Mass extinctions have devastated biodiversity many times over the past 540 million years, according to scientists. After each cataclysmic event the species that survived diversified and filled the planet with life again.
Until now the fossil record supported the theory that species that survived extinction events–which ranged from meteorite impacts to an eruptions of super volcanoes (and in our time the mass destruction of ecosystems by humans)–did so in much smaller forms. This “Lilliput effect,” in which post-extinction life is downsized, was believed to have persisted for millions of years.
But now a new fossil discovery in the U.S. by a team of French, German, Swiss, and American scientists may change what we know about the evolution of species after an extinction crisis.
Giant gastropods found in marine sediments in Utah dating from only about 1 million years after the P-T mass extinction. The scale bar represents 1 centimeter (0.4 inch).
© A. Brayard/J. Thomas/CNRS
The team discovered “giant” gastropods, mollusks that lived on the seabed and are related to present-day land snails. The gastropods, found in Utah, date from only 1 million years after the greatest mass extinction of all time, the Permian-Triassic extinction which wiped out about 90 percent of marine species about 250 million years ago.
The newly discovered gastropods call into question the existence of a “Lilliput effect,” the reduction in the size of organisms inhabiting postcrisis biota, normally spanning several million years, said the French National Center for Scientific Research (CNRS) in a news release this week.
The finding, published in the February 2010 issue of the journal Geology, has “drastically changed” paleontologists’ current thinking regarding evolutionary dynamics and the way the biosphere functions in the aftermath of a mass extinction event, CNRS said.
“The history of life on Earth has been punctuated by numerous mass extinctions, brief periods during which biodiversity is considerably reduced, followed by phases of re-conquest of the biosphere, corresponding to the diversification of those species that survived.
“Over the last 540 million years, around 20 mass extinctions, of greater or lesser intensity, have succeeded one another. The most devastating of these, the Permian-Triassic (P-T) mass extinction, which decimated more than 90 percent of the marine species existing at the time, occurred 252.6 million years ago with a violence that is still unequaled today.
After extinction events “the oceans become less oxygenated, water becomes poisonous, there is increased competition, collapse of food chains.”
“In the aftermath of such events, environmental conditions are severely disrupted: the oceans become less oxygenated, water becomes poisonous, there is increased competition, collapse of food chains,” CNRS explained.
Until now, it has generally been accepted that certain marine organisms, such as gastropods or bivalves, were affected by a drastic reduction in size in response to major disruptions of this nature, both during and after the event, CNRS said.
“It took several million years for such organisms to return to sizes comparable to those that existed prior to the crisis. This is what scientists call the ‘Lilliput effect,’ in reference to the travels of Gulliver who was shipwrecked on the island of the same name, inhabited by very small Lilliputians.”
Fossil-bearing outcrop in Utah where lgiant gastropods dating from the Early Triassic have been discovered along with ammonoids and bivalves.
© A. Brayard/CNRS
The researchers who discovered the large-size gastropods in Utah have been studying the re-conquest phase that followed the P-T crisis. “By focusing their efforts on fossil-bearing outcrops in Utah dating from the Early Triassic, which have not yet been studied in detail, they have uncovered some outstanding specimens of gastropods, up to 7 centimeters [2.75 inches], which can be termed as ‘giants’ in comparison to those generally found, normally no bigger than 1 centimeter [0.4 inch],” CNRS said.
“Complementary studies of these new gastropod fauna also indicate that they are not any smaller than older or present-day fauna,” CNRS added.
“This discovery therefore refutes the existence of a Lilliput effect on gastropods during the major part of the Early Triassic or, at the very least, suggests that its importance has been overestimated.”
Re-conquest of the ocean
The presence of the large gastropods also coincides with an explosive re-conquest of the ocean by organisms such as ammonites, CNRS said. “Ammonoids, related to present-day nautilus, cuttlefish and squid, are free-swimming cephalopod mollusks with external shells. They disappeared from the world’s oceans at the same time as the dinosaurs, 65 million years ago, after having been a major part of the marine fauna for nearly 400 million years,” CNRS explained.
“Taken together, these events therefore suggest that restructuring of marine ecosystems was already well underway only one million years after the P-T crisis, a very short time after a mass extinction of such magnitude.”
The researchers plan to continue to study the fossils discovered in Utah while searching for other species and groups, such as bivalves, to confirm the new data. “However, these findings already suggest that paleontologists are going to have to re-think the immediate and long term impact of mass extinctions on species,” CNRS said.