Abstract
The monodactyl horses of the genusEquusoriginated in North America during the Pliocene, and from the beginning of the Pleistocene, they have been an essential part of the large ungulate communities of Europe, North America and Africa. Understanding how body size ofEquusspecies evolved and varied in relation to changes in environments and diet thus forms an important part of understanding the dynamics of ungulate body size variation in relation to Pleistocene paleoenvironmental changes. Here we test previously published body mass estimation equations for the family Equidae by investigating how accurately different skeletal and dental measurements estimate the mean body mass (and body mass range) reported for extant Grevy's zebra (Equus grevyi) and Burchell's zebra (Equus quagga). Based on these tests and information on how frequently skeletal elements occur in the fossil record, we construct a hierarchy of best practices for the selection of body mass estimation equations inEquus. As a case study, we explore body size variation in Pleistocene EuropeanEquuspaleopopulations in relation to diet and vegetation structure in their paleoenvironments. We show a relationship between diet and body size inEquus: very large-sized species tend to have more browse-dominated diets than small and medium-sized species, and paleovegetation proxies indicate on average more open and grass-rich paleoenvironments for small-sized, grazing species ofEquus. When more than one species ofEquusco-occur sympatrically, the larger species tend to be less abundant and have more browse-dominated diets than the smaller species. We suggest that body size variation in PleistoceneEquuswas driven by a combined effect of resource quality and availability, partitioning of habitats and resources between species, and the effect of environmental openness and group size on the body size of individuals.
Highlights
The evolution of equids leading to the extant genus Equus is of particular interest due to their rich fossil record in all continents except Australia and Antarctica
We present a temporal trend of Equus species during the Pleistocene, and compare dietary variation between the body size classes
The results show that mesowear and body mass data of Pleistocene Equus are bivariate normally distributed according to Anderson-Darling test (p = 0.147) and Chi-squared type tests for multivariate normality (McCulloch test: p = 0.36, Nikulin-Rao-Robson test: p = 0.24, DzhaparidzeNikulin test: p = 0.20)
Summary
The genus Equus is a relatively monotypic large mammal genus, with modest eco-morphological variation between species, and it provides an excellent case for exploring body size variation between palaeopopulations. The benefit of this approach is that it provides generalized information of body size variation, which is not obscured by evolutionary lineages with high morphological disparity, in response to diet, habitats and the presence of competing, closely related species. Several methods for estimating the body mass of living and extinct species and populations of mammals have been developed, with those based on regressions between body mass and bone/tooth measurements in living taxa being the most practical for application to fossil species
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