Printer friendly version
Reefs made rapid comeback after the greatest mass extinction of all time
05 October 2011
CNRS (Délégation Paris Michel-Ange)
In the aftermath of the extinction that marked the end of the Paleozoic Era, 252.6 million years ago, reefs made by multicellular animals took less than two million years to reappear and diversify. Until now, it was thought that recovery had taken around ten million years. These results were obtained by an international team led by four French researchers from the Laboratoire Biogéosciences (CNRS/Université de Bourgogne) and from the Laboratoire de Géologie de Lyon: Terre, Planètes et Environnement (CNRS/Université Claude Bernard Lyon 1/ENS Lyon). They were published on October 1 in the journal Nature Geoscience.
252.6 million years ago, the Earth went through the greatest mass extinction event ever recorded: the Permian-Triassic crisis (1). Over 90% of species present at the time disappeared suddenly, especially in the oceans, leaving behind a devastated biosphere that was to take 10-30 million years to recover the same levels of biodiversity as before the crisis. A large number of sedimentary and geochemical records support this 'global ecocide (2)' scenario, and show evidence of major environmental disturbance throughout the entire Early Triassic (the five million years following the mass extinction), such as an abnormal carbon cycle, and acidic oceans that were oxygen-depleted and enriched in carbon dioxide and sulfides. The consensus reached by the scientific community after half a century of research is that reefs, which are havens of marine diversity, disappeared during the entire Early Triassic. They were replaced by massive deposits of carbonates of exclusively microbial origin called microbialites, evidence of ecosystems that lacked multicellular organisms.
In 2009, a French-Swiss group challenged this 'slow, delayed rediversification' paradigm by showing that, following the Permian-Triassic extinction, the diversity of ammonites had started to increase again 10-30 times faster than previously estimated (3). Now, the same team has found evidence of the rapid (on geological time scales, i.e. after 1-2 million years) reappearance of reefs made by multicellular animals (metazoans). These findings are the result of several years of methodical exploration of the valleys of the South-western United States (Utah, Nevada and California). Field observations, together with microscopic examination of hundreds of samples in the laboratory, made it possible to reach the following conclusion: reefs formed by the association of microbes and sponges of various shapes and sizes developed in different places and at different times during the early Triassic. The reefs were home to a diverse fauna including foraminifera, serpulids, gasteropods, bivalves, ammonites, ostracods, brachiopods, echinoderms and conodonts.
The rapid recovery of diverse metazoan reefs from the very beginning of the Early Triassic rekindles the debate on the pace and conditions of biological rediversification following the Permian-Triassic crisis. Although it is now clearer how mass extinctions are triggered, the way and the rate at which the biosphere recovers and rediversifies after such crisis remain poorly understood. Now that humans are in all likelihood driving the Earth into its sixth mass extinction, these findings are a reminder that the recovery of diverse ecosystems following a mass extinction may be relatively fast on the geological timescale (several hundred thousand years), but that it is a long process on the human timescale, spanning at least tens of thousands of human generations.
This research was funded by CNRS, the Fondation pour la Recherche sur la Biodiversité, the Bourgogne Region, and the Swiss National Science Foundation.
(1) Named after the two geological periods that it spans, the Permian (299 – 252.6 million years ago (Ma)) and the Triassic (252.6 – 201.6 Ma), this crisis marks the end of the Paleozoic Era and the beginning of the Mesozoic Era. It was probably related to intense volcanic activity in China and Siberia.
(2) An ecocide (neologism constructed from the words 'ecosystem' and 'genocide') is the systematic and total destruction (whether natural or human-induced) of an ecosystem.
(3) See Brayard et al. (2009 - Science, 235 : 1118-1121) and the related press release.