Abstract
The Arkadelphia Formation-Midway Group contact (Maastrichtian-Paleocene) near Malvern, Arkansas preserves one of the youngest plesiosaur assemblages yet reported from the Gulf Coastal Plain of the United States. The assemblage consists of three vertebrae, a pubis bone and two teeth recovered by scuba diving an outcrop along a meander bend of the Ouachita River. These plesiosaur remains are preserved in mollusk, coral and ammonoid coquina and may derive from a single individual achieving a total overall length of 10 meters. Taphonomic conditions under which this coquina was deposited indicate that plesiosaurs may have inhabited a shallow, biohermal patch reef environment in southwestern Arkansas where they preyed upon an abundance of ammonoids, bonyfish and chondrichthyans such as Placenticeras sp., Enchodus sp. and Serratolamna serrata. The Arkadelphia Formation-Midway Group plesiosaur assemblage extends the known geographic range of plesiosaurs in North America and indicates that these apex marine reptiles were living at, or near, the K-Pg mass extinction boundary in the region.