Reassessment of the iconic Oligo‐Miocene heterodont dolphin <i>Squalodon</i>: a redescription of the type species <i>S. grateloupii</i>

Abstract Since its description in 1840, the toothed whale Squalodon was conceptualized as a transitional form between archaeocetes‐like Basilosaurus and the modern toothed whales (Odontoceti). The cranial morphology of Squalodon approximates that of modern odontocetes; however, their dental patterning more closely resembles that of archaeocetes. Despite representing a transitional form, the Squalodontidae have been neglected in scientific study. The poor taxonomy of the family is the primary cause: significant specimens have not been redescribed, including the type species, S . grateloupii . Complicating matters, the holotype of that species has been missing for over a century, and this calls for the designation of a neotype by the ICZN. We present a neotype for S . grateloupii and synapomorphies for the genus and family, which include several features in the posterior basicranium and the periotic; we identified a strongly conserved dental pattern for the Squalodontidae. Given that the Squalodontidae has been underrepresented in phylogenetic analyses, our analysis included representatives from nearly all clades within Odontoceti, as well as every squalodontid species. Our results demonstrate that the Squalodontidae is monophyletic; unlike previous iterations of this phylogenetic analysis, the Squalodontidae were not recovered in crown Odontoceti. This result supports the hypothesis that homodonty is a synapomorphy of crown Odontoceti and did not evolve convergently, which is supported by four dental synapomorphies for crown odontocetes. Our modern revision of the iconic Squalodon lays the foundation for research into interspecies variation, ontogeny and biogeography of a key transitional form, so that we better understand the course of odontocete evolution.