Awakening Patagonia's sleeping sperm whale: a new description of the Early Miocene Idiorophus patagonicus (Odontoceti, Physeteroidea)

Abstract

Idiorophus patagonicus is one of the oldest extinct sperm whales known, and was recovered from the Early Miocene Gaiman Formation (Chubut, Argentina). It was described in the late 19th century by Lydekker based on an incomplete skull and has not been reviewed since its original description. Thus, many of its key anatomical features remain unknown, preventing a better understanding of the evolutionary trends in Physeteroidea. Here, we shed light on the anatomical features of Idiorophus patagonicus, its phylogenetic relationships and the palaeobiological aspects of its body size and feeding methods. Phylogenetically, Idiorophus is recovered as the basalmost member of the Physeteridae, or as a stem physeteroid; the most crownward physeteroid known from Patagonia. Idiorophus is a longirostrine species with a peculiar rostrum: it has a shape like the neck of a wine bottle, is tube like, and has the left premaxilla overhanging the right one, suggesting a structural advantage as a reinforcement of the rostrum from external pressures. Idiorophus is also a novelty in the evolution of sperm whale body size, being the oldest physeteroid (Burdigalian) with a large body size (6.61 m). The body size, skull morphology and the teeth wear facet of Idiorophus suggest a raptorial feeding method, which differs markedly from those inferred for the other Miocene Patagonian sperm whales (Diaphorocetus poucheti and Cozzuoliphyseter), suggesting a high ecomorphological disparity of the Patagonian sperm whale assemblage.