Chapter 16

Abstract The morphology and dimensional parameters of the cranium and upper and lower cheek tooth dentition of the early Barstovian aged three-toed horse, Merychippus insignis, the genotypic species, are analyzed based on specimens from Echo Quarry in the Olcott Formation of western Nebraska. Specimens of early Barstovian age from the Trinity River Pit 1 quarry, Texas, and of late Barstovian age from Deep Creek, Nebraska, are utilized to corroborate the association of upper and lower dentitions of the Echo Quarry sample to Merychippus insignis. A revised definition of Cormohipparion is utilized. Cormohipparion goorisi is characterized on the basis of cranial and dental information based on material from Trinity River Pit 1, Texas (early Barstovian). Based on these species, Merychippus and Cormohipparion are distinctly different taxa.