EXCEPTIONALLY PRESERVED SMALL CARBONACEOUS FOSSILS (SCFS) IN THE CLARKIA LAGERSTÄTTE (EARLY MIOCENE): DIVERSITY, ECOLOGY AND TAPHONOMIC PATHWAYS

The early/middle Miocene (ca. 15 Ma) Clarkia Lagerstätte is most famous for its fossil plants, including pigmented leaves and chloroplast ultrastructure. Low manipulation processing/picking of samples from the P-33 locality has also yielded exceptionally preserved SCFs, representing a diverse range of arthropods, fungi, and non-vascular plants. Branchiopod crustaceans are particularly well represented, not only as isolated carapaces but also fully articulated feeding apparatus and whole organisms. Aquatic midge larvae (chaoborids and chironomids), mayfly larvae and oribatid mites are also recovered as whole organisms, commonly exhibiting exquisite morphological detail. By contrast, the remains of non-aquatic arthropods (e.g., adult dipterans, lepidopterans) are conspicuously less common and more fragmentary. The principal difference appears to be the susceptibility of aquatic invertebrates to predation, alimentary repackaging, and rapid export out of the taphonomically active zone. Recurrent accumulations of cladoceran carapaces, chaoborid larvae and other remains are readily recognized as fecal associations, produced by both predatory insect larvae and zooplanktivorous fish. Such conventional biostratinomic processing, however, fails to explain the unprecedented quality of organic preservation in Clarkia fossils, which most likely derives from unique diagenetic circumstances related to volcanogenic sedimentation.