Article: A cladistic analysis of the horses of the tribe Equini

The Equini tribe with seven genera forms a monophyletic group defined by one synapomorphy: protocone connected to the protoloph. Fourteen species are considered as the terminal taxa: Protohippus (two species), Calippus (two), Pliohippus (one), Hippidion (three), Dinohippus (three), Astrohippus (one) and Equus (two). A cladistic analysis was performed using 20 characters from cranial morphology, upper and lower teeth, and appendicular skeletons. Polarity of characters was based on outgroup criterion using the Hippotheriini tribe. For some characters, apomorphic states were identified using Merychippus, which was the sister group of both tribes. One parsimonious cladogram of 31 steps and a consistency index of 0-77 was produced, from which a classification of the tribe Equini was constructed. We recognize two subtribes: Protohippina (Protohippus and Calippus) and Pliohippina subtrib. nov. (Pliohippus, Hippidion, Dinohippus, Astrohippus and Equus). Pliohippus was the sister group to the rest of the subtribe. The species of Hippidion form a monophyletic group and there is no evidence of a relationship between ' Onohippidium' galushai, from North America, and the Hippidion group, from South America. Astrohippus stocki was the sister species of the Equus-group, which includes 'Dinohippus' mexicanus. The analysis shows that characters mostly from the cranial morphology and upper teeth characterize the suprageneric taxa. High congruence between the stratigraphical record and the phylogenetic hypothesis is observed.