The later evolution of horses can be problematic and this is perhaps best illustrated in the Americas, where more than 50 species of Pleistocene equids have been named, most of them during the 19th and early 20th centuries (Weinstock et al., 2005: 0001). Whilst it has been argued that this number should be drastically revised, no consensus has as yet been reached about the number of valid species or their phylogenetic relationships (Weinstock et al., 2005: 0001). It was to address this problem that a recent study by Weinstock et al., looked at a segment of between 349 and 716 base pairs of the mitochondrial control region of fossil equid remains from a number of different localities in North and South America and Eurasia ranging in date from c. 53,000 years ago to historical times (Weinstock et al., 2005: 0002).
Using maximum likelihood, phylogenetic analysis resolved Hippidion, New World ‘stilt-legged’ horses (NWSL) and caballines (including the domestic horse, Equus caballus, and the extant wild Przewalskii horse) as three genetically divergent species within a monophyletic group (Weinstock et al., 2005: 0002). African zebras and asses and Asian asses (onager and kiang) form a basal polytomy (Weinstock et al., 2005: 0002). However, the close phylogenetic relationship between Hippidion and caballine horses is in direct contrast to palaeontological models of hippidiform origins (Weinstock et al., 2005: 0002; Benton, 1997: 342).
It is suggested that the origin of Hippidion should not be seen as a descendent of Miocene pliohippines; instead, its origins appear to be more recent, probably during the last stages of the Pliocene (c. 3 Ma), which is close to the time of the first fossil occurrence of this genus (Weinstock et al., 2005: 0003). This is in opposition to views expressed by authors such as McFadden (1997) who claim that hippidiforms (genera Hippidion and Onohippidium) were present in North America as early as the late Miocene (c. 7 – 8 Ma).
The situation with the phylogenetic position of the NWSL is also apparently resolved with the molecular evidence, which places them as North American endemics tather than Eurasian migrants (Weinstock et al., 2005: 0003). Specimens from both north (Alaska/Yukon) and south (Wyoming/Nevada) of the Pleistocene ice sheets clearly belong to the same taxon. This apparent large geographic distribution raises the possibility that other Late Pleistocene NWSL currently described as different species (E. francisci, E. tau, E. quinni, E. cf. hemionus, E (Asinus) cf. kiang) may in fact represent the same taxon (Weinstock et al., 2005: 0003). Their origins probably lie south of the Pleistocene ice sheets, where Mid-Pleistocene remains (c. 0.5 Ma) with similar limb characteristics have been found (Weinstock et al., 2005: 0003). Frequencies of NWSL are much lower in the north and they appear to have a restricted temporal distribution. It would appear that, despite their presence in eastern Beringia (unglaciated Alaska/Yukon), they failed to disperse through the Bering land-bridge into western Beringia (north-east Siberia) (Weinstock et al., 2005: 0003).
All caballine horses from western Europe to eastern Beringia – including the domestic horse – would appear to be a single Holarctic species (Weinstock et al., 2005: 0003). This group can be split into two major clades. The first is broadly distributed from central Europe to North America north and south of the ice. The second clade appears to have been restricted to North America. If present in the Old World at all, it would appear to have disappeared before domestication of the horse took place, around 5,000 years BP, as all domestic horses cluster in the first clade (Weinstock et al., 2005: 0004).
Benton, MJ. 1997.Vertebrate Palaeontology. Second Edition. London: Chapman and Hall.
McFadden, BJ. 1997. Pleistocene horses from Tarija, Bolivia, and the validity of the genus Onohippidium (Mammalia: Equidae). Journal of Vertebrate Palaeontology 17: 199-218
Weinstock, J, Willerslev, E, Sher, A, Tong, W, Ho, SYW, Rubenstein, D, Storer, J, Burns, J, Martin, L, Bravi, C, Prieto, A, Froese, D, Scott, E, Xulong, L, and Cooper, A. 2005. Evolution, systematics, and phylogeography of Pleistocene horses in the New World: a molecular perspective. PLoS Biology 3 (8): 0001-0007.