SEM IMAGING STUDY OF RARE ARTHROPODS FROM THE BARSTOW FORMATION, CALICO MTS., CALIFORNIA

The middle member of the Barstow Formation (Miocene) in the Calico Mountains of San Bernardino County, California contains silicified arthropods preserved inside calcareous concretions, deposited in a lacustrine setting within an extensional basin. Formic acid dissolution of concretions yields specimens preserving intricate anatomic detail at the micron scale. Scanning electron microscopy allows us to describe previously unseen detail on new specimens of the predaceous diving beetle Calicovatellus petrodytes and the spider Argenna fossilis. Exceptional preservation yields images of characters not previously described. We are able to document taxonomically and functionally important features such as tarsal claws, lyriform organs and trichobothria on a spider referred to Argenna fossilis. The sternite morphology of a specimen referred to C. petrodytes conflicts with its earlier description, suggesting that this species is either more plastic than previously thought, or that the differences are significant enough to recognize more than one species of Calicovatellus.