The ancient evolutionary origins of Scleractinia revealed by azooxanthellate corals

By adding new data from a large and representative range of deep-water species to existing molecular datasets and applying a relaxed molecular clock, we show that two exclusively deep-sea families, the Gardineriidae and Micrabaciidae, diverged prior to the Complexa/Robusta coral split around 425 Ma, thereby pushing the evolutionary origin of scleractinian corals deep into the Paleozoic.The early divergence and distinctive morphologies of the extant gardineriid and micrabaciid corals suggest a link with Ordovician "scleractiniamorph" fossils that were previously assumed to represent extinct anthozoan skeletonized lineages. Therefore, scleractinian corals most likely evolved from Paleozoic soft-bodied ancestors. Modern shallow-water Scleractinia, which are dependent on symbionts, appear to have had several independent origins from solitary, non-symbiotic precursors. The Scleractinia have survived periods of massive climate change in the past, suggesting that as a lineage they may be less vulnerable to future changes than often assumed.The two most popular hypotheses put forward to account for scleractinian origins are that they are either descendants of late Paleozoic rugose corals that survived the mass extinction at the Permian/Triassic boundary [1-3] or, that they evolved from soft-bodied (corallimorpharian-like) ancestors by gaining the ability to deposit a calcified skeleton [4-6]. Difficulties with the former hypothesis include that it requires major changes in both the composition of the skeleton, which was calcite in the case of Rugosa, but is aragonite in Scleractinia, and the symmetry of septal insertion [4], characters that are otherwise highly conserved. By contrast with Rugosa, some Permian fossils (known as scleractiniamorphs) appear to have had aragonite skeletons (Numidiaphyllum, Houchnagocyathus) and may be the immediate ancestors of some Triassic scleractinian coral lineages [7,8]. Intriguingly, some early Paleozoic "scleractiniamorphs" (kilbuchophyl