Gzhelian–Asselian Palaeoaplysina–microencruster reef community in the Taishaku and Akiyoshi limestones, SW Japan: Implications for Late Paleozoic reef evolution on mid-Panthalassan atolls

A Palaeoaplysina–microencruster reef community is recognized in the Gzhelian–Asselian interval of the Taishaku and Akiyoshi limestones in SW Japan, which represent accreted Panthalassan atoll carbonates. In a reef-core area of the atoll, the alga Palaeoaplysina acted as a framebuilder and was associated with various binders (e.g., Tubiphytes, Archaeolithoporella, cystoporate bryozoans, and microbialites) and sediment-bafflers such as fenestrate and cryptostomate bryozoans and phylloid algae. In a subtidal environment of the back-reef area, Palaeoaplysina and phylloid algae flourished as sediment-bafflers. A microbial community, including Tubiphytes and sessile foraminifers, was predominant in a very shallow, peritidal environment of the back-reef area. aplysina was distributed mainly along the northern margin of Pangea and is regarded as a boreal element. During Gzhelian–Asselian time, its distribution extended to the tropical or subtropical Panthalassa Ocean due to global cooling. On the Akiyoshi atolls, the Palaeoaplysina–microencruster community was succeeded by a sponge–microencruster community in the late Early Permian. The timing of this biotic turnover is similar to that of a change in climate from icehouse to greenhouse conditions, and coincides with superplume activity beneath the mid-Panthalassa Ocean.