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.
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