Calippus and Protohippus (Mammalia, Perissodactyla, Equidae) from the Miocene (Barstovian-early Hemphillian of the Gulf Coastal Plain

Abstract

Nine species of Calippus and four species of Protohippus are recognized from the late Miocene (late Barstovian to early Hemphillian, from about 14.0 ma to 6.0 ma) of the Gulf Coastal Plain. Two subgenera of Calippus are recognized: C. (Calippus) which includes four small species, C. proplacidus n. comb. (senior synonym of C. francisi), C. placidus, C. regulus and C. elachistus n. sp.; and C. (Grammohippus) n. subgen. which contains five medium-sized species: C. circulus n. comb., C. martini, C. cerasinus n. sp., C. hondurensis n. comb., and C. maccartyi n. sp. Protohippus is a valid genus, not synonymous with nor related to Merychippus sensu stricto, and not ancestral to Pliohippus, Dinohippus, or Eguus. Four successive species of Protohippus are recognized from the Gulf Coastal Plain, P. vetus, P. perditus, P. supremus (senior synonym of P. simus), and P. gidleyi n. sp. Eight of these 13 species also ranged north into the Great Plains, and are useful in faunal and chronological correlations between the two regions. Phylogenetic analyses of cranial and dental characters suggest the following hypotheses: (1) Protohippus and Calippus are more closely related to each other than either is to any other equid genus, and the two form a monophyletic group, the Subtribe Protohippina Quinn new rank; (2) Protohippus and Calippus form the sister group to a second monophyletic radiation of hypsodont equids, the equines; and (3) the common ancestor shared by protohippines and equines was a relatively small, late Hemingfordian-early Barstovian taxon, dentally of primitive merychippine-grade. In these clades, dental characters are more subject to parallelism than cranial characters, thus the latter are often more useful for deducing phylogenetic relationships.