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Insights into cranial morphology and intraspecific variation from a new subadult specimen of the pan-cheloniid turtle Euclastes wielandi Hay, 1908


We describe a nearly complete skull and mandible of a subadult of Euclastes wielandi, a pan-cheloniid turtle recently recovered at the Jean and Ric Edelman Fossil Park at Rowan University in Mantua Township, New Jersey, which yields new information about the osteology, ontogeny, and intraspecific variation of this taxon. The specimen was collected from the earliest Danian Main Fossiliferous Layer (MFL) of the Hornerstown Formation. Although discovered immediately adjacent to remains of two pleurodires, Taphrosphys sulcatus and Bothremys sp., the skull and mandible can be definitively assigned to Pan-Cheloniidae based on its V-shaped basisphenoid and rod-like rostrum basisphenoidale. Among three pan-cheloniid taxa known from the MFL, the specimen is assigned to Eu. wielandi based on its low skull with dorsally-directed orbits, symphyseal swelling in the mandibular triturating surface, and high dorsum sellae. Comparisons with other specimens of Eu. wielandi and adults and juveniles of other pan-cheloniids revealed variations in the type and timing of cranial ontogenetic changes in the clade, as well as anatomical traits subject to intraspecific variation, such as the depth of the sella turcica, paths of the foramina nervi hypoglossi, and development of a precolumellar fossa. The relative contribution of the frontal to the orbital margin and precise path of the prefrontal-supraorbital scale sulcus are subject to individual variation in Eu. wielandi, as well as ontogenetic variation and bilateral asymmetry in other cryptodirans, signifying that the widespread use of frontal retraction in taxon diagnoses and as a phylogenetic character should be reconsidered. As in multiple other taxa, the mandibular triturating surface expands through growth in Eu. wielandi, demonstrating that increased durophagy with age was a common life strategy among Cryptodira.

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