Abstract
Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record. Despite the abundant material available for study, there is still considerable disagreement over the number of species of Equus that inhabited the different regions of the continent and on their taxonomic nomenclature. In this study, we investigated cheek tooth morphology and ancient mtDNA of late Pleistocene Equus specimens from the Western Interior of North America, with the objective of clarifying the species that lived in this region prior to the end-Pleistocene extinction. Based on the morphological and molecular data analyzed, a caballine (Equus ferus) and a non-caballine (E. conversidens) species were identified from different localities across most of the Western Interior. A second non-caballine species (E. cedralensis) was recognized from southern localities based exclusively on the morphological analyses of the cheek teeth. Notably the separation into caballine and non-caballine species was observed in the Bayesian phylogenetic analysis of ancient mtDNA as well as in the geometric morphometric analyses of the upper and lower premolars. Teeth morphologically identified as E. conversidens that yielded ancient mtDNA fall within the New World stilt-legged clade recognized in previous studies and this is the name we apply to this group. Geographic variation in morphology in the caballine species is indicated by statistically different occlusal enamel patterns in the specimens from Bluefish Caves, Yukon Territory, relative to the specimens from the other geographic regions. Whether this represents ecomorphological variation and/or a certain degree of geographic and genetic isolation of these Arctic populations requires further study.
Highlights
Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record [1,2,3]
The caballine equid remains we studied from Alberta, Natural Trap Cave, the American Southwest, and northeastern Mexico have been identified under several names including Equus caballus caballus Linnaeus, 1758, E. caballus laurentius Hay 1913, E. excelsus Leidy, 1858, E. laurentius Hay, 1913, E. mexicanus (Hibbard), 1955, E. midlandensis Quinn, 1957, E. niobrarensis, and E. scotti Gidley, 1900 [4, 10, 22,23,24, 26, 27, 29,30,31,32]
The restriction of E. cedralensis to lower latitudes of the Western Interior may seem surprising compared to the distribution of E. ferus and E. conversidens; our study evaluated specimens from a paleontologically short time interval and it is possible that this species had a wider distribution at other times
Summary
Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record [1,2,3]. One of the first large-scale quantitative studies of the genus Equus in North America was undertaken by Winans [4, 10]. She conducted a multivariate analysis using linear measurements of cranial and metapodial remains. Her study sample consisted of equid specimens of Blancan to late Rancholabrean North American Land Mammal Ages (NALMA) largely from the Great Plains and the Western United Sates, and smaller samples from Florida, and Mexico. The other two species groups are E. simplicidens Cope, 1892 and E. scotti Gidley, 1900 [10]
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