New remains and new insights on the Gondwanan meiolaniform turtle Chubutemys copelloi from the Lower Cretaceous of Patagonia, Argentina

Abstract

New cranial and postcranial remains of the Gondwanan meiolaniform turtle Chubutemys copelloi Gaffney, Rich, Vickers-Rich, Constantine, Vacca and Kool 2007 from the Lower Cretaceous Puesto La Paloma Member of the Cerro Barcino Formation (Chubut province, Patagonia, Argentina) are presented, described, and compared in this contribution. Carapace and plastral remains, pectoral and pelvic girdle, cervical and thoracic vertebrae, and the left maxilla and jugal are the new recovered elements from C. copelloi. These new remains were found in 2008 in Turtle Town locality, the same site where the holotype and referred specimens of C. copelloi had been found in 1997. The facies analysis of the sedimentary succession in Turtle Town and surrounding areas suggests a paleoenvironment constituted by a broad alluvial plain with influx of volcanic ash-falls, whose deposits were disturbed by pedogenesis or reworked by current flows that supplied water and sediment in ponded areas. The presence of a very thin shell, broad vertebrals (broader than pleurals), the presence of a shallow nuchal notch with the vertebral 1 almost reaching the anterior border of the carapace, the presence of costo-peripheral fontanelles, and the presence of an ectepicondylar canal in the humerus, are the most notable features provided by the new remains. A cladistic analysis has been performed to test the phylogenetic position of C. copelloi. This phylogeny corroborates the basal position of C. copelloi among Meiolaniformes. The plausible phylogenetic relationships of the clade Meiolaniidae with other Gondwanan (e.g., Otwayemys cunicularius Gaffney, Kool, Brinkman, Rich, and Vickers-Rich 1998) or Laurasian Lower Cretaceous taxa (e.g., sinochelyids) are also discussed in this contribution. Taxa included in Meiolaniformes have been mainly found in Gondwana and as the most basal taxon was found in Patagonia, paleobiogeographic studies suggest this clade originated in South America. Due to the basal position of C. copelloi among Meiolaniformes the discovery of more and previously unknown remains of this species sheds light on the origin and early evolution of this curious clade.