SMALL SHELLY FOSSIL-STYLE PRESERVATION FROM THE LOWER TRIASSIC VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION, LOST CABIN SPRINGS LOCALITY, WESTERN UNITED STATES

A new assemblage of fossils exhibiting small shelly-fossil style preservation—minute fossils replaced by apatite, glauconite and other minerals was recently described in samples from the Lower Triassic Virgin Limestone Formation in nearshore settings of the western United States. The occurrence of small shelly-style preservation in the insoluble residues of Virgin Limestone samples in the >400 µm, >250 µm and >177 µm size fractions indicate that this taphonomic mode was present in rocks deposited in the aftermath of the largest mass extinction in Earth history. Here, we describe a more diverse assemblage extracted from eight samples of fossiliferous packstone from the more offshore Lost Cabin Springs locality of the Virgin Limestone. These insoluble residues contained brachiopods, echinoid spines, ophiuroid elements, crinoid ossicles and rare bivalves and ostracods. Energy-dispersive X-ray spectra (EDS) of individual fossils confirms size selectivity of phosphatization, as the smaller fossils (<300 µm) are preserved by apatite, whereas the larger fossils (>300 µm) are replaced or molded by silica and dolomite. We propose a phosphatization mechanism in which the small particle size of the skeletons or skeletal elements coupled with an extremely warm marine setting created porewater environments that experienced periodic oscillations in oxygen levels, contributing to the phosphatization of minute shells. The warm, low-oxygen seas of the Early Triassic shallow marine realm were an ideal environment in which the formation of small shelly-style fossils could occur.