Publication Type:Book Chapter
Source:Geological Society of America, 2000 annual meeting, Geological Society of America (GSA), Boulder, CO, United States, Volume 32, p.198 (2000)
Keywords:Anthozoa, Ashgillian, biodiversity, biologic evolution, carbonate platforms, Cnidaria, Coelenterata, controls, cycles, Devonian, ecosystems, extinction, Famennian, glaciation, Invertebrata, mass extinctions, Metazoa, middle Paleozoic, Ordovician, Paleozoic, Porifera, Reefs, Rugosa, Scleractinia, Silurian, species diversity, Stromatoporoidea, symbiosis, Tabulata, tropical environment, Upper Devonian, Upper Ordovician, Zoantharia
Comparative biodiversity plots for three major groups of tropical, reef-inhabiting metazoans show remarkable parallelism with reef abundance, and sizes of reefal carbonate platforms during mid-Paleozoic cycles, sandwiched between the late Ashgill and Famenne glaciations. More than 200 genera of calcite-secreting tabulate and rugose corals evolved during the 80 myr long Siluro-Devonian global super greenhouse interval, most of which were equatorial, reefal and peri-reefal in distribution, and structurally dominated the reef ecosystem (compared to ca. 150 genera of Mesozoic-Cenozoic stony scleractinians). The tabulate-rugose hegemony was briefly interrupted during the glaciated late Ashgill (Hirnantian) for a short decline, and collapsed catastrophically at the end of Givetian (50% loss) and Frasnian times (70% loss), the latter marked by global cooling. Maximum skeleton sizes, integrated Bauplane, and growth rates for mid-Paleozoic colonial coral taxa are comparable to modern corals, strongly supporting photosymbioses as evolving no later than Late Ordovician time. The aragonite-secreting stromatoporoids reached peak diversity in the Givetian (50 genera), but were consistently high from the late Llandovery through Frasne with more than 30 genera: only 16 genera are known from the Famennian, and none from the Carboniferous. Stromatoporoids periodically replaced corals as reef builders in the late Silurian and Frasnian. Comparison with the exclusively tropical shelly brachiopod clade of the order Atrypida over the same time interval shows peaks in the late Llandovery-Ludlow (ca. 32 genera), and maximal abundance in reefal settings in the late Emsian and Eifelian (41 genera). Atrypid diversity was down to 9 genera in the late Ashgill, with progressive losses to 22 genera in the Givet and 12 in the Frasne. No Atrypida survived the Frasne-Famenne extinction events. Paleodiversity tracked reef abundance and distribution: there is little evidence that hypoxia played a role in controlling either diversity or reef abundance, even during mass extinction episodes.
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