PLESIOSAUR REMAINS FROM THE ARKADELPHIA FORMATION-MIDWAY GROUP CONTACT (MAASTRICHTIAN-PALEOCENE) HOT SPRING COUNTY, NEAR MALVERN, ARKANSAS

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.