DEAD CLADE WALKING: SMALL ARCHAEOCYATHAN REEF MOUNDS IN THE LOWER CAMBRIAN (UPPER STAGE 4) MULE SPRING LIMESTONE, SPLIT MOUNTAIN, NEVADA

Archaeocyath sponges, the first animal reef builders of the Phanerozoic, suffered catastrophic and perhaps total extinction during Cambrian Stage 4. Two later occurrences in the Guzhangian and Paibian of Antarctica arguably represent Elvis taxa—sponges that independently evolved calcified skeletons. However, the exact timing of the archaeocyath extinction varies regionally. On the Siberian paleocontinent, this extinction occurred at the regional Toyonian-Amgan stage boundary (mid-Stage 4). That event more-or-less coincides with the demise of archaeocyath reefs within the regional mid-Dyeran Stage of Laurentia. Here, we report stratigraphically younger archaeocyaths from small reef mounds in the upper unit of the Mule Spring Limestone near Split Mountain, Nevada (upper Dyeran; upper Stage 4). Thin section analysis revealed the presence of archaeocyaths and associated microbial fabrics, like stromatolites, along with other reef-dwelling organisms. Archaeocyathus is the only archaeocyath genus present; individuals assigned to that taxon were found in all six of the sampled reef mounds. Trilobite and echinoderm debris was sparse within the reef mounds but was abundant in beds below and between the mounds. These buildups represent the last gasp of archaeocyathan reef building in the early Cambrian of the Laurentian Cordillera, and they were unlike those of the underlying Harkless Formation, which are characterized by higher biodiversity and abundance of archaeocyaths and reef-dwellers. Thus, this occurrence of Archaeocyathus is interpreted as an example of a dead clade walking—the last true archaeocyaths that locally persisted, albeit in a reduced ecological role, into upper Stage 4 after the decimation of this sponge group during mid-Stage 4. As the youngest-known archaeocyathan reefs of Laurentia, these provide a critical opportunity to better understand the environmental and ecological conditions surrounding animal reefs in the early Cambrian.