The ancient evolutionary origins of Scleractinia revealed by azooxanthellate corals
1 Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, PL-00-818 Warsaw, Poland
2 ARC Centre of Excellence for Coral Reef Studies and Coral Genomics Group, James Cook University, Townsville, QLD 4811, Australia
3 Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560 USA
4 Department of Chemistry, Laboratory of Electrochemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland
5 Muséum National d'Histoire Naturelle, Laboratoire de Mineralogie et Cosmochimie du Museum, LMCM UMR 7202, Case Postale 52, 61 rue Buffon, 75005 Paris, France
BMC Evolutionary Biology 2011, 11:316 doi:10.1186/1471-2148-11-316Published: 28 October 2011
Scleractinian corals are currently a focus of major interest because of their ecological importance and the uncertain fate of coral reefs in the face of increasing anthropogenic pressure. Despite this, remarkably little is known about the evolutionary origins of corals. The Scleractinia suddenly appear in the fossil record about 240 Ma, but the range of morphological variation seen in these Middle Triassic fossils is comparable to that of modern scleractinians, implying much earlier origins that have so far remained elusive. A significant weakness in reconstruction(s) of early coral evolution is that deep-sea corals have been poorly represented in molecular phylogenetic analyses.
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.