PRESERVATION OF FOSSIL ARTHROPODS IN THE MIDDLE MIOCENE BARSTOW FORMATION SOUTHERN CALIFORNIA

Concretions from the middle Miocene (13.4Ma) Barstow Formation of southern California are widely recognized for the exceptionally preserved three-dimensional fossil arthropods found inside. In this study, we examine 236 concretions from the Barstow Formation to gain a greater understanding of concretion formation and fossil preservation. More specifically we ask, are organic remains necessary for concretion formation? Is there a correlation between the size of arthropod remains and the size of the original concretion? Concretions were measured along three axes: length, width, and height using digital calipers. The concretions were then dissolved using two separate methods: 146 were dissolved in 10% acetic acid solution, and 90 were dissolved in 10% formic acid solution. The resultant contents were then examined under a compound microscope. The contents were separated into two broad categories, those containing organic remains and those that contained non-organic remains. Concretions that had organic remains were then further categorized as those containing arthropods and those without arthropods. If a concretion contained an arthropod, that arthropod was photographed using a Leica microscope camera and its diameter and length were measured using Image J. The presence of organic material does not seem to be necessary for concretion formation as we found that 71% of the dissolved concretions had no organic material inside. The presence and absence of arthropod remains did not correlate with the size of concretions (F_{1, 233} = 0.0107, p = 0.9179) and we found no correlation between concretion size and size of the arthropod remains inside (R² = 0.000012, n = 62, p = 0.9786). Therefore, one cannot assume apriori that concretions, even if large in size, will yield fossils. Nor can one assume that large concretions contain large fossils or vice a versa. Therefore, all concretions collected, regardless of size, will need to be dissolved.