A new genus of Cryptodiran turtles (Testudinoidea, Chelydridae) from the Upper Cretaceous Hell Creek Formation of Montana

Emarginachelys cretacea is described as a new genus and species of cryptodiran turtle and is placed in the Chelydridae on the basis of shared derived characters.The holotype is a nearly complete skeleton from the Hell Creek Formation (Maestrichtian) of Montana.Emarginachelys is the oldest known member of the superfamily Testudinoidea as defined by Gaffney (1975a).The Chelydridae are hypothesized to be a monophylctic group, sharing a cruciform plastron ligamentously attached to the carapace, a reduced entoplastron, long costiform processes on the nuchal bone, an elongate jugal, and the pectineal processes of the pubis not strongly divergent.Emarginachelys is hypothesized to be the most primitive genus of the Chelydridae since it does not have the derived characters shared by advanced chelydrids.Advanced chelydrids have the frontal bones separated from the orbital rim, a premaxillary "hook," constriction of the otic bridge, a closed incisura columella auris, a serrated carapacial margin, and peripheral fontanelles.