AUTHOR=Barrón-Ortiz Christina I. , Avilla Leonardo S. , Jass Christopher N. , Bravo-Cuevas Víctor M. , Machado Helena , Mothé Dimila 

TITLE=What Is Equus? Reconciling Taxonomy and Phylogenetic Analyses

JOURNAL=Frontiers in Ecology and Evolution

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2019.00343

DOI=10.3389/fevo.2019.00343

ISSN=2296-701X

ABSTRACT=<p>Interest in the origin and evolution of <italic>Equus</italic> dates back to over a century, but there is still no consensus on the definition of the genus or its phylogenetic position. We review the placement of <italic>Equus</italic> within several phylogenetic frameworks and present a phylogenetic analysis of derived Equini, including taxa referred to <italic>Equus, Haringtonhippus, Dinohippus, Astrohippus, Hippidion</italic>, and <italic>Boreohippidion</italic>. A new, morphology-based phylogenetic tree was used as an initial hypothesis for discussing what taxa <italic>Equus</italic> encompasses, using four criteria previously used to define the genus category in mammals: phylogenetic gaps, uniqueness of adaptive zone, crown group definition, and divergence time. According to the phylogenetic gaps criterion, <italic>Equus</italic> encompasses clade 6 (<italic>Ha. francisci</italic> = <italic>E. francisci, E. conversidens, E. quagga, E. hemionus, E. mexicanus, E. ferus, E. occidentalis</italic>, and <italic>E. neogeus</italic>) based on morphological synapomorphies. <italic>Equus</italic> is assigned to clade 6, or possibly clade 7, according to the uniqueness of adaptive zone criterion. The crown group criterion places <italic>Equus</italic> at clade 6. Based on the time-calibrated phylogeny of Equini, the divergence time criterion suggests that <italic>Equus</italic> encompasses clade 9. This clade comprises all taxa traditionally assigned to <italic>Equus</italic> analyzed in our study, including the eight taxa listed above as well as <italic>E. stenonis, E. idahoensis</italic>, and <italic>E. simplicidens</italic>; the latter two are sometimes referred to the subgenus <italic>Plesippus</italic> and the former to the subgenus <italic>Allohippus</italic>. With the exception of the divergence time criterion, the results of our evaluation are congruent in identifying clade 6 as the most suitable position for <italic>Equus</italic>. The taxonomic implications of delimiting <italic>Equus</italic> to clade 6 in our phylogenetic tree include elevation of <italic>Allohippus</italic> and <italic>Plesippus</italic> to generic rank, assignment of a new genus to “<italic>Dinohippus</italic>” <italic>mexicanus</italic>, and synonymy of <italic>Haringtonhippus</italic> with <italic>Equus</italic>.</p>