THE FIRST METAZOAN BUILD-UPS FOLLOWING THE END-PERMIAN MASS EXTINCTION

Lower Triassic sections of the southwestern United States contain the first metazoan bioherms that formed in the aftermath of the end-Permian mass extinction. These small build-ups (< 0.5 m) constructed by the cementing bivalve Placunopsis accreted in shallow, high-energy conditions atop an oolitic hardground on a carbonate ramp. The build-ups exhibit both columnar and mound morphologies, perhaps as a consequence of hydrodynamic conditions in a high-energy, shallow subtidal setting. Placunopsis bivalve build-ups have previously been described from Middle Triassic successions in European Tethys; their presence in Lower Triassic sections of the southwestern United States demonstrates that cementing, mound-building bivalves were geographically widespread, even early in their evolutionary history. The late Early Triassic age of the mounds indicates that a complex cementing ecology of bivalves had evolved prior to the Middle Triassic. However, the phylogenetic position of Placunopsis and its relationship to Paleozoic cementing bivalves will need to be resolved to determine whether the absence of Placunopsis mounds during much of the Early Triassic represents a true stratigraphic gap in the record of cementing bivalves or, rather, whether the Placunopsis mounds within the Virgin Limestone record the re-invention of a cementing, mound-building life mode among bivalves. Despite their mound-building ecology, Placunopsis bivalves do not occur in Middle Triassic platform-margin reefs, underscoring the decoupling of the recovery of framework-building metazoans from the return of large carbonate platform-margin reefs in the wake of the end-Permian mass extinction.