A POSSIBLE ORIGIN FOR RABBITS

Abstract

The relationship of lagomorphs (rabbits, hares, and pikas) to other mammals has been obscure ever since general acceptance of Gidley's (1912) conclusion that their few special similarities to rodents are not necessarily indicative of a close common ancestry. The most recent authoritative opinions are those of A. E. Wood (1957), who has tentatively derived the lagomorphs from the periptychid condylarths, and L. S. Russell (1959), who has derived them from the zalambdodont insectivores. The latter view is discussed and rejected by Van Valen (in press). I have not found clearcut evidence as to the actual ancestry of lagomorphs, but I do think that the Mongolian genus Pseudictops (late Paleocene, early Eocene, or both) provides the most probable indication of this ancestry. The earliest known lagomorph is Eurymylus from the Gashato Formation (approximately late Paleocene) of Mongolia, in which Pseudictops also occurs; the basic reference is Wood (1942). A second eurymylid, Mimolagus, was described by Bohlin (1952) from deposits of unknown but very probably later age (cf. also McKenna, 1963) of Kansu in northwest China. Comparison will be mainly with Eurymylus, although unworn upper teeth are not known in either eurymylid. Eurymylus is known from two moderately worn sets of upper cheek teeth with adjoining parts of the maxilla, and the anterior parts of three mandibles containing nearly or quite unworn P3 and P4, worn M1 and M2, and a broken incisor. Mimolagus is known from the muzzle with incisors, most of a set of worn cheek teeth with nearby maxilla and anterior zygoma, and several probably associated limb bones. Pseudictops is known from nearly unworn P3-M3, the mandible below most of this region and slightly anterior and posterior to it, and two maxillary fragments with moderately worn P4 (possibly DP4), M1, and M2, and the roots of M3. A palate with teeth or alveoli posterior from J?2 was poorly figured by Trofimov (1952). The descriptions and important discussion of Pseudictops are by Matthew, Granger, and Simpson (1929) and Trofimov (1952). Except for the specimens of Mimolagus, and those of Pseudictops described by Trofimov, all the known specimens (or important samples) of the genera discussed in the present paper are in the American Museum of Natural History (A.M.N.H.) and have been compared directly with each other. The existence of two species of Pseudictops is questionable. They are the same size, and I can see (from Trofimov's figures of P. ariloplio,don) no difference in morphology. The three characters given in Trofimov's diagnoses do not seem to me to distinguish the two species (although they do in part distinguish Trofimov's figures of P. arilophiodon from the slightly inaccurate figures of P. lophiodon), but direct comparison of specimens or casts will probably be necessary for a final resolution of this question.