CLADOCERANS (BRANCHIOPODA: ANOMOPODA) FROM THE MIOCENE BARSTOW FORMATION: SAN BERNARDINO COUNTY, CALIFORNIA

This is the first report of three-dimensional cladoceran fossils from the Miocene Barstow Formation. Exceptional SEM images of silica preserved soft-tissue structure allows study of these fossils as if they were modern organisms (sub-micron detail). Both body fossils and ephippia occur in early diagenetic carbonate nodules associated with lacustrine sediments of the Middle Member of the Barstow Formation. The new fossils are found in Rainbow Basin and are confidently correlated with known biostratigraphic and chronostratigraphic markers. The cladoceran fossils occur stratigraphically between the Rak Tuff and the Oreodont Tuff.

The cladocerans are 0.7 mm to 1.0 mm in length. The carapace covers the thorax, abdomen and thoracic legs. The carapace margin is curved ventrally. The carapace surface exhibits a fine reticulate pattern. The first antennae are five times longer than they are wide and are attached to the head away from the carapace margin. The second antennae are biramous. The dorsal branch contains four segments and four swimming setae. The ventral branch contains three segments and five swimming setae. There is a row of feathered lateral setae on the post-abdomen and a distal bident tooth near the base of the post-abdominal claw. The ephippia are 0.5 mm in length. Two embryos are evident. These characteristics are consistent with placement of this cladoceran in the family Moinidae.

The occurrence of cladoceran “resting eggs” may indicate changing paleoenvironmental conditions such as temperature, pH, or even periodic lake desiccation. All specimens observed are fully articulated implying high sedimentation and/or high mineralization rates.