Cormohipparion sp.

Woodburne, Michael O., 2007, Phyletic Diversification Of The Cormohipparion Occidentale Complex (Mammalia; Perissodactyla, Equidae), Late Miocene, North America, And The Origin Of The Old World Hippotherium Datum, Bulletin of the American Museum of Natural History 2007 (306), pp. 1-138 : 86-89

Cormohipparion sp.

Two samples are referred to Cormohipparion sp. , unallocated. The first is LACM 150080, from Punchbowl Formation, Devil’s Punchbowl, Valyermo, California. The second is derived from the Dove Spring Formation, El Paso Basin, California.

VALYERMO: The locality (V, fig. 1) and stratigraphic setting for LACM 150080 are presented in Woodburne (2005). As discussed there, the complexity of the fossette borders and cheek tooth plis caballin, the subovate outline of the protocone (rather than very elongate; fig. 25) and its likely moderate unworn MSTHT (ca. 50–55 mm) are prophetic of the morphology displayed in upper cheek teeth of Hippotherium primigenium . LACM 150080 seems to be morphologically the closest to the Old World taxon among North American samples. Note that the preservation of the specimen apparently masks the complexity of the plis caballin in P4–M2. The connection of the protocone to the protoloph in P2 at about 50% wear may be a relatively plesiomorphic retention, but, as discussed below, it also occurs in other species of Cormohipparion as well as in Old World material referred to Hippotherium sp. and H. primigenium . In the current report, the age of LACM 150080 is suggested as medial Clarendonian, although an early Clarendonian age cannot be ruled out.

As indicated in tables 13 and 21 and in figure 25, the complexity of the fossette borders (especially at the posterior border of the prefossette and the anterior border of the postfossette in P3 and M1) rivals or exceeds that of C. occidentale as well as samples of Hippotherium primigenium . At the same time, the ovate protocones and their width-to-length ratios are generally comparable to those seen in samples of H. primigenium . The confluence of the pre- and postfossettes in P2 is seen also in those taxa, as well as in other species of Cormohipparion , apparently most abundantly in C. fricki .

EL PASO BASIN: Whistler and Burbank (1992) described the litho-, magneto-, and biostratigraphy and geochronology of the fossiliferous succession of the Dove Spring Formation of the El Paso Basin, California (E, fig. 1). Whistler and Burbank (1992: fig. 4) recorded C. occidentale as occurring from the base of the Cupidinimus avawatzensis / Paracosoryx furlongi Assemblage Zone to just within the base of the Paronychomys / Osteoborus diabloensis Assemblage Zone. Based on the specimens discussed below, this range is modified to about 11.8–10.3 Ma, as shown in figure 2. All of the specimens are isolated teeth (or, in one case, in a lower mandible) and have no associated cranial material. Thus, for the purposes of this report, the specimens are allocated as Cormohipparion sp. but, as also discussed below, it appears unlikely that they would pertain either to C. occidentale as defined herein or to the Valyermo taxon.

As summarized in table 19, these teeth overall are of larger size, greater dental complexity, and, likely, taller unworn MSTHT than Hipparion forcei and Hipparion tehonense , as documented by MacFadden (1984), which also occur in the Dove Spring Formation ( Whistler and Burbank, 1992: fig. 4). The lower dentition, LACM 138755, shows protostylids present in p3–m3, also characteristic of Cormohipparion . Most show elongate protocone proportions, with only LACM 140854 being more nearly ovate than elongate. In general, crown height (in which the unworn MTSHT could have been on the order of 50–55 mm), protocone index, and fossette complexity in these specimens is similar to the XMas-Kat materials of C. matthewi , but further material from each site is needed before a more definite comparison can be made. In that the material spans an interval of nearly 1 m. y., it is possible that more than one taxon could be represented.

Cormohipparion quinni figure 26; table 20

here. F:AM 108535 confirms the presence of this species in the MacAdams Quarry sample, and the persistence of this species, as shown in figure 2, is contemporaneous with medial Clarendonian descendants of the speciation event responsible for the genesis of the C. occidentale group of taxa.