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 endPermian 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 s...