Outcrops of the Lower Triassic (Spathian) Virgin Limestone Member (Moenkopi Formation) in the southwestern United States contain the oldest known metazoan bioherms formed in the aftermath of the end-Permian mass extinction. These small buildups, up to 1.0 m in diameter and 0.2 m high, were constructed by cementing bivalves. The bivalve bioherms accreted in a shallow, subtidal environment above storm wave base atop an oolitic hardground on a carbonate ramp. Similarities in shell microstructure and bioherm morphology between the Lower Triassic buildups reported here and previously described Middle Triassic occurrences suggest that the Lower Triassic bioherms were likely built by a bivalve assignable to Placunopsis. Although taxonomic assignment of the bivalve remains uncertain, the presence of cemented bivalve bioherms in Lower Triassic sections of the southwestern United States demonstrates that cementing bivalves were geographically widespread, even early in their Mesozoic evolutionary history. Despite their bioherm-building ecology, cementing 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. These first Mesozoic bioherms built by metazoans represent a significant ecological advance in the evolutionary history of bivalves in that the cementing life-mode had reappeared before the end of the Early Triassic.

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