Ankle morphology of the earliest Cetaceans and its implications for the phylogenetic relations among ungulates.

Recent molecular studies are inconsistent with ungulate phylogenetic trees that are based on morphological traits. These inconsistencies especially relate to the position of cetaceans and perissodactyls. Evaluation of the close phylogenetic ties between artiodactyls and cetaceans has been hampered by the absence of tarsal bones of primitive cetaceans, as artiodactyls are often diagnosed on the basis of their tarsus. We here describe newly discovered tarsal bones that are the oldest cetacean tarsals known. We present a character analysis for primitive ungulate tarsals and evaluate their impact on the ungulate phylogenetic tree. Tarsal data are consistent with some molecular studies in suggesting that the extant sister group of Cetacea is Artiodactyla or that Cetacea should be included within the latter order. Tarsal data do not support Cete (Mesonychia plus Cetacea) and are consistent with the exclusion of perissodactyls from paenungulates as suggested by some molecular studies.