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Comments on the origin and generic status of Equus

1942; Cambridge University Press; Volume: 16; Issue: 5 Linguagem: Inglês

1937-2337

R. A. Stirton,

Wildlife Ecology and Conservation

Most American and many foreign paleontologists hold that Equus is the descendant of a line carrying back through Pliohippus to the protohippine species of Merychippus. On the other hand, some authorities contend that at least some of the species of Equus descended from Hipparion. A careful review of the characters which have been presented in argument of an Hipparion ancestry for Equus reveals that this point of view is not tenable. Evidence from different parts of the skeletons of late Tertiary horses conclusively demonstrates the descent of Equus through Parahippus, Merychippus and Pliohippus. Characters in the teeth are discussed in detail. The category represented by the North American upper Pliocene horses is not sufficiently different from Equus to warrant a recognition of the genus Plesippus. The characters listed for this genus are considered here as perhaps slightly more primitive than in Recent and Pleistocene Equus but certainly not worthy of the systematic rank ascribed to other Tertiary genera of horses. THOUGH THE phyletic lines of the horses are better known than in any other family of Tertiary mammals, there are still a few differences in opinion on the lines of descent and genetic relationships of the genera. Some of these pertain to the Pliocene line of descent and the generic status of the genus Equus. In reference to the immediate ancestry of the genus, two main points of view have been presented. Most American and many foreign paleontologists hold that Equus is the descendant of a line carrying back through Pliohippus to the protohippine species of Merychippus. On the other hand some authorities contend that at least some of the species of Equus descended from Hipparion. It is intended here to review the problem in the light of our more recent information of the groups bearing on these supposed relationships. IS HIPPARION THE ANCESTOR OF EQUUS? Kovalevsky (1873) was the first to include Hipparion in the direct ancestry of Equus. Then in 1888 Pavlov (1888A) considered Hipparion as representing a side branch and placed Protohippus in the line of descent to the living horse. The evidence for this interpretation, though slightly modified, was increased tremendously by American paleontologists during the early part of the twentieth century-so much so that most vertebrate paleontologists have accepted this point of view. In the meantime, however, Schlosser (19031; 1907) again supported the position of Hipparion in the Equus line of descent. This was followed by Abel (1909; 1926; 1928) and Antonius (1919A). Some facts bearing on the characters used by these European authors should be reviewed anew. Most of the reasons for including Hipparion as the ancestor of at least some species of Equus were mentioned by Schlosser (19031; 1907). At that time he relied on Leidy's (1869A) report and miscellaneous small papers by Cope for his information on North American fossil horses. Obviously the critical annectent forms in the New World had not been found. Schlosser, however, recognized a distinction between the American hipparions (Neohipparion) and the European forms. Hipparion, he thought, was of American origin, and Anchitherium he considered as not directly ancestral to Hipparion. Schlosser admits that forms less specialized in the dentition, like Equus, are seldom derived from animals like Hipparion with more complicated teeth, but he thinks that the Hipparion-Equus descent is a probable exception. The following reasons are taken from his (19031) paper: (a) Equus stenonis Cocchi is morphologically intermediate between Hipparion and Equus, s.s., as seen in its complicated fossette borders and less compressed protocone.