Tabulate corals across the Frasnian/Famennian boundary: architectural turnover and its possible relation to ancient photosymbiosis

The Frasnian-Famennian biodiversity crisis has traditionally been considered one of the “Big Five” mass extinctions; among deep changes in the biosphere, it almost completely erased shallow-water marine ecosystems like coral-stromatoporoid reefs, which were widespread on the carbonate platforms in the Devonian. The high biodiversity of bioconstructors during the Givetian-Frasnian acme of Palaeozoic reef development contrasts sharply with very low biodiversity levels during the Famennian. One of major bioconstructing groups in the Palaeozoic was tabulate corals. A statistical analysis of colony architecture of Givetian-Frasnian and Famennian tabulates from the Holy Cross Mountains (Poland) reveals that Givetian and Frasnian tabulate faunas had smaller corallite diameters and higher colony integration, compared to the Famennian faunas. A similar pattern occurs in the Devonian of the Ardennes. In the Holy Cross Mountains the biodiversity drastically decreases in the Late Frasnian. We interpret Givetian-Frasnian tabulate communities as generally photosymbiotic, and their Famennian counterparts as generally aposymbiotic. In Recent corals temperatures above 32 °C are generally considered as a threshold for bleaching (expulsion of photosymbionts), a literature review indicates that in the late Frasnian sea surface temperatures exceeded this threshold. This leads to a discussion of the late Frasnian rise of temperature as a contributing cause of extinction of photosymbiotic bioconstructors, and possibility of a Late Devonian “bleaching event”.