Awakening Patagonia’s sleeping sperm whale: a new descriptionof the Early Miocene Idiorophus patagonicus (Odontoceti,Physeteroidea)

Abstract

diorophus patagonicus is one of the oldestextinct sperm whales known, and was recovered from theEarly Miocene Gaiman Formation (Chubut, Argentina). Itwas described in the late 19 th century by Lydekker based onan incomplete skull and has not been reviewed since its ori-ginal description. Thus, many of its key anatomical featuresremain unknown, preventing a better understanding of theevolutionary trends in Physeteroidea. Here, we shed light onthe anatomical features of Idiorophus patagonicus, its phylo-genetic relationships and the palaeobiological aspects of itsbody size and feeding methods. Phylogenetically, Idiorophusis recovered as the basalmost member of the Physeteridae, oras a stem physeteroid; the most crownward physeteroidknown from Patagonia. Idiorophus is a longirostrine specieswith a peculiar rostrum: it has a shape like the neck of a wine bottle, is tube like, and has the left premaxillaoverhanging the right one, suggesting a structural advantageas a reinforcement of the rostrum from external pressures.Idiorophus is also a novelty in the evolution of sperm whalebody size, being the oldest physeteroid (Burdigalian) with alarge body size (6.61 m). The body size, skull morphologyand the teeth wear facet of Idiorophus suggest a raptorialfeeding method, which differs markedly from those inferredfor the other Miocene Patagonian sperm whales (Diaphoroce-tus poucheti and Cozzuoliphyseter), suggesting a high ecomor-phological disparity of the Patagonian sperm whaleassemblage.