VERTEBRATE PALEONTOLOGY AND REVISED AGE OF THE OSO SAND MEMBER, CAPISTRANO FORMATION, ORANGE COUNTY

The Oso Sand Member is the highly fossiliferous, nearshore facies of the Capistrano Formation, which spans the southeastern rim of the Los Angeles Basin in Orange County, California. Over 20 vertebrate taxa have been identified from this unit, including well preserved fossils of marine taxa such as a nearly complete skull of a blue marlin and the most complete fossil walrus found to date. In addition to other marine mammals (whales and seacows), terrestrial mammals are also known from the Oso Sand Member including gomphotheriids, rhinocerotids, antelocaprids, canids, cricetids, and lagomorphs. Despite the abundance of material from Oso Sand Member sites, just three papers have reported on this unit: one paper focused on the skull of the blue marlin mentioned above, the other two mentioned mammal fossils in passing. We provide an overview of all known vertebrate fossils from Oso Sand Member, and establish a more refined age for the Oso Sand Member, which will help provide a temporal framework for ongoing paleontological studies. Based on stratigraphic correlation, the Capistrano Formation is reported as Upper Miocene to Lower Pliocene. Previous workers have referred to undescribed specimens to place the Oso Sand Member in the Hemphillian North American Land Mammal Age. Partial camelid teeth are identified as Alforjas, known from the late early to latest Hemphillian (Hh2 to Hh4). Horse teeth previously referred to Pliohippus (Barstovian to Hemphillian) are reidentified as Dinohippus interpolatus, which is characteristic of the early late Hemphillian (Hh3). Based on these identifications, we can constrain the age of the Oso Sand Member to the early late Hemphillian (Hh3). By better defining the age of the Oso Sand Member, we can place the marine and terrestrial vertebrate fossils from this unit into a more precise chronostratigraphic framework that allows us to make more detailed comparisons to other late Neogene faunas in California.

Grant Information

This research was funded by the CSU-Louis Stokes Alliance for Minority Participation program (NSF grant # HRD-1302873).