What Is Equus? Reconciling Taxonomy and Phylogenetic Analyses

Abstract

Interest in the origin and evolution of Equus 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 Equus within several phylogenetic frameworks and present a phylogenetic analysis of derived Equini, including taxa referred to Equus, Haringtonhippus, Dinohippus, Astrohippus, Hippidion, and Boreohippidion. A new, morphology-based phylogenetic tree was used as an initial hypothesis for discussing what taxa Equus 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, Equus encompasses clade 6 (Ha. francisci = E. francisci, E. conversidens, E. quagga, E. hemionus, E. mexicanus, E. ferus, E. occidentalis, and E. neogeus) based on morphological synapomorphies. Equus is assigned to clade 6, or possibly clade 7, according to the uniqueness of adaptive zone criterion. The crown group criterion places Equus at clade 6. Based on the time-calibrated phylogeny of Equini, the divergence time criterion suggests that Equus encompasses clade 9. This clade comprises all taxa traditionally assigned to Equus analyzed in our study, including the eight taxa listed above as well as E. stenonis, E. idahoensis, and E. simplicidens; the latter two are sometimes referred to the subgenus Plesippus and the former to the subgenus Allohippus. 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 Equus. The taxonomic implications of delimiting Equus to clade 6 in our phylogenetic tree include elevation of Allohippus and Plesippus to generic rank, assignment of a new genus to "Dinohippus" mexicanus, and synonymy of Haringtonhippus with Equus.

Testing Equid Body Mass Estimate Equations on Modern Zebras—With Implications to Understanding the Relationship of Body Size, Diet, and Habitats of Equus in the Pleistocene of Europe

“…Historically, most scholars agree that North American E. simplicidens can be identified as the oldest species of genus Equus (Skinner and Hibbard, 1972;Azzaroli, 1992Azzaroli, , 2000Azzaroli, , 2002Azzaroli and Voorhies, 1993;Alberdi et al, 1998;Palombo and Alberdi, 2017;Bernor et al, 2018Bernor et al, , 2019Rook et al, 2019;Cirilli et al, 2020a,b); this hypothesis has also been supported by some recent paleogenomic analyses (Orlando et al, 2013;Vilstrup et al, 2013). Nevertheless, this hypothesis is not shared by some scholars, which prefer to identify the North American species into the genus Plesippus (Matthew, 1924;Gazin, 1936), and the Eurasian and African species into Allohippus (Kretzoi, 1938;Gromova, 1949); in their opinion, the genus Equus should be recognized as younger than 1 Ma (Eisenmann and Baylac, 2000;Eisenmann and Deng, 2005;Barrón-Ortiz et al, 2019). In this work, we consider the North American and Eurasian fossil species as being members of the genus Equus, following Skinner and Hibbard (1972), Azzaroli (1992Azzaroli ( , 2000Azzaroli ( , 2002, Azzaroli and Voorhies (1993), Alberdi et al (1998), Vilstrup et al (2013), Orlando et al (2013), Palombo and Alberdi (2017), Cantalapiedra et al (2017), Bernor et al (2018Bernor et al ( , 2019, Rook et al (2019), and Cirilli et al (2020a,b).…”

Enamel hypoplasia and dental wear of North American late Pleistocene horses and bison: an assessment of nutritionally based extinction models

“…The generic name Haringtonhippus was recently proposed for the non-caballine horse (Heintzman et al 2017). However, a recent phylogenetic analysis identified Haringtonhippus within the group that includes species traditionally assigned to Equus (Barrón-Ortiz et al 2018). Regardless of how the caballine and non-caballine species are named, several studies consistently identify them as distinct lineages (e.g., Weinstock et al 2005; Barrón-Ortiz et al 2017; Heintzman et al 2017); therefore, their recognition as separate taxonomic units in the present study is well supported.…”

Species Diversity and Paleoecology of Late Pleistocene Horses From Southern Mexico

“…We preferred not to synonymize Haringtonhippus to Equus, as suggested by Barrón-Ortiz et al (2019), given the profound discrepancies between the genomic and morphologic analyses about the phylogenetic position of H. francisci, which reflect that the definition of Equus is a work still in progress ( Barrón-Ortiz et al, 2019). Future analyses combining morphology and genomics will shed light about the phylogenetic relationships of H. francisci.…”

“…This species was reported (as Equus conversidens) from San Josecito Cave, Nuevo Léon, based upon a short mitochondrial DNA sequence (Barrón-Ortiz et al, 2017); this record was subsumed into H. francisci by Heintzman et al (2017). Finally, Barrón-Ortiz et al (2019) based on a morphology-based tree, determined that Equus comprises eight species, including H. francisci, and suggested that Haringtonhippus should be considered a synonym of Equus, but the authors do not formally synonymize both taxa.…”