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Evolutionary history

The first organisms that might be called scleractinians are known from Paleozoic fossils from China and Scotland, but the earliest proliferation of organisms that were clearly ancestral Scleractinia are Middle Triassic and consisted of at least seven, but possibly nine, suborders. These corals did not build reefs; they were small solitary or phaceloid organisms of the shallow Tethys of southern Europe and Indo-China. A Family Tree of Scleractinia. The width of the branches indicates the overall abundance of whole eras, not variations within them, so the branches do not indicate extinction events unless entire families went extinct. Many of these branches would be joined at their base if the fossil record had adequate information, but they remain separate where this information is missing. A Family Tree of Scleractinia. The width of the branches indicates the overall abundance of whole eras, not variations within them, so the branches do not indicate extinction events unless entire families went extinct. Many of these branches would be joined at their base if the fossil record had adequate information, but they remain separate where this information is missing.

Mesozoic

During the Middle and Late Triassic, corals became widespread throughout the Tethys region and their fossils are now found around much of the equatorial Panthalassa Ocean rim. There was a time interval of 20-25 million years between the earliest Triassic corals and the earliest widespread coral reefs. Most noteworthy of Triassic corals is that they had a wide range of skeletal micro-structures, suggesting that any common ancestry would have been remote. Nevertheless, Triassic corals were not the ecological equivalents of modern corals; corallites were large and poorly integrated so that phacelloid growth forms (where branches are composed of individual corallites) were dominant. Reconstruction of a Late Triassic coral community. The taxa portrayed may not have occurred in the same geographic region at the same time. The dominant growth forms of the corals are massive and phacelloid. There were no intricately branching corals such are found today and most corals had large corallites. Painting: Geoff Kelly. Reconstruction of a Late Triassic coral community. The taxa portrayed may not have occurred in the same geographic region at the same time. The dominant growth forms of the corals are massive and phacelloid. There were no intricately branching corals such are found today and most corals had large corallites. Painting: Geoff Kelly.

There was a 5-8 million year hiatus between the collapse of Triassic reef development and the onset of Jurassic reefs, a time of origin of many new scleractinian families. The Late Jurassic was probably the all-time global maximum of Mesozoic coral diversity with at least 150 genera recorded in the European Tethys and 51 in the Panthalassa. Paleobiogeographic provinces can be recognized which reflect continental plate movements, especially the increasing width of the Protoatlantic. By the Late Jurassic the paleobiogeographic pattern that had developed was the precursor to the pattern that persisted into the Cenozoic. It was dominated by massive reef development throughout the Tethys, the Atlantic, and also the far eastern Panthalassa. The vast expanse of the eastern Panthalassa was probably a barrier to east-west dispersion, just as the far eastern Pacific is today.

A high proportion of the families of extant Scleractinia have their origins in the Middle to Late Jurassic. For most, the fossil record is not clear and thus there are few links between the main branches of the Family Tree. The Jurassic was the time of the proliferation if not origin of two of the most major groups of corals, the Fungiina and the Faviina. The Fungiina dominated much of the Jurassic as well as the Cretaceous. As a group it was greatly diminished by the end-Cretaceous and the families attributed to it today have uncertain affinities. The Faviina, on the other hand, are a well-defined group and the Faviidae have remained a major family for 150 million years.

Early Cretaceous corals are broadly similar to those of the Late Jurassic. However, relatively little is known of the Middle Cretaceous corals. The continuity of families is largely due to extrapolations between Early and end-Cretaceous fauna, with poorly-known families omitted. This is because, by Middle Cretaceous, reefs worldwide had become dominated by rudist bivalves and environmental perturbations greatly affected reef development. It was not until the very late Cretaceous, following an unexplained total extinction of the rudists that corals returned to a position of dominance. At this time reefs probably again occurred worldwide, but there are few remains of them today.

Cenozoic

One-third of all families and over 70% of all genera became completely extinct at or near the end-Cretaceous boundary. The Faviidae and the Caryophylliidae are the only families that were major component of Mesozoic reefs that also proliferated in the Cenozoic.

The evolutionary history of modern corals is divisible into three geological intervals (1) the Paleogene, when the survivors of end-Cretaceous and Late Palaeocene extinctions proliferated into a diverse cosmopolitan fauna, (2) the Miocene, when this fauna became subdivided into the broad biogeographic provinces we have today and pre-cursors of most extant species evolved, and (3) the Plio-Pleistocene to present, when the world went into full glacial mode and modern distribution patterns emerged.

The Miocene is the time of origin of non-Oligocene extant genera (primarily Indo-Pacific) and the immediate ancestors of extant species. It is also the time of obliteration of the Tethys, the extinction of non-zooxanthellate corals from the Mediterranean, and the start of the separate evolutionary histories of Atlantic and Indo-Pacific species.

Compared with most other major groups of animals, coral genera are long-lived in geological time and have low extinction rates: nearly half of all extant genera extend as far back as the Oligocene and nearly ΒΌ extend back to the Eocene.

Contours of average age of all extant genera (in millions of years). The central Indo-Pacific centre of diversity has an average generic age of 30 million years, about half that of the Caribbean. The likely reason for this is that Tethyan genera are older than non-Tethyan genera, not that evolution has been faster in the Indo-Pacific. The average age of genera in peripheral regions of both the Atlantic and Indo-Pacific is the outcome of a small number of highly dispersed species, it is not created by older genera having more species nor is it created by displacement of species as has been suggested. The peripheral pattern is therefore created by dispersion, not evolution. Contours of average age of all extant genera (in millions of years). The central Indo-Pacific centre of diversity has an average generic age of 30 million years, about half that of the Caribbean. The likely reason for this is that Tethyan genera are older than non-Tethyan genera, not that evolution has been faster in the Indo-Pacific. The average age of genera in peripheral regions of both the Atlantic and Indo-Pacific is the outcome of a small number of highly dispersed species, it is not created by older genera having more species nor is it created by displacement of species as has been suggested. The peripheral pattern is therefore created by dispersion, not evolution. The history of corals subsequent to the Miocene becomes decreasingly visible in the fossil record and increasingly visible in the taxonomy and distribution of living corals. The Plio-Pleistocene fossil record of the Caribbean is better than that of the Indo-Pacific and it is in the Caribbean that the impacts of the Pleistocene glaciations were greatest. The progressive closure of the Central American Seaway was one of the most important events in the history of modern corals. Before the closure there may have been no distinction between the corals of the far eastern Pacific and those of the Caribbean. After the closure (3.4 million years ago), the corals of the Pacific side of the Isthmus were extinguished, or nearly so. There are no zooxanthellate scleractinian species common to the Indo-Pacific and the Caribbean today.

J.E.N. Veron