The ancient evolutionary origins of Scleractinia revealed by azooxanthellate corals.
BACKGROUND
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.


RESULTS
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.


CONCLUSIONS
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.
Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, PL-00-818 Warsaw, Poland. stolacy@twarda.pan.pl
Jarosław
J
Stolarski
Marcelo V
MV
Kitahara
David J
DJ
Miller
Stephen D
SD
Cairns
Maciej
M
Mazur
Anders
A
Meibom
D000818:Animals; D037421:Anthozoa / Q000033:anatomy & histology* / Q000235:genetics*; D005075:Biological Evolution; D057894:Coral Reefs; D005580:Fossils; D010802:Phylogeny