New remains of the aquatic turtle <i>Hydromedusa casamayorensis</i> (Pleurodira, Chelidae) from the middle Eocene of Patagonia: taxonomic validation and phylogenetic relationships

Abstract

Abstract Twenty‐seven new specimens recovered from lower levels of the Sarmiento Formation (Casamayoran SALMA , middle Eocene) in the ‘Cañadón Hondo’ area, Chubut (Argentina), bring new information on the Palaeogene long‐necked chelid Hydromedusa casamayorensis . The discovery of cranial and postcranial remains allows a complete description of the only known extinct species of the genus Hydromedusa recognizing diagnostic characters that revalidate the taxonomic status of this species. Hydromedusa casamayorensis is incorporated into a cladistic framework of chelid turtles. The unconstrained and constrained phylogenetic analyses recovered H . casamayorensis as a sister taxon of Hydromedusa maximiliani plus H . tectifera . A Danian age is proposed at least for the origin of the Hydromedusa clade, while a Late Cretaceous age is suggested for the split from the Yaminuechelys clade. Great differences in size are documented for H . casamayorensis based on the various specimens recovered from this study. The osteohistological study in H . casamayorensis and H . tectifera shows similarities in the internal cortex and cancellous bone of both taxa, but some differences are observed in the external cortex of H . casamayorensis (e.g. a parallel fibred bone on the more external surface of this cortex).