How Naked Coral Solved a Mystery
Professor George Stanley coined the term “naked coral” for his theory that the reef-forming stony corals (order Scleractinia) lost their hard skeletons when ocean acid levels rise.
Stanley studies modern and ancient reefs and the mass extinctions that decimated them. He set out to understand why coral fossils appear to leapfrog geologic time. Luxuriant corals and reefs all died out at the end of the Paleozoic era, during the great Permian crisis, most likely due to ocean acidification. Then, after 10 to 14 million years, during the Middle Triassic period, a new group of scleractinian corals appeared in the fossil record.
Many scientists consider modern scleractinian corals as a newly evolved group replacing extinct Paleozoic corals. However, earliest modern corals are not simple forms but show advanced colonies and are divided into many different groups. This suggests these early modern coral have a history before their geologic appearance. The DNA of modern corals places them back into Paleozoic time, millions of years before the first fossil corals. So where were the coral for those 10 to 14 million years?
Professor Stanley’s Solution
In an article in Science in 2001, Stanley showed that, following the end-Permian mass extinction when 95% of all sea life died off, CO2 levels skyrocketed. Ocean conditions of low oxygen and pH continued for millions of years. Stanley said, “Later, with favorable pH, lineages of soft-bodied, anemone calcified, producing stony skeletons.”
In 2010 Journal of Earth Science, Stanley called this the Naked Coral Effect. Corals may have persisted in naked form during much of the Paleozoic but are hidden from the fossil record, which is strongly biased toward organisms with hard parts.
Stanley hypothesized that the anemone became the modern corals that build reefs today.
Examining the subsequent 230 million-year history of modern corals, Stanley noted unexplained gaps between evolving coral lineages which stand out like sore thumbs. Some could be explained by the Naked Coral idea.
Stanley’s hypothesis received wide coverage but being based on negative evidence, some colleagues were skeptical. Scientific hypotheses are rigorously tested, operating in an arena of being guilty until proven innocent. He needed more confirmation.
Stanley Finds More Evidence for Naked Coral
In 2005, he traveled to China to uncover the first soft-bodied fossil anemone. Hailing from the famous Lower Cambrian Chengjiang biota, Stanley calls this UN World Heritage site in Yunnan “simply amazing” because of the wealth of well-preserved soft-bodied sea creatures preserved with their tissues, stomachs, and organs.
Stanley and Chinese colleagues wrote a paper describing the fossil anemone and also he published other research in PLoS One, reporting older fossil anemone from China. Employing a technique of axial topography, it revealed a common body plan with modern corals. Could such anemone be the long-sought “naked” ancestors?
Confirmation of the naked coral hypothesis came from aquaria experiments in Israel, where researchers subjected reef corals to lower pH, more acidic sea water, simulating conditions of a future world. The skeletons dissolved but, surprisingly, the remaining soft polyps continued to live. When the experimenters returned the sea water to normal levels of pH, the naked corals reacquired their skeletons, confirming Stanley’s naked coral hypothesis.
Coral without skeleton from the Israeli experiment. Photo: Maoz Fine, Bar-Ilan University
Support also came from the DNA clock used to trace evolutionary relationships and date their divergences.
What Ocean Acidification Means for the Future of Coral
Stanley’s findings have resolved the 10-14 million year coral gap and the naked coral hypothesis can explain how corals might respond to future change. Global climate change and consequences of rising CO2 are a concern of both scientists and policymakers. Scientists predict that in 20 years most of the world’s reefs will have died or be unable to recover.
Rising ocean temperatures, changes in nutrients, and ocean acidification will hasten the demise and humans will be impacted economically, ecologically, and socially. Most poignant is the thought that corals might again go naked and disappear forever from the fossil record. Stanley was one of hundreds of scientists at the International Coral Reef Symposium in Australia who signed a statement requesting immediate action to stop reef degradation. “We can learn from the fossil record,” Stanley said. Looking back hundreds of millions of years into the deep past, we might be motivated to act quickly to save our future world.
George Stanley is a Professor at UM and Director of UM Paleontology Center