PRESERVATION AND DISTRIBUTION OF OPHIUROIDS IN THE LOWER TRIASSIC VIRGIN LIMESTONE MEMBER, MOENKOPI FORMATION, SOUTHERN NEVADA

The End-Permian Mass Extinction (~251 Ma) was the largest extinction in the history of life. Global warming and high levels of atmospheric carbon dioxide increased the ocean’s temperature and led to surface acidification. After this boundary-defining extinction, Early Triassic ecosystems were characterized by small, opportunistic taxa, and recent work suggests that some of these organisms were preserved in unusual ways – as fossils molded by apatite, glauconite, and other iron-bearing minerals. This work examines the taxonomy and taphonomy of ophiuroid fossils preserved through some of these pathways from the Virgin Limestone Member of the Moenkopi Formation. We collected samples from three localities of the Virgin Limestone in Southern Nevada across a depth gradient from Lost Cabin Springs at the deepest, Blue Diamond at an intermediate depth, and Muddy Mountains at the shallowest. From each of these localities we have collected fragments that are likely lateral arm plates of the ophiuroid Ophiosuperstes praeparvus. The specimens from each of these locations are preserved in three ways —phosphatization, silicification, and with iron-rich minerals— and this preservation varies between localities. At Lost Cabin Springs, all the specimens of ophiuroids have been phosphatized, while at Muddy Mountains, all the fossils collected to date have been silicified. At the Blue Diamond locality, we have found phosphatized, and silicified fossils, and some molded by iron oxides. This variation in preservation of ophiuroid fossils likely points to small-scale environmental differences as a control on taphonomy. Additionally, the presence of Ophiosuperstes praeparvus across environments of the Virgin Limestone suggests it was especially adapted to the persistent stressful conditions of the Early Triassic.