American Hyracotherium (Perissodactyla, Equidae). Bulletin of the AMNH ; v. 110, article 1

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New York : [American Museum of Natural History]
"The teeth of Hyracotherium are in most respects quite primitive. They do, however, possess well-developed lophs and lophids, and in one species there is a definite tendency towards the molarization of P[superscript 3]. Although the teeth of Hyracotherium are in most respects more primitive than those of the phenacodontids, the possession of a mesostyle excludes all phenacodontid genera except Desmatoclaenus from the ancestry of Hyracotherium. The skull of Hyracotherium is not strikingly similar to the skulls of later horses. The orbit is relatively more posteriorly located than it is in any of the condylarths, and in the arrangement and relationship of the skull bones Hyracotherium closely resembles Mesohippus. The lumbar neural spines are almost vertical, indicating a relative unimportance of the longissimus muscle. Some of the lumbar zygopophyses were apparently embracing. In these characters Hyracotherium resembles the later horses. It is probable that the lumbar series was relatively inflexible. The scapula and fore limb are remarkably primitive, while the pelvis and hind limb exhibit most of the fundamental equid specializations. The feet, although strongly mesodactyl, were digitigrade. In view of the structure of the vertebral column, pelvis, and hind limb it is probable that Hyracotherium ran more or less in the typical ungulate manner, gaining most the propulsive force from the action of the hind limb relative to the pelvis rather than from an alternate flexing and straightening of the back. All the Lower Eocene perissodactyls are very similar in general structure, and it is only in the structure of the teeth and the astragalus and calcaneum that Hyracotherium is more horse-like than its closely related contemporaries. A study of large samples of teeth has shown that most of the specific characters cited by earlier authors show great variation within single populations. On the basis of the structure of P[superscript 3] and P[superscript 4] three species have been recognized, one from the Gray Bull and equivalents, and two from the Lost Cabin and Lysite and equivalents. Subspecies have been defined on the basis of significant mean tooth size differences. It is possible that two subspecies found in the Almagre member of the San Jose formation, H. a. etsagicum and H. a. angustidens, which were partially isolated but apparently in competition, gave rise to the two late Wasatchian species H. vasacciense and H. craspedotum. Hyracotherium v. venticolum from the Lost Cabin member of the Wind River formation is clearly the most advanced form of the genus and is undoubtedly ancestral to the genus Orohippus"--P. 58.
60 p., 7 p. of plates : ill. ; 28 cm.
Thesis (Ph. D.)--Columbia University.
Includes bibliographical references (p. 58-60).