382 Paleobiology, Paleoecology, and Systematics of Solemyidae

Paleobiology, paleoecology, and systematics of Solemyidae (Mollusca: Bivalvia: Protobranchia) from the Mazon Creek Lagerstatte, Pennsylvanian of Illinois - BAP #382 The most abundant bivalve of the Essex biofacies (Mazon Creek fauna, Pennsylvanian of Illinois), misidentified by past authors as the marine pholadomyoid Edmondis de Koninck, 1841, is herein named Mazonomya mazonensis n. gen., n. sp., and assigned to the family Solemyidae. The second most abundant Essex solemyid (Solemya radiata of past authors), showing traces of the periostracal frifi and external ligament, is emended as Acharax radiata (Meek & Worthen, 1860) n. comb. Systematic revisions herein challenge open-marine and open-estuary depositional models of the Essex biofacies.  Within the Essex, the prevalence ofsolemyids along with an admixture of thin-shelled euryhaline bivalves and growth-inhibited stenohaline bivalves is suggestive of a transitional paleoenvironment, such as a drowned coal-swamp or restricted es