Post-Devonian re-emergence and demise of stromatoporoids as major reef-builders on a Carboniferous Panthalassan seamount

Stromatoporoids were major reef-builders during the middle Paleozoic; however, no Carboniferous stromatoporoid reefs have been reported. The Akiyoshi Limestone Group of southwest Japan formed on a seamount in the Panthalassa Ocean from the Mississippian (Visean) to the middle Permian (Capitanian). The Early Pennsylvanian (Bashkirian) reef-core setting was well developed and laterally differentiated into several reef environments. Laminated skeletons made up of cystose or flat laminae and pillar-like vertical structures were abundant and, together with Chaetetes, contributed greatly to reef construction in most reef-core environments. The morphology of the laminated skeletons clearly indicates a stromatoporoid, probably labechiid, affinity. Thus, a lineage of reef-building stromatoporoids reappeared on a Panthalassan seamount in a locally warm-water tropical setting after the Late Devonian extinctions. Ongoing global glaciation may have resulted in enhanced ocean circulation, upwelling, and nutrient supply, especially around shallow-water seamounts, culminating in elevated carbonate saturation, which should have favored hypercalcified stromatoporoids and Chaetetes. The Chaetetes−stromatoporoid reefs remained in the Moscovian, but probably died out during the Kasimovian with intensive global cooling and frequent subaerial exposure, to be replaced by Palaeoaplysina−phylloid algal reefs. The occurrence of Bashkirian reef-building stromatoporoids indicates that Paleozoic stromatoporoids continued as reef-builders long after the Late Devonian extinction, at least in Panthalassa. This occurrence emphasizes the significance of rarely preserved open, but isolated oceanic settings like Akiyoshi for global biogeography and evolution.