Abstract
In this paper we describe the first cheloniid turtle fossils from the Miocene (Barstovian) of California, USA. All specimens are from Sharktooth Hill, Round Mountain Silt Member of the marine Temblor Formation, in Kern Country. The material includes two species: (1) a form with a sculptured carapace (cf. Syllomus) known from a single specimen; (2) a form with unusually plesiomorphic characters including a wide plastron, a Toxochelys-like humerus, femoral trochanters separated by a deep fossa, a broad sutural contact between the vomer and premaxillae on the palatal surface, and a single facet on the anterior end of the eighth cervical vertebra. This second sea turtle can be differentiated from other cheloniid taxa and so is named a new species, Euclastes hutchisoni. A cladistic analysis of 13 cheloniid taxa and 34 morphological characters suggests a phylogenetic position of Euclastes hutchisoni far-removed from the living cheloniids. Euclastes hutchisoni appears to be the last member of a Cretaceous-Paleocene radiation of durophagous stem cheloniids.