Diaphorocetus poucheti (Cetacea, Odontoceti, Physeteroidea) from Patagonia, Argentina: one of the earliest sperm whales

Abstract

Sperm whales (Physeteroidea) are the basal-most surviving lineage of odontocetes, represented today by just three highly specialized, deep-diving suction feeders. By contrast, extinct sperm whales were relatively diverse, reflecting a major Miocene diversification into various suction feeding and macroraptorial forms. The beginnings of this diversification, however, remain poorly understood. The Atlantic coast of South America provides a crucial window into early physeteroid evolution and has yielded some of the oldest species known from cranial material, Idiorophus patagonicus and Diaphorocetus poucheti – both of which are in need of re-description and phylogenetic reappraisal. Here, we re-examine Diaphorocetus in detail and, in light of its complex taxonomic history, declare it a nomen protectum. Phylogenetically, the species forms part of a polytomy including ‘Aulophyseter’ rionegresis and the two crown lineages (Physeteridae and Kogiidae) and demonstrates that facial asymmetry and a clearly defined supracranial basin have characterized this lineage for at least 20 Ma. With a total body length of 3.5–4 m, Diaphorocetus is one of the smallest physeteroids yet known. Its cranial morphology hints at an intermediate raptorial/suction feeding strategy and it has a moderately developed spermaceti organ and junk.

Keywords:

• Physeteroidea
• Gaiman Formation
• phylogeny
• anatomy
• body size
• Miocene

Acknowledgements

We wish to thank A. Jäkel and W. Ferrari for taking the photographs in Figures 2–8; M. A. Reguero, A. Scarano, M. L. de los Reyes, O. Lambert and O. Pauwels for access to the collections under their care; C. de Muizon for providing photographs of Diaphorocetus; M. Farro and A. Mones for access to historical information on Lydekker’s publication; C. Deschamps and S. Bargo for providing information on the original drawing of Diaphorocetus; and O. Lambert and an anonymous reviewer for their constructive comments. This work was supported by Research Project PICT 2015-0792 to MB and JC, Projects PICT 2016-1039 and N749 UNLP to MF, and an FNRS postdoctoral fellowship (32795797) to FGM.

Supplemental material

Supplementary material for this article can be accessed at: https://doi.org/10.1080/14772019.2019.1605544.

Associate Editor: Adrian Lister