A new genus and species of kekenodontid from the late Oligocene of New Zealand with comments on the evolution of tooth displacement in Cetacea.

The Kekenodontidae are late-surviving archaeocetes from the Late Oligocene of Southwest Pacific that includes a single-named species, Kekenodon onamata. Tohoraonepu nihokaiwaiu is a new genus and species of small body-sized kekenodontid from the upper Oligocene (Chattian) Kokoamu Greensand of Otago, South Island, New Zealand. Phylogenetic analyses recover T. nihokaiwaiu within a monophyletic Kekenodontidae, forming a clade with an unnamed provisional kekenodontid, OU 22023. Kekenodontids are recovered crownward to basilosaurids and stemward to a paraphyletic group of toothed 'mysticetes' that are excluded from Neoceti. The analyses confirm the identification of kekenodontids as the latest-diverging archaeocetes that persisted into the Late Oligocene. The holotype OU 22394 is a juvenile individual preserving several isolated heterodont teeth with characteristics of deciduous teeth, including unmineralized pulp cavities and cheek teeth with lower-lying triangular crowns that are different from all known kekenodontids. Diphyodonty is known from Eocene archaeocetes but is unknown from geologically younger toothed cetaceans, with monophyodonty being hypothesised for all Neoceti. Inferences of diphyodonty in T. nihokaiwaiu would be the first instance in Cetacea from rocks geologically younger than the Eocene and would indicate diphyodonty persisted in some Late Oligocene archaeocetes. [ABSTRACT FROM AUTHOR]