THE DEAD CLADE IS STILL WALKING: ARCHAEOCYATHS FROM UPPER STAGE 4 STRATA OF THE WESTERN UNITED STATES

Archaeocyaths were the first animal reef builders of the Phanerozoic, appearing in the lower Cambrian mid-Stage 2. Archaeocyaths eventually suffered extinction during Cambrian Stage 4, but the exact timing of this extinction varies globally. Here, we report two new occurrences of some of the youngest archaeocyaths from the upper Stage 4 of Laurentia. These are within the upper unit of the Mule Spring Limestone at Split Mountain (Nevada) and the correlative Thimble Limestone Member of the Carrara Formation in Echo Canyon (California). These archaeocyaths occur stratigraphically above the last archaeocyath reefs of the western US in the Harkless Formation. The only species from both localities are modular archaeocyathides belonging to the genus Archaeocyathus. In the Mule Spring Limestone, archaeocyaths were observed in situ within small stromatolitic patch reefs, while in the Carrara Formation they were found in storm bed accumulations. These occurrences of Archaeocyathus are interpreted as examples of a “dead clade walking”, because these archaeocyaths—with a reduced ecological presence and role—are some of the last to locally persist into the upper Stage 4 following the disappearance of reefs during mid-Stage 4. With the youngest-known archaeocyaths from western Laurentia, this work provides insight into the ecological and environmental context of the regional decline of these reef-builders.