Access

You are not currently logged in.

Access your personal account or get JSTOR access through your library or other institution:

login

Log in to your personal account or through your institution.

If You Use a Screen Reader

This content is available through Read Online (Free) program, which relies on page scans. Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.

Variation, Sexual Dimorphism, and Social Structure in the Early Eocene Horse Hyracotherium (Mammalia, Perissodactyla)

Philip D. Gingerich
Paleobiology
Vol. 7, No. 4 (Autumn, 1981), pp. 443-455
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/2400696
Page Count: 13
  • Read Online (Free)
  • Download ($12.00)
  • Subscribe ($19.50)
  • Cite this Item
Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Preview not available

Abstract

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.

Page Thumbnails

  • Thumbnail: Page 
[443]
    [443]
  • Thumbnail: Page 
444
    444
  • Thumbnail: Page 
445
    445
  • Thumbnail: Page 
446
    446
  • Thumbnail: Page 
447
    447
  • Thumbnail: Page 
448
    448
  • Thumbnail: Page 
449
    449
  • Thumbnail: Page 
450
    450
  • Thumbnail: Page 
451
    451
  • Thumbnail: Page 
452
    452
  • Thumbnail: Page 
453
    453
  • Thumbnail: Page 
454
    454
  • Thumbnail: Page 
455
    455