Abstract
BackgroundScleractinian 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.ResultsBy 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.ConclusionsThe 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.
Highlights
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
Beyond implying that most extant scleractinians fall into two major clades (Robusta and Complexa) that are assumed to have diverged in the Late Carboniferous, ca. 300 Ma [11,12], molecular data have so far not added significantly to our understanding of early coral evolution
One reason for this may be that molecular phylogenetics has focused primarily on shallow-water corals, most of which harbor symbiotic dinoflagellates commonly known as zooxanthellae, whereas azooxanthellate, deep-water corals that account for approximately half of extant scleractinian species, have largely been ignored in these analyses [13]
Summary
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. Another study based on COX1 [15] found that members of the Gardineriidae and Micrabaciidae families formed a deeply diverging clade that may represent the oldest extant scleractinian lineage and that modern deep-water species diverge at or near the bases of both the Robusta and Complexa, implying that the evolutionary origin of scleractinians is best sought in deep-water rather than shallow-water (primarily zooxanthellate) coral species These contradictory interpretations motivated us to extend phylogenetic analyses of a large and representative range of deep and shallow water corals (more than 10% of all extant deep-sea species; see Additional file 1) beyond COX1, to include data for the mitochondrial 12S and 16S rDNAs, and the nuclear 28S rDNA, in an attempt to clarify scleractinian origins and relationships. The divergence time estimates generated here bridge the gap with fossils, allowing the integration of the morphologically similar Paleozoic “scleractiniamorphs” into Scleractinia
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