisons.