A new Miocene cetothere (Mammalia: Cetacea) from the Monterey Formation at Laguna Niguel, Orange County, California

Phylogenetic relationships among the fossil and living "whalebone" (baleen) whales remain poorly understood. Until recently, the extinct family Cetotheriidae, a former grade taxon of edentulous (toothless) whales of the suborder Mysticeti, was thought to be the evolutionary source of the modem-baleen whales, which include the families Balaenopteridae, Balaenidae, Neobalaenidae, and Eschrichtiidae. Following attempts to identify the most valid "cetotheres" to justifiably include in a strict family clade, the taxon is recognized as a derived group with uncertain relationships to modern taxa. The monophyly of the order Cetacea, suborder Mysticeti, and clade comprised of all edentulous baleen-bearing mysticetes is, nevertheless, assumed to be valid. Fossil baleen-bearing mysticetes commonly exhibit a mosaic of primitive and derived characters, hinting at transitional forms that link them to ancestors among more basal-toothed mysticetes. Furthermore, the toothed mysticetes show affinities with the archaeocetes, the third and most primitive suborder of the cetaceans. The evolutionary chronology of the mysticetes is not transparently linear, but branches stepwise, in radiative spurts. Relict groups commonly survived alongside groups with more derived characters. This thesis involved the preparation and description of a fossil cranium belonging to a new species of cetothere assigned to the genus Mixocetus. The specimen is of late Miocene age and is from the Monterey Formation at Orange County, southern California. The fossil whale is here placed in evolutionary context within the family Cetotheriidae sensu stricto (in the strict sense) and suborder Mysticeti.