Skull anatomy of the Oligocene toothed mysticete Aetioceus weltoni (Mammalia; Cetacea): implications for mysticete evolution and functional anatomy THOMAS A. DEMÉRÉ 1 * and ANNALISA BERTA 2 1 Department of Paleontology, San Diego Natural History Museum, PO Box 121390, San Diego, CA 92112, USA 2 Department of Biology, San Diego State University, San Diego, CA 92182, USA Received 20 September 2007; accepted for publication 25 September 2007 Toothed mysticetes of the family Aetiocetidae from Oligocene rocks of the North Pacific play a key role in interpretations of cetacean evolution because they are transitional in grade between dorudontine archaeocetes and edentulous mysticetes. The holotype skull of Aetiocetus weltoni from the late Oligocene (28–24 Ma) of Oregon, USA, has been further prepared, revealing additional morphological features of the basicranium, rostrum and dentary that have important implications for mysticete evolution and functional anatomy. The palate of Aetiocetus weltoni preserves diminutive lateral palatal foramina and associated delicate sulci which appear to be homologous with the prominent palatal foramina and sulci that occur along the lateral portion of the palate in extant mysticetes. In modern baleen whales these foramina allow passage of branches of the superior alveolar artery, which supplies blood to the epithelia of the developing baleen racks. As homologous structures, the lateral palatal foramina of A. weltoni suggest that baleen was present in this Oligocene toothed mysticete. Cladistic analysis of 46 cranial and dental characters supports monophyly of the Aetiocetidae, with toothed mysticetes Janjucetus and Mammalodon positioned as successive sister taxa. Morawanacetus is the earliest diverging aetiocetid with Chonecetus as sister taxon to Aetiocetus species. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154, 308–352. ADDITIONAL KEYWORDS: palaeontology – phylogeny – systematics. INTRODUCTION Aetiocetidae (Cetacea, Mysticeti) comprises a rela- tively diverse family of archaic, small, toothed mys- ticetes found in Oligocene sedimentary rocks on both sides of the North Pacific (Barnes et al., 1995). They represent an important transitional group that pre- serves morphologies intermediate between basilo- saurid archaeocetes and edentulous mysticetes. The transitional status of aetiocetids should not be viewed as suggesting they were ancestral to all later diverg- ing edentulous mysticetes, but instead that aetio- cetids provide a glimpse of the morphological changes that occurred in early members of the mysticete clade as they evolved from bite-and-swallow predators to specialized bulk, filter-feeders. To date, aetiocetids are only known from the North Pacific and can be considered an endemic lineage in that ocean basin until remains are discovered in other regions. The recent report of a new species of early Oligocene cetacean, Willungacetus aldingensis, from the Port Willunga Formation in South Australia (Pledge, 2005) provisionally assigns this southern hemisphere taxon to the Aetiocetidae. This record, however, still needs to be confirmed. Two of the four nominal aetiocetid genera are monotypic, Ashorocetus and Morawanocetus, both from the Morawan Forma- tion near Ashoro, Hokkaido, Japan (Barnes et al., 1995). Chonecetus includes two species, C. sookensis (Russell, 1968) from the Hesquiat Formation on *Corresponding author. E-mail: tdemere@sdnhm.org Zoological Journal of the Linnean Society, 2008, 154, 308–352. With 11 figures © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154, 308–352 308