Evolution of the Family Equidae, Subfamily Equinae, in North, Central and South America, Eurasia and Africa during the Plio-Pleistocene

Abstract

Simple Summary The family Equidae enjoys an iconic evolutionary record, especially the genus Equus which is actively investigated by both paleontologists and molecular biologists. Nevertheless, a comprehensive evolutionary framework for Equus across its geographic range, including North, Central and South America, Eurasia and Africa, is long overdue. Herein, we provide an updated taxonomic framework so as to develop its biochronologic and biogeographic frameworks that lead to well-resolved paleoecologic, paleoclimatic and phylogenetic interpretations. We present Equus' evolutionary framework in direct comparison to more archaic lineages of Equidae that coexisted but progressively declined over time alongside evolving Equus species. We show the varying correlations between body size, and we use paleoclimatic map reconstructions to show the environmental changes accompanying taxonomic distribution across Equus geographic and chronologic ranges. We present the two most recent phylogenetic hypotheses on the evolution of the genus Equus using osteological characters and address parallel molecular studies. Studies of horse evolution arose during the middle of the 19th century, and several hypotheses have been proposed for their taxonomy, paleobiogeography, paleoecology and evolution. The present contribution represents a collaboration of 19 multinational experts with the goal of providing an updated summary of Pliocene and Pleistocene North, Central and South American, Eurasian and African horses. At the present time, we recognize 114 valid species across these continents, plus 4 North African species in need of further investigation. Our biochronology and biogeography sections integrate Equinae taxonomic records with their chronologic and geographic ranges recognizing regional biochronologic frameworks. The paleoecology section provides insights into paleobotany and diet utilizing both the mesowear and light microscopic methods, along with calculation of body masses. We provide a temporal sequence of maps that render paleoclimatic conditions across these continents integrated with Equinae occurrences. These records reveal a succession of extinctions of primitive lineages and the rise and diversification of more modern taxa. Two recent morphological-based cladistic analyses are presented here as competing hypotheses, with reference to molecular-based phylogenies. Our contribution represents a state-of-the art understanding of Plio-Pleistocene Equus evolution, their biochronologic and biogeographic background and paleoecological and paleoclimatic contexts.