Study of the materials in the American Museum of Natural History that pertain to the taxon Cormohipparion occidentale Skinner and MacFadden (1977) reveals that at least six species are represented. This is based on data from the cranium, upper dentition, lower dentition, and mandible. The taxa embrace a period of time from about 12.5 Ma to 10 Ma. Major sites or faunas include the Dove Spring Formation (medial Clarendonian, California; Burge Local Fauna (early Clarendonian, Nebraska); Minnechaduza Fauna, Nebraska; MacAdams Quarry, Texas (early medial Clarendonian); Gidley Horse Quarry (late medial Clarendonian, Texas); XMas-Kat, Hans Johnson, and Machaerodus quarries (late medial Clarendonian, Nebraska); and Ash Hollow Formation (late medial Clarendonian, South Dakota). The name Cormohipparion occidentale is restricted to the larger of two species that occur contemporaneously in medial Clarendonian sites in Nebraska and, alone, at the Ed Ross Ranch Quarry, South Dakota. The second species in the XMas-Kat and related quarries is assigned to Cormohipparion matthewi, n.sp. A taxon from the Burge Local Fauna is assigned to Cormohipparion merriami, n.sp. Another taxon from the Burge Local Fauna is assigned to Cormohipparion johnsoni, n.sp. Materials from the Texas sites are allocated as Cormohipparion fricki, n.sp., and Cormohipparion skinneri, n.sp., from the MacAdams Quarry and Gidley Horse Quarry, respectively. Cormohipparion fricki, n.sp., also is represented in the Minnechaduza Fauna, Nebraska. Cormohipparion is restricted geographically to North America.
Based on this review, Cormohipparion johnsoni, n.sp., is the most plesiomorphic species of the C. occidentale group of taxa. Cormohipparion quinni is a plausible ancestor (sister-taxon) for the C. occidentale group, but also persisted with it until about 12 Ma. During their radiation, elements of the C. occidentale group demonstrate an increase in upper cheek tooth crown height and complexity of the enamel pattern, as well as an increase in overall cranial size, with each species showing its own mosaic of parameters. The interval of 12.5–10 m.y. witnessed the initiation of a period of climatic cooling and an eventual expansion of vegetation communities toward more open associations, in part showing an increase in grassy areas. Apparently, the C. occidentale group developed and maintained a mixed-feeding adaptation to these conditions, even though it had evolved very hypsodont cheek teeth by about 10 Ma (C. occidentale, s.s.; C. skinneri, n.sp.).
Cormohipparion johnsoni, n.sp., and C. merriami, n.sp., are followed by C. fricki, n.sp., at about 12–11.5 Ma, which demonstrates an increased crown height and complexity of the upper cheek teeth along with the persistence of a functional dP1 into the adult condition. At least in C. fricki, n.sp., and likely also C. merriami, n.sp., the pre- and postfossettes of P2 commonly were confluent. All of these features are to be found in early Pannonian C members of Hippotherium of the Old W