Revision of the Hyrachyidae. Bulletin of the AMNH ; v. 67, article 5

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New York : The American Museum of Natural History
"1. The Perissodactyla show a very early and very fundamental dichotomy into two suborders, for which the names Solidungula and Tridactyla could be appropriately revived. The family Hyrachyidae is a very primitive group of the Rhinocerotoidea, and still closely resembles the other Eocene families of the suborder Tridactyla. The family is known from the Eocene (Lost Cabin to Uinta, inclusive) of Wyoming, Utah, Colorado and Montana. It includes four genera, of which Hyrachyus is by far the best known. Hyrachyus has the greatest time range of any American Eocene ungulate genus, and has an unusually long range for a placental mammal. The taxonomy, stratigraphic distribution and probable phylogeny are summarized in figure 1. 2. The genus Hyrachyus apparently includes three contemporary main series (consisting of small, medium-sized and large forms, two of which series have side branches, of which a few are rather aberrant in one or more characters). Each series contains successive grades of organization which are best treated as species. It is significant that when known from sufficiently numerous specimens, the individual measurements of the representatives of each species form normal curves of distribution, with relatively slight, or no, overlapping with other contemporary forms, or with earlier or later forms. Certain of these forms are now sufficiently well documented to deserve rank as index fossils for their respective horizons. H. modestus is an index fossil for the Lower Bridger, H. eximius and H. princeps for the Upper Bridger, and the genus Hyrachyus is very characteristic of the Bridger in general, although not confined to it. 3. Paired horns appear independently in the second and third genera of the family, Colonoceras and Metahyrachyus. Colonoceras is close to H. affinis. Metahyrachyus is a rather aberrant form, particularly as to P[superscript]2, in which the peculiar 'amphiloph' parallels the similar development in P[superscript]3 of the horses. 4. The fourth genus, Ephyrachyus, not previously separated from Hyrachyus, is the most advanced member of the family, and shows progress toward a rhinocerine grade of organization. It could hardly be ancestral to any of the American or European true rhinoceroses, but might conceivably lead to Caenolophus of Mongolia, especially to C. obliquus. I doubt Matthew's view that Caenolophus might be ancestral to the true rhinoceroses, since it is rather too late in time, nor can I accept his alternative hypothesis that primitive amynodonts were the ancestors of the true rhinoceroses. On the contrary, this study confirms the traditional view that the Hyrachyidae are approximate, morphological ancestors of the other rhinocerotoids, although the actual common ancestor has probably not yet been discovered, and should be found in the early Lower Eocene, in view of the presence of a true rhinoceros, Prohyracodon, in the Auversian of Transylvania (Wood, 1929b). 5. The Hyrachyidae agree with other lines (Matthew, 1909) in suggesting a hiatus between the Lower and Upper Bridger. The following new stratigraphic names are proposed, to represent units which are already rather widely, although tacitly, accepted: Black's Fork, Twin Buttes, Wagonhound, and Myton. Emmons Peak is proposed as a substitute for 'Uinta' Quartzite, and Green Cove for 'so-called Bridger.' The Sage Creek Beds are apparently of either Lower Uinta or Upper Bridger age. Cook Ranch is proposed for new Middle Oligocene Beds in the Sage Creek region of Montana. 6. Study of the brain casts of the hyrachyids, as compared with more primitive and more progressive forms, illustrates the same tendencies seen in other lines. 7. It is possible to follow the evolution of the permanent premolars, deciduous premolars, and molars. In some of the lines, the metamorphosis of the upper premolars may be followed step by step, from one horizon and its species to the next. It is not possible to do this for the lower premolars and deciduous premolars, but general trends may be recognized. Although the molars are usually notable for their conservatism, some changes may be observed. Evolution of the upper and lower teeth usually moves pari passu, as would be predicted, thereby safeguarding the occlusal relations, or if aberrant changes do occur, they are of such a character as not to interfere with the occlusion. All phases of the evolution of the cheek teeth conform to mechanical principles, but there is no evidence favoring orthogenetic or neo-Lamarckian interpretations of this accordance. On the contrary, the data may best be explained on the strictly neo-Darwinian basis of natural selection from among random mutations"--P. 276-277.
p. 181-295, [7] leaves of plates (2 folded) : ill., maps ; 25 cm.
Includes bibliographical references (p. 290-295).