The age of the Oso Member, Capistrano Formation, and a review of fossil crocodylians from California

Abstract

Fossils from the late Miocene Oso Member of the Capistrano Formation in Orange County, California, are underreported in the scientific literature. Mitigation activities in the Oso Member have resulted in collections of fossils in museums. We provide a preliminary list of identified faunal elements from the Oso Member and determine its age using biostratigraphy. The presence of the fossil horse Dinohippus interpolatus allows us to constrain the age of the Oso Member to the early late Hemphillian (Hh3, 6.6–5.8 Ma). We provide a review of other Hemphillian terrestrial vertebrate sites from Southern California. Our age assessment for the Oso Mem- ber allowed us to recognize a significant temporal range extension for crocodylians on the Pacific Coast of the United States. Including the Oso Member, the fossil record of crocodylians in California is based on fragmentary material from 13 formations ranging from the Paleocene to the late Miocene (including seven Eocene units). Of these records, the Oso Member fossils represent the youngest record of fossil crocodylians in California. Fossil crocodylians from Orange County extend the record of crocodylians in California ~10 million years from the middle Miocene to the late Miocene.