As stated by Quinn (1957) ‘although the type of E. semiplicatus is a molar tooth and the type of E. calobatus is a metatarsal … they belong to closely related forms’. Unlike Quinn I think, however, that the metapodial proportions are different enough to not put the two species in synonymy. They seem to be both represented at Hay Springs and Rock Creek.
The cranium UNSM 1349 (locality SH 2) and the cranium bearing the number "13" (unknown locality) at the AMNH very probably belong to these ‘closely related’ species. Apart from the choanal length (at all times difficult to measure and especially on the badly preserved crania) the cranium UNSM 1349 of Hay Springs seems similar to the species from Channing and is referred here to E. semiplicatus. The cranium AMNH 8600 from San Diego may also belong to E. semiplicatus. The larger Hay Springs cranium AMNH ‘13’ is referred here to E. calobatus (see ratio diagrams).
(Azzaroli referred UNSM 1349 to E. fraternus. When compared to E. "fraternus" of Cedar Meadow, however, the size of UNSM 1349 is small and the muzzle is shorter and wider. It is not as rounded as in the Cedar Meadow specimen and I do not think it is just because the latter is an old individual. I do not think either that the Hay Springs specimens belong to the species of Cedar Meadow Spring).
By size and enamel pattern the upper cheek teeth of some specimens from Rock Creek, San Diego, and Hay Springs appear similar. The series from Channing are smaller.
Ratio diagrams of metapodials show the differences between E. calobatus and E. semiplicatus and the fact that they both are present at Hay Springs.
Azzaroli, A. 1995. - A synopsis of the Quaternary species of Equus in North America. Bolletino della Societa Paleontologica Italiana, 34(2), 205-221.
Azzaroli A., 1998. The Genus Equus in North America. The Pleistocene species. Palaeontographia Italica, 85 : 1-60, 21 pl.
Eisenmann V., Howe J., Pichardo M. 2008 (paru 2011). Old World Hemiones and New World slender species (Mammalia, Equidae). Palaeovertebrata 36(1-4): 159-233, 73 fig., 4 append. fig., 8 tabl.160.Eisenmann V., Howe J., Pichardo M. 2008 (paru 2011). Old World Hemiones and New World slender species (Mammalia, Equidae). Palaeovertebrata 36(1-4) : 159-233, 73 fig., 4 append. fig., 8 tabl.
Quinn J. H., 1957. Pleistocene Equidae of Texas. Bureau of Economic Geology, University of Texas, Report of Investigations 33 : 51pp.