Fossil horses from “Eohippus” (Hyracotherium) to Equus: scaling, Cope's Law, and the evolution of body size

Abstract

The evolution of body size in fossil horses is frequently depicted as a gradual, progressive trend toward increased body size (Cope's Law). Body size (actually body mass) was estimated for 40 species of fossil horses using dental and skeletal characters and regression equations derived from the same characters in extant species of Equus with known body mass. After body sizes were estimated, rates of morphological evolution, in darwins (d), were calculated between known ancestral and descendant fossil horse species. For the first half of horse evolution (from ca. 57 to 25 ma) body mass remained relatively static between about 25 and 50 kg with very slow evolutionary rates of 0.003–0.04 d. During the early–middle Miocene (from ca. 25 to 10 ma) there was a major diversification of body mass to about 75–400 kg and consistently higher evolutionary rates between 0.04 and 0.24 d. Since the late Miocene, body mass has generally increased with a maximum seen (in natural populations) in Equus scotti (ca. 500 kg) during the middle Pleistocene. Therefore, for horses, the traditional interpretation of gradual increase in body size through time is oversimplified because: (1) although the exception to the rule, 5 of 24 species lineages studied are characterized by dwarfism; and (2) the general trend seems to have been a long period (32 ma) of relative stasis followed by 25 ma of diversification and progressive (although not necessarily gradual) change in body size.