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Variation, Sexual Dimorphism, and Social Structure in the Early Eocene Horse Hyracotherium (Mammalia, Perissodactyla)
Philip D. Gingerich
Vol. 7, No. 4 (Autumn, 1981), pp. 443-455
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/2400696
Page Count: 13
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Patterns of dental and cranial variation are analyzed in a unique sample of early Eocene equid Hyracotherium from one fossil quarry of late Wasatchian age in the Huerfano Basin of southern Colorado. The sample includes remains of two species differing principally in size: H. vasacciense and H. tapirinum. The larger species, H. tapirinum, is represented by 24 individual specimens that show marked bimodality in cranial size and robustness and in upper and lower canine size. These differences are attributed to sexual dimorphism. Males tend to be about 15% larger than females in cranial dimensions and 40% larger than females in canine dimensions suggesting, by comparison with modern ungulates, that early Eocene horses were probably polygynous. The appearance of Hyracotherium in North America and its rise to dominance in early Eocene mammalian communities coincides with development of an environmental mosaic including open park woodland and savanna habitats. The evolutionary success of Hyracotherium may reflect, in part, the adaptive superiority of a social structure in which females in open habitats are grouped for mutual protection from predators, for optimal foraging in patchy environments, or both.
Paleobiology © 1981 Paleontological Society