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

BONA, P.; PEREYRA, E.; GELFO, J.; ACOSTA HOSPITALECHE, C.; GARCÍA LÓPEZ, D.; IRAZOQUI, F.; REGUERO, M.

Salta

Congreso; RCAPA; 2022

APA

Among archosaurs, the Avemetatarsalia is well-documented in the Antarctic fossil record. However, the presence of crocodyliforms was tangentially mentioned but never accurately supported. Herein we describe a partial osteoderm, coming from the basal coquina of Cucullaea I Allomember (Ypresian) of La Meseta Formation, at Seymour/Marambio Island. It consists of the half of a huge dermal plate (approximately 58 mm of anteroposterior length and 17 mm of maximum thickness) which preserves the interdigitated latero-medial contact between axial osteoderms. The dorsal surface (highly eroded) is typically ornamented by a combination of small and large cells, delimited by thin, sharp, and anastomosed ridges, irregularly distributed. 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 plate. The combination of these anatomical features allows us to identify this material as a dorsal osteoderm of a crocodyliform. This osteohistological pattern is similar to the osteoderms of extant and extinct crocodylians. The Crocodylia is the clade of eusuchians crocodyliforms that survived the massive extinction of the end of the Cretaceous and diversified during the Cenozoic. They are known since the Late Cretaceous to the present in all continents except Antarctica. Traditionally this absence was partially explained by the latitudinal position of the continent, due to the Antarctic polar-night and the restrictions imposed by the presumed ectothermy of the crocodilians. This new finding extends the paleobiogeographic distribution of crocodiles to the southernmost Gondwanan landscapes. It also raises new questions related to a greater physiological plasticity for this group.