NEW RECORDS OF IRVINGTONIAN TAPIRUS FROM THE AMERICAN SOUTHWEST

Middle Pleistocene vertebrate faunas from southwestern North America infrequently include fossil remains of the tapir, Tapirus. Fossils of this genus from Irvingtonian assemblages in northwestern Sonora, Mexico and inland southern California, US, can be separated into two size classes, large and small. Large tapir fossils from Sonora fall within the size range of Tapirus haysii Leidy, but are otherwise nondiagnostic. Small tapir fossils from California and Sonora fall in or near the size range of Tapirus californicus (Merriam) and Tapirus veroensis Sellards, based upon tooth measurements. Fossils from both regions also exhibit a dental morphology resembling the holotype of T. californicus, an isolated left m1 from California.

Newly recovered fossils of small Irvingtonian tapirs from Sonora include craniodental remains of multiple individuals. Based upon this material, small tapirs from Sonora can be distinguished from Tapirus veroensis based upon a number of cranial characters including: 1) the development of the posterodorsal process of the maxilla; 2) the width of the maxillary bar between the infraorbital foramen and the lacrimal; and 3) the morphology of the lacrimal.

The new tapir fossils from Sonora can also be distinguished from small Irvingtonian tapirs from inland southern California on the basis of: 1) the presence of strong anterolabial cingula on the upper premolars; 2) the morphology of the meatal diverticulum; 3) the orientation of the lacrimal foramina; and 4) the degree of development of the anterior lacrimal process. Whether these observed morphologic differences are sufficient to delineate two species, or instead represent variability within a single species, remains to be determined. If two small tapir species are present, the taxonomic validity of Tapirus californicus requires revision.