Mesozoic and Cenozoic fossil reefs

No reef building occurred for 12-14 million years after the end-Permian mass extinctions. Only stromatolites remained common and it was not until the Middle Triassic that that the first reef-like structures first appeared, these having a fauna dominated by sponges, algae, bryozoa and Tubiphytes, much like those of the Late Permian. However, by Late Triassic massive scleractinian reefs were abundant, especially in the Tethys, which remained a center of reef development throughout the Mesozoic. A Late Triassic Reef. Dolomite Ranges, Italy. Photograph: Michael Hautmann. The Late Triassic showing the distribution maximum of coral reefs. The end of the Triassic was marked with a mass extinction that was not the equal of the extinctions that marked the end of the Paleozoic 45 million years earlier, but it may have rivaled the extinctions at the end of the Mesozoic. The main impact on reefs was the collapse of the reefs of the Tethys, perhaps because of anoxic conditions correlated with sea-level and climatic changes. Whatever the cause, reef building did not recover for 6-8 million years. The inheritance of the Jurassic was a remnant of these extinctions, a depauperate although diverse suite of scleractinian genera. Early Jurassic reefs are rare everywhere in the world and all Triassic genera had become extinct by then.

The Jurassic recovery was slow but by Middle Jurassic reef development had again proliferated in the Tethys of Europe and in the Mediterranean. Complex ecosystems underwent a major radiation, enhanced by a warming climate and extensive flooding of shallow shelf areas. Elsewhere reefs remained poorly developed, especially in the Panthalassa and thus it may have remained throughout most of the Jurassic. Jurassic Calcareous Ranges. Austria. Photograph: Harald Krenn. The Late Jurassic showing the distribution maximum of coral reefs.

The beginning of the Cretaceous was not marked by any extinction event that had a major impact on reefs, which continued to thrive throughout the Early Cretaceous. There was, however, an increasingly drastic change in coral communities over this time. Rudist bivalves, a previously obscure group of molluscs, progressively displaced corals as the dominant reef biota, and thus it remained for 30 million years. During this interval, zooxanthellate corals coexisted with rudists, but largely in separate, deeper habitats. The reefs of that time probably resembled inshore fringing reefs of today, repeatedly destroyed by changing sea levels and consisting mostly of banks of entrapped sediment with no algal cementation. Rudists were probably zooxanthellate and, as they had a lesser amount of aragonite in their shells than corals, probably survived acidic conditions better than the corals. The Late Cretaceous showing the distribution maximum of coral reefs.

Repeated environmental upheavals probably had a greater influence on evolutionary change in the Cretaceous than did continental movements. The Late Cretaceous was a time of extreme sea level change, periodically flooding up to 40% of the continents and creating a Super-Tethys Ocean over much of present-day Europe. Consequences for reefs are unknown, but the continually decreasing sea levels of the Late Cretaceous would have had a great impact on coral communities.

Much of the Middle Cretaceous was characterised by release of carbon dioxide due to extensive volcanism around the continental plate margins and this, together with the accumulation of organic matter associated with sea level changes, may have increased the acidity of much of the ocean surface. Ocean and atmospheric temperatures over a range of latitude from the equator to the poles were much higher than they are now. This would have varied greatly over time, but subtropical conditions may have periodically extended to 45°N and possibly 70°S, and there were no polar ice caps. These conditions would have resulted in weaker ocean currents than we have today. Corals would have been far more widely dispersed and there would have been a much greater development of distinctive regional provinces.

By the close of the Mesozoic, the flooding of the continents had ceased and the warm climates that had dominated the Cretaceous had begun a long and irregular decline towards a glacial mode. Reefs did not proliferate anywhere for long after the end-Cretaceous extinctions. Eocene reefs are mostly poorly preserved, but were extensive in area. An end-Eocene extinction event, combined with progressive blockages of the Tethys Sea in the Early Oligocene created a hiatus in reef development, but did not greatly diminish coral diversity. The Late Eocene showing the distribution maximum of coral reefs.

There were two post-Miocene events which greatly affected reef development. The first was the closure of the Central American Seaway, finally separating the faunas of the Indo-Pacific and Caribbean. The Late Miocene showing the distribution maximum of coral reefs. The second was the Pleistocene glaciations which, because of low sea temperatures and episodes of freshwater inundation stopped reef development in the far eastern Pacific, much of the Caribbean as well as high latitude locations of the western Pacific. More importantly, low sea levels repeatedly aerially exposed all coral reefs world-wide.

J.E.N. Veron