Bivalves (Mollusca) from the Imo Formation (Mississippian, Chesterian) of north-central Arkansas

The Chesterian Imo Formation in north-central Arkansas contains an abundant and diverse bivalve fauna which is dominated in numbers by nuculoids. The pterioids, pholadomyoids, and nuculoids show the greatest, and nearly equal, species diversity. Ecologically, on a generic level, the fauna is dominated by infaunal, burrowing, suspension feeders with attached suspension feeders a close second. Among the 40 taxa identified, the following new taxa are proposed: Economolopsis gordoni n. gen. and sp.; Spathelopsis browni n. gen. and sp.; Paleyoldia angustia n. sp.; P. bruta n. sp.; Phestia corrugata n. sp.; P. wortheni n. sp.; P. obtusa n. sp.; Imoella obscura n. gen. and sp.; Prothyris (Lophoprothyris) crista n. subgen. and sp.; Ectogrammysia crassatis n. gen. and sp.; Asketomorpha grandis n. gen. and sp.; and Sphenotus pisinnus n. sp. On the basis of ammonoids, the upper portion of the Imo Formation falls within the Elvirian Stage, E2b–c undifferentiated. Foraminifera place the formation in Mamet zone 19. The bivalve fauna discussed herein was collected from the E2b–c portion of the Imo.