THE EVOLUTION OF SOME MORPHOLOGICAL CHARACTERS OF THE UPPER CHEEK TEETH OF THE FOSSIL HORSES

The connection of the metaloph to the ectoloph and the development of and connection of the crochet to the protoloph are phylogenetically important characters in the upper cheek teeth of the Equidae (Fig. 1). They show a gradual increase in frequency from absence in early forms to complete or almost complete development in some later taxa, and fluctuating frequencies in groups of horses of an intermediate evolutionary stage. The spread of the crochet and the connected metaloph, and the probable genetic mechanisms of their development were briefly discussed by Simpson (1953, p. 105-106); more recent data reveal the evolution of these morphological characters, which are commonly used in the classification of fossil Equidae (Fig. 2). The metaloph is not connected to the ectoloph in genera of the sub-family Hyracotheriinae (Hyracotherium, Orohippus, Epihippus, and Haplohippus), nor are there indications of a crochet. In early representatives of the Anchitheriinae the metaloph is still disconnected, but there are specimens with a crochet, although the crochet does not connect to the protoloph. This is the situation in Chadronian and Brulean Mesohippus bairdi; in Chadronian M. texanus a crochet is absent. The first appearance of the crochet in the Equidae took place slightly earlier than the connection of the metaloph. In rare specimens of Brulean Miohippus antiquus the metaloph may connect to the ectoloph (Table 1), but the frequency of the crochet is as in M. bairdi or only slightly higher (Table 2). In Anchitherium the frequency of the crochet probably remained as it was in Miohippus antiquus, fluctuating around a mean of some 10% in P3-M2, but the frequency of the connected metaloph increased rapidly and reached 100% in P3M2 and M3 in the Early Hemingfordian or in the Late Arikareean; only in P2 did the metaloph occasionally remain disconnected. Whereas in Anchitherium the frequency of the connected metaloph reached 100% in P3-M2 and M3, in contemporary samples of Parahippus teeth with a disconnected metaloph were still quite common (Table 1). White (1942) described some small teeth from Thomas Farm as Miohippus sp. on the basis of the disconnected metaloph, but Bader (1956) synonymized Miohippus sp. with Archaeohippus blackbergi, although he commented on this character as atypical of the genus. In A. blackbergi a disconnected metaloph is slightly less common than in some forms of Parahippus (Table 1). Even in the Equinae a disconnected metaloph does occur; it is common in P2 especially in early wear and also occurs, although very infrequently, in the other cheek teeth. In Pleistocene Equus I have seen an upper molar with a disconnected metaloph. Thus the connected metaloph distinguished contemporaneous groups of horses; it was nearly universal in Anchitherium long before becoming universal in the parahippine ancestors of the more advanced equines. In Hypoiippus-Megahippus the frequency of the crochet increased somewhat over that of Anchitherium (Table 2), probably as the result of the crochet becoming segregated in this line of equid evolution, although derivation from an ancestral form with higher frequencies of the crochet than Anchitherium can not be excluded. In my data the frequency of the crochet in the primitive anchitheriines