From the beginning of the Late Ordovician (Sandbian: 460.9myr) through end Devonian (Famennian: 359.2myr), coral-stromatoporoid sponge reefs formed a remarkable, evolving ecosystem that dominated sediment production on tropical carbonate platforms in a calcitic ocean. This was a time of maximal and unparalleled reef development in the Phanerozoic, with reef tracts vastly exceeding in size and biodiversity of those in the Holocene (e.g., the Great Barrier Reef). Within this circum-equatorial niche, the calcitic tabulate and rugose corals, and the aragonitic (or high Mg calcite) stromatoporoid sponges, were the primary Middle Paleozoic reef frame builders. These were supplemented ecologically and skeletally by now extinct groups of calcitic bryozoans, crinoids, brachiopods, and red algae, alongside aragonitic green algae, and enigmatic CaCO3 precipitating and binding calcimicrobes. This 100 myr long Middle Paleozoic reef consortium thrived under SST averages of 30°+, to latitudes as high as 45°–55°, under high atmospheric CO2 conditions of 6000+ ppm, and sealevels 150–200 m higher than today. This reef ecosystem was disrupted by several relatively short duration south polar glacial episodes, centered around northern Gondwana, defining the O/S boundary Mass Extinction Events (MEEs). Nearly all coral and stromatoporoid families survived this MEE: there were losses at the genus level. Reef-building stopped nearly everywhere, and during the ‘recovery’ interval, solitary rugose corals initially prevailed, and stromatoporoids were small. Full global re-establishment of the reef ecosystem, and biodiversity, took another 3–4 million years (not until the late Aeronian, Early Silurian). This was followed by a remarkable reef expansion in the Middle Silurian (Wenlock), then by declines in the latest Silurian (Ludlow-Pridoli), and earliest Devonian (Lochkovian) possibly due to sealevel lowstands, tectonic plate re-assembly, and ocean current re-direction. Maximal Phanerozoic reef success was during the Emsian-Givetian, when some 15 barrier reef tracts more than 1100 km long flourished in tropical shallow seas. Reef-building coral diversity exceeded 200 genera, and the calcifying stromatoporoids evolved 60+ genera, especially in the ‘Old World’ faunal province (Euramerica, Cathaysia, northern Australia). Near the end of the Middle Devonian (mid- to late Givetian), the primary reef dwellers declined sharply in diversity, marked generally by sealevel lowstand, followed in the Frasnian (Late Devonian) by shrinking latitudes for carbonate platforms, and reduced reef accommodation space. Sharp cooling, with the arrival of a global Icehouse climate, and aragonitic oceans, led to the second largest Phanerozoic Mass extinction around the Frasnian/Famennian boundary, with reef builder and reef inhabitant losses exceeding those of the O/S MEE. The global absence of coral-sponge reefs persisted for nearly all of the 16 myr long Famennian, as total CaCO3 production fell some 60–90%, as aragonitic oceans took over. Only small and scattered Famennian coral-stromatoporoid patch reefs are known, with the last of these in the late Famennian (Strunian), punctuated by total disappearance of the whole keystone reef-building order. Famennian and Strunian corals belonged to Carboniferous families. During the Famennian, calcimicrobes, the first calcifying foraminiferans, and select ‘lithistid’ calcareous sponges dominated a highly stressed reef ecosystem, lacking barrier reef tracts. Biodiversity and reef construction were decoupled under global climatic stress during the succeeding icehouse Late Paleozoic.