New evidence of a toothed mysticete from the Vaqueros Formation of California fills a gap in the palaeobiogeographic range of Aetiocetidae

Abstract Extant baleen whales (Mysticeti) are toothless aquatic predators that use keratinous baleen plates to capture and filter smaller prey. Although all present-day baleen whales are edentulous obligate filter feeders, the fossil record documents several extinct lineages with teeth from the Eocene through to the Oligocene. Aetiocetidae represents one group of toothed mysticetes found in Oligocene rocks along the North Pacific Ocean, with occurrences of named genera from Japan, British Columbia (Canada), Washington State and Oregon (USA), and Baja California (Mexico). Here, we fill a gap in the geographical range of the family by reporting a possible aetiocetid from California. We describe a portion of the mandible and two associated teeth from the Vaqueros Formation in the Central Santa Monica Mountains near Malibu, California, USA. The associated postcanine tooth is most similar in size and shape to the postcanine dentition of Salishicetus meadi from Washington State. We refer the Santa Monica material to Aetiocetidae cf. Salishicetus. Given that the age of the Vaqueros rocks is poorly constrained, we cannot confirm whether the referred specimen is from the Late Oligocene or Early Miocene. However, a possible Miocene occurrence would extend the upper stratigraphic range of toothed mysticetes across the Oligocene–Miocene boundary.