Are Petaluma Horse Teeth Reliable in Correlation?
When the Petaluma horse, Neohipparion gidleyi Merriam, was described in 1915 knowledge of the sequence of equine species had not progressed far enough to permit age or stage refinement in correlation. Three genera are now recognized among the hipparion-like horses that were supposedly indicative of lower Pliocene age in 1915. Furthermore as certain phyletic lines are recognized in each of these genera, it is possible to correlate early or late divisions of the provincial ages now used by vertebrate paleontologists. There are two distinct species groups listed under the genus Neohipparion. The occidentale group is of Clarendonian age (early Pliocene) with the possibility of rare representation in the earliest Hemphillian (middle Pliocene). The eurystyle group is composed of Hemphillian species. N. gidleyi is clearly referable to the second group and its characters show that it is one of the most advanced species in the group. Recent revaluations of the plants (Axelrod, 1944) and of the
