In This Issue

Fresh light on horse evolutionThe evolutionary history of horses is relatively well understood, with Hyracotherium, or the dawn horse (Eohippus), a now-extinct mammal roughly three to five hands high, thought to be the oldest known horse.However, the early diversification of horses remains shrouded in mystery.Jrmy Tissier and Thierry Smith performed a large-scale phylogenetic analysis of the fossil remains of perissodactyls-the group of odd-toed ungulate mammals that includes horses, tapirs, and rhinoceroses.The analysis suggested that Hyracotherium and other taxa previously named Hyracotherium were not closely related to horses-or even to the taxonomic subgroup to which horses belong-but rather were basal perissodactyls.Further, the analysis pointed to an Asian origin of perissodactyls, with the most basal taxon in the phylogenetic tree, Gazijhippus, traced to Pakistan and the two closest sibling groups of perissodactyls traced to the India-Pakistan region.Perissodactyls are thought to have first appeared on the three continents of the Northern Hemisphere roughly 56 million years ago, during a geological event called the Paleocene-Eocene Thermal Maximum (PETM).The authors' analysis indicated rapid dispersals of two of the earliest known genera-Pliolophus and Cardiolophus-across North America, Europe, and Asia during the PETM, similar to other modern mammals.According to the authors, the study chips away at the systematic and geographic mysteries surrounding Hyracotherium and sheds fresh light on early perissodactyl evolution.-P.N.