Silurian reef episodes, changing seascapes, and paleobiogeography

Eight global Silurian reef-building episodes coincide with climatic and oceanic conditions characterized by inferred, warmer, high-latitude climates; salinity-dense bottom waters; and accompanying low-diversity, planktic and nektic faunas. Periodic removal of reef and level-bottom community habitats by tectophases and relative sea-level falls appears to have stimulated reorganization and evolution of invertebrate communities during subsequent transgressive intervals. Latest Ordovician and early–middle Llandovery metazoans–parazoans gradually re-established shallow- and deeper-water reef ecosystems. Evolutionary radiations of coral and stromatoporoid faunas are evident in the upper Llandovery and lower Wenlock. Although corals and stromatoporoids reached their Silurian acmes in the Wenlock, stromatoporoids maintained similar diversities in the Ludlow. Numerous coral species disappeared by the early Ludlow, in part coinciding with end-Wenlock extinctions of different planktic and benthic faunas. Calcimicrobial communities and calcareous algae were important constructors in many early–middle Llandovery reefs, are less conspicuous in many late Llandovery–early Wenlock reefs, and were volumetrically important reef constructors in many Late Silurian reefs. Morphological innovations of selected Ludlow benthos and associated lithofacies show a "Devonian carbonate bank archetype", with distinguishable forereef, reef, backreef, and lagoonal facies. Partially reef-rimmed, late Ludlow, distally-steepened, carbonate banks reflect a change in reef patterns from the patchiness that characterized most Early Silurian flat-topped carbonate bank seascapes. Wenlock and late Ludlow reef tracts were larger in areal extent than modern reef tracts and were concentrated in subtropical and equatorial climatic belts.