Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles

Abstract

Chinlechelys tenertesta is a turtle from the Upper Triassic Bull Canyon Formation of the Chinle Group of eastern New Mexico, USA, which has largely been ignored in recent studies of the phylogenetic position of turtles (Testudinata) within the Tetrapoda. Here, we present the first comprehensive description of the morphology of Chinlechelys tenertesta and reaffirm its unequivocal support for the composite model of costal bone formation in turtles. We also document the presence in Chinlechelys tenertesta of separate ribs reminiscent of those of Odontochelys semitestacea, a Late Triassic prototurtle from China, and costal plates, as would be expected under the composite model. This indicates that the two-phase embryological model of costal and plastral bone formation approximately corresponds to the formation of the endochondal rib and plastral primary ossifications and then the later dermal formation of the plastral and costal plates, as is suggested by the composite model. We challenge the identification of Permian Eunotosaurus africanus as a stem turtle and instead suggest that it is a caseid synapsid. We do not consider Pappochelys rosinae to be a close relative of turtles. It more closely resembles the basal placodont sauropterygians, particularly Palatodonta bleekeri. Indeed, phylogenetic analysis based on correctly coded character states places Pappochelys rosinae in the Sauropterygia as the sister taxon of placodonts. The morphology of Chinlechelys tenertesta supports the placement of Testudines outside of crown Sauria, as a taxon derived from pareiasaurs based on the morphology of the dorsal osteoderms and skull roof. Asher J. Lichtig. New Mexico Museum of Natural History & Science, 1801 Mountain Road N. W., Albuquerque, New Mexico 87104, USA. ajlichtig@gmail.com Spencer G. Lucas. New Mexico Museum of Natural History & Science, 1801 Mountain Road N. W., Albuquerque, New Mexico 87104, USA. spencer.lucas@state.nm.us