The famous ‘Bone Wars’ between Edward Drinker Cope and Othniel Charles Marsh resulted in the naming of numerous vertebrate taxa from the Cretaceous formations of the Atlantic Coastal Plain, including many species that have since been shown to be of indeterminate affinities or synonyms of each other. Here, we describe a new specimen of the pan-cheloniid turtle Euclastes named by Cope in 1867 which demonstrates that another pan-cheloniid he named in 1870, Catapleura, is actually the same taxon. Each of these genera were originally based on incomplete and fragmentary remains from the Paleocene of New Jersey, with Euclastes named for a partial skull and Catapleura named for partial carapace bones, and their poor fossil records have generated considerable taxonomic confusion and debate for over 150 years. The new skeleton, recovered from the earliest Danian Main Fossiliferous Layer (MFL) of the Hornerstown Formation, includes a rostrum and mandible exhibiting autapomorphies of Euclastes wielandi (Hay, 1908) (e.g. dentary with an elongated symphysis) and a carapace and plastron exhibiting autapomorphies of Catapleura repanda Cope, 1870 (e.g. no contact between second suprapygal and eleventh peripheral), demonstrating that they are synonyms. Taxonomic priority is given to the senior synonym Euclastes. By incorporating character scorings based on the new specimen into a revised phylogenetic analysis, we found Euclastes to occupy a derived position within Pan-Cheloniidae, with close relationships to Mexichelys Brinkman et al., 2009 and Corsochelys Zangerl, 1960. Euclastes wielandi was found to be the most basal species of the genus owing to its broad contribution of the frontal to the orbital margin in adult individuals. These insights reduce the apparent diversity of chelonians in the MFL and highlight how the discovery (and description) of new specimens including both cranial and shell remains is critical to advancing our understanding of the evolutionary relationships and palaeoecology of extinct turtles.
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