LACUSTRINE OSTRACODS AND OTHER MICROCRUSTACEANS WITH PRESERVED APPENDAGES FROM THE MIOCENE BARSTOW FORMATION

Ostracod fossils are ubiquitous, however three-dimensional fossil ostracod soft-tissues (non-biomineralized tissues) are rarely reported. This is the first report of three-dimensional ostracod appendages from the Neogene, the first report of abundant articulated ostracod appendages from North America and the first report of abundant three-dimensionally preserved ostracod appendages from a lacustrine paleoenvironment. The ostracod collection contains three male and 25 female specimens. This sex ratio may indicate parthenogenesis as well as sexual reproduction in these Neogene freshwater podocopid ostracods. The soft-tissue morphology is virtually identical to some Recent and Cretaceous Cipriodontid ostracods. The fossils were recovered from carbonate concretions associated with lacustrine sediments of the Miocene Barstow Formation of southern California. A single carbonate concretion from the study site yielded eight silicified ostracods (with preserved appendages) and over 500 associated microcrustaceans (copepods, fairy shrimp and cladocerans). These silicified (non-biomineralized) tissues were found 17 km north of Barstow, California near the type section of the Barstow Formation in the Mud Hills. Air fall tuff beds that bracket the fossil site have been dated at 15.8 Ma (Oreodont Tuff) and 16.3 Ma (Red Tuff) by other authors. This is a remarkably diverse, silicified, three-dimensional zooplankton assemblage that includes abundant resting cysts and ephippa of several species. For example, two cladoceran (Branchiopoda: Anomopoda) species and three cladoceran ephippial morphotypes have been identified. Preservation of delicate zooplankton non-biomineralized tissues coupled with the complete lack of fish fossils suggests that the microcrustaceans lived in small vernal pools or in a large saline lake similar to Mono Lake, California.