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McHorse, Brianna

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McHorse

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Brianna

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McHorse, Brianna
Author's Publications: search.filters.isAuthorOfPublication.2a577df9-8d1c-4f51-bde0-7a239e4f070b

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    Mechanics of evolutionary digit reduction in fossil horses (Equidae)
    (The Royal Society, 2017) McHorse, Brianna; Biewener, Andrew; Pierce, Stephanie
    Digit reduction is a major trend that characterizes horse evolution, but its causes and consequences have rarely been quantitatively tested. Using beam analysis on fossilized centre metapodials, we tested how locomotor bone stresses changed with digit reduction and increasing body size across the horse lineage. Internal bone geometry was captured from 13 fossil horse genera that covered the breadth of the equid phylogeny and the spectrum of digit reduction and body sizes, from Hyracotherium to Equus. To account for the load-bearing role of side digits, a novel, continuous measure of digit reduction was also established—toe reduction index (TRI). Our results show that without accounting for side digits, three-toed horses as late as Parahippus would have experienced physiologically untenable bone stresses. Conversely, when side digits are modelled as load-bearing, species at the base of the horse radiation through Equus probably maintained a similar safety factor to fracture stress. We conclude that the centre metapodial compensated for evolutionary digit reduction and body mass increases by becoming more resistant to bending through substantial positive allometry in internal geometry. These results lend support to two historical hypotheses: that increasing body mass selected for a single, robust metapodial rather than several smaller ones; and that, as horse limbs became elongated, the cost of inertia from the side toes outweighed their utility for stabilization or load-bearing.
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    A new genus of horse from Pleistocene North America
    (eLife Sciences Publications, Ltd, 2017) Heintzman, Peter D; Zazula, Grant D; MacPhee, Ross DE; Scott, Eric; Cahill, James A; McHorse, Brianna; Kapp, Joshua D; Stiller, Mathias; Wooller, Matthew J; Orlando, Ludovic; Southon, John; Froese, Duane G; Shapiro, Beth
    The extinct ‘New World stilt-legged’, or NWSL, equids constitute a perplexing group of Pleistocene horses endemic to North America. Their slender distal limb bones resemble those of Asiatic asses, such as the Persian onager. Previous palaeogenetic studies, however, have suggested a closer relationship to caballine horses than to Asiatic asses. Here, we report complete mitochondrial and partial nuclear genomes from NWSL equids from across their geographic range. Although multiple NWSL equid species have been named, our palaeogenomic and morphometric analyses support the idea that there was only a single species of middle to late Pleistocene NWSL equid, and demonstrate that it falls outside of crown group Equus. We therefore propose a new genus, Haringtonhippus, for the sole species H. francisci. Our combined genomic and phenomic approach to resolving the systematics of extinct megafauna will allow for an improved understanding of the full extent of the terminal Pleistocene extinction event.