Free swimmers came back first after Great Dying

by Mary Caperton Morton
Thursday, January 17, 2019

In the Early Triassic, following the end-Permian extinction, nektonic marine life recovered fastest, according to new research, resulting in a reversed functional pyramid. Percentages represent the proportion of marine genera belonging to each class of organisms. Credit: courtesy of Haijun Song.

About 252 million years ago, at the boundary between the Permian and Triassic periods, the vast majority of marine and terrestrial life died out in the most devastating extinction event in Earth’s history. Earth’s ecosystems eventually recovered, but not in the way — or as quickly as — scientists thought. In a new study looking at how marine species reemerged in the Triassic, researchers report a surprising trend of recovery from the top of the food chain down.

Unlike some other extinction events, in which certain classes of organisms were more impacted than others, the end-Permian event, known as the Great Dying, severely impacted nearly all life, both on land and in the oceans, including long-dominant species such as trilobites and rugose corals. “Almost all animal and plant groups were impacted in some way, except some microbes such as cyanobacteria and sulfate-reducing bacteria, which bloomed during the event,” says Haijun Song, a paleontologist at the China University of Geosciences in Wuhan and lead author of the new study in Science Advances.

The broad swath of devastation on land and at sea is thought to have been caused by widespread aridity on land and anoxia and acidification in the water. The underlying triggers are still debated: The consensus hypothesis is that massive volcanism associated with eruption of the Siberian Traps flood basalts led to the catastrophe, but asteroid strikes and toxic methane releases have not been ruled out. Ecological recovery from the end-Permian extinction has been thought to have taken 5 million to 10 million years, with new species initially evolving to fill niches at lower trophic levels before upper levels recovered. But the new analysis from Song’s team suggests that recuperation may have taken place from the top down, with populations of free-swimming animals higher in the food chain, such as fish and reptiles, recovering fastest.

To study the array of organisms populating Triassic seas, Song and his colleagues analyzed global fossil data from the Paleobiology Database, including more than 51,000 occurrences of key marine species in the late Permian and Early Triassic fossil records. The researchers grouped species into three categories based on their mobility: nonmotile species like plankton and coral, motile animals such as crustaceans and bivalves, and free-swimming, or nektonic, creatures such as fish and reptiles.

The team found that it took 50 million years for marine ecosystems to fully recover — five to 10 times longer than previous studies have found. “This means that the impact on the marine ecosystem by the Permo-Triassic extinction was more severe than we previously thought,” Song says.

He and his colleagues also found that the three functional groups experienced very different fates during and after the Permian-Triassic boundary: Nonmotile animals suffered the most severe losses and were slowest to recover, with biodiversity levels not starting to rebound until the Middle Triassic. Motile animals also suffered dramatic losses, though not to the same degree as nonmotile animals, but rebounded faster, by the Middle Triassic. Nektonic animals saw far fewer losses in the extinction, and species diversity in this group increased steadily throughout the Early Triassic.

“We found that the oceans were dominated by nonmotile animals during both the Permian and Middle-Late Triassic, but during the Early Triassic, after the crisis, the oceans were dominated by free-swimming animals,” Song says. This could be the only documented case of a “reversed functional pyramid,” in which there’s greater biodiversity higher in a food chain than lower, in the past 500 million years, he adds.

“The motility of animals has a lot to do with their community function, which tells us something about how the community is functioning as a whole,” says Peter Roopnarine, a paleoecologist at the California Academy of Sciences in San Francisco who was not involved in the new study. The researchers present “an interesting way to divide the ecological community, but there are probably more nuanced ways to do it” beyond just motility, he says, such as by considering the specific roles different animals play in the food web.

“We need to think about how the energetics of this proposed sequence of recovery would work,” Roopnarine says. “How can organisms at a higher trophic level rebound if the bottom of the food web has been knocked out? It raises all sorts of evolutionary questions about how things adapt to a radically changing food web structure.”

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