H erpetocetus morrowi ( C etacea: M ysticeti), a new species of diminutive baleen whale from the Upper P liocene ( P iacenzian) of C alifornia, USA, with observations on the evolution and relationships of the C etotheriidae

The extinct edentulous mysticete family C etotheriidae historically has been viewed as a notoriously paraphyletic group, and only recently have rigorous studies been executed to rectify this issue. These problems do not necessarily just stem from lack of phylogenetic analyses, but are in part because of a general lack of complete specimens, poor descriptions of taxa, and long‐lived taxonomic instability issues. The fossil mysticete genus H erpetocetus is a poster child of these problems as it is primarily only known from a few relatively incomplete and poorly described specimens. A new species of H erpetocetus from the upper P liocene of C alifornia, Herpetocetus morrowi sp. nov. , provides an archetypal model for the genus based on a multitude of well‐preserved specimens. These specimens reveal a diminutive mysticete characterized by an elongate rostrum and roughly quadrate cranium. A mosaic of primitive and derived features preserved in this new species underscores its potential value in helping to resolve a number of taxonomic and phylogenetic problems. The occurrence of specimens assignable to juvenile through to mature adult individuals provides a basis for investigating ontogenetic changes. Functional analysis of the unusual craniomandibular anatomy of H . morrowi suggests a limited degree of mandibular gape and an enhanced capacity for longitudinal rotation of the dentary, features that support a hypothesis of suction feeding convergent with that of living grey whales. A phylogenetic analysis provides support for recognition of a redefined and monophyletic C etotheriidae and H erpetocetinae, and also serves as a basis for evaluating the recent proposal that the pygmy right whale ( C aperea marginata ) is a living cetothere. Morphological features of H erpetocetus morrowi , including features of the cranium and petrosal, suggest that a number of the purported synapomorphies supporting a C aperea −cetothere grouping are either symplesiomorphies, nonhomologous features, or are highly variable. © 2014 The Linnean Society of London