BENTHIC FORAMINIFERAL ASSEMBLAGES AND TAPHONOMY FROM THE LOWER TRIASSIC VIRGIN LIMESTONE FORMATION, WESTERN UNITED STATES

The end-Permian mass extinction was the largest mass extinction in Earth’s history. Biotic recovery from this event was a slow process that, for some groups, was delayed for much of the Early Triassic because of environmental stress, including ocean acidification, anoxia, and hot surface ocean temperatures. Insights into biotic recovery are provided by examination of the Lower Triassic Virgin Limestone Member in southern Nevada. Here, we report new foraminifera assemblages, their abundance, and preservation style within the upper Virgin Limestone at two localities, Lost Cabin Springs and Blue Diamond. Insoluble residues from both Lost Cabin Springs and Blue Diamond produced internal molds of benthic foraminifera. Abundance of these internal molds is relatively low compared to other fossils in residue, but as many as 50 forams can be found in the residues where they are present. A variety of internal mold morphologies were found, including planispiral evolute chambers, uniserial chambers in U and V shapes, and uniserial oval and round shaped chambers. They are preserved by two distinct minerals, with the larger forams (~200µm to ~400µm) typically preserved by apatite and the smaller forams (~75µm to ~200µm) with iron oxides. All morphologies occur across these size ranges and across localities. Thin sections of these fossiliferous samples have not yielded any foraminifera to date, underscoring the low abundance of these organisms relative to other Virgin Limestone fossils. The discovery of various morphologies of foraminifera in insoluble residues from the Lower Triassic Virgin Limestone Member expands the range of these benthic organisms to outside the Tethyan realm. This work also demonstrates that insoluble residues of Lower Triassic limestone continue to yield new fossils, related to the unusual taphonomic window that was present in this setting that is likely related to broader climatic conditions.