A recent discovery shields light on the presence of crocodiles in the Eocene of West Antarctica

PAULA BONA; EUGENIA PEREYRA; JAVIER GELFO; CAROLINA ACOSTA HOSPITALECHE; DANIEL GARCÍA LÓPEZ; FACUNDO IRAZOQUI; MARCELO REGUERO

Salta

Congreso; Reunión de comunicaciones de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

Among archosaurs, the clade Avemetatarsalia is well documented in the Antarctic fossil record. However, the presence of crocodyliforms was mentioned but never accurately supported. Herein we describe by first time a partial osteoderm coming from the La Meseta Formation (Ypresian), at Marambio/Seymour Island. It is a half huge dermal plate (anteroposterior length: 58 mm, maximum thickness: 17 mm) which preserves the interdigitated latero-medial contact between axial osteoderms. Its highly eroded dorsal surface is typically ornamented by small and large cells delimited by thin, sharp, and anastomosed ridges. The longitudinal section of the plate is exposed by fracture, spindle-shaped and ventrally concave. Microstructurally is observed a deep region composed by poorly vascularized lamellar bone with many growth marks, a core formed by large resorption spaces and compacted coarse cancellous bone, and a superficial region composed of highly vascularized woven bone tissue. Traces of Sharpey's fibers are abundant, mainly in the superficial region and on the laterals of the dermal plate. The combination of these anatomical features allows us to identify this material as a dorsal osteoderm of a crocodyliform with an osteohistological pattern similar to that of extant and extinct crocodylians. The clade Crocodylia is known since the Late Cretaceous to the present in all continents except Antarctica. This apparent absence was traditionally explained by the latitudinal position of the continent, due to the Antarctic polar-night and the restrictions imposed by the presumed ectothermy of the group. This new finding situates crocodiles in southernmost Gondwanan Paleogene landscapes and raises new questions related to their physiological plasticity.