About 252 million years ago, almost all life disappeared during Earth’s worst mass extinction, an event sometimes called the “Great Dying.” Paleontologists call the event the Permian-Triassic (PT) mass extinction, after the two geologic periods it delineates.
Scientists have known about this extinction for decades, and they know that the effects were especially severe in the oceans, where about 96% of all marine species died out. But the details of how life recovered in the cataclysm’s aftermath have remained fuzzy.
Now, through research that uses a new global fossil database, scientists are shedding light on how life in the seas crawled back from the brink. Their findings appeared last month in the journal Science Advances.
The results seem a bit counterintuitive: The first types of organisms to emerge in the oceans, it seems, were animals at the top of the marine food chain. Known as nekton, these kinds of animals included free-swimming predators like the dolphin-like ichthyosaurs and coiled cephalopods known as ammonoids. In light of this new work, the PT extinction appears to buck the usual recovery trend: Ordinarily, the first creatures to rebound after an ecosystem is annihilated tend to be those that live toward the bottom of the food chain. These creatures help lay the foundations for the overlying ecosystem to build upon.
“In the early Triassic, the nekton was diversifying fast,” headed toward “a full recovery,” said Paul Wignall, a paleontologist at the University of Leeds in the United Kingdom who is a coauthor of the new work. At the same time, he noted, “life on the seafloor was just beginning to recover.”
The findings are a bit of a puzzle, noted Michael Benton, a vertebrate paleontologist at the University of Bristol who was not involved in the new research. “It’s kind of a top-down recovery, rather than bottom-up, as might be expected,” he said.
Database Deep Dive
The trigger of the PT extinction, at least in the oceans, was likely massive volcanism in what is today Siberia, explained Wignall. The Siberian volcanoes ejected about 3 million cubic kilometers of lava, as well as greenhouse gases like carbon dioxide. The latter would have helped to warm the ancient world.
When Earth warms, its oceans warm as well. And warmer oceans, Wignall explains, have a tougher time retaining dissolved oxygen in their waters than colder oceans do. This situation leads to ocean waters that lack sufficient oxygen to sustain life, thus the mass extinction.
The extinction “happened really quite quickly,” Wignall says. “Not instantaneous in the sense of a meteor impact, but certainly fast geologically speaking, happening in thousands and thousands of years.”
To reconstruct how the ocean recovery happened after the dust settled, Wignall and his team created a new database that catalogs fossil occurrences from around the world. Their new database houses data gleaned from the Paleobiology Database as well as from literature sources.
Wignall and colleagues’ database includes rich details on fossil species that occur in Triassic rock layers above the extinction boundary. By tracking which fossils occur and when those fossils appear and disappear in the rock layers, the researchers were able to unravel the tempo of life’s recovery.
The Tempo of the Dying
The team found that after the extinction, it took about 5 million years for animals at the top of the food chain to emerge, but it took about 50 million years for the underlying ecosystem to bounce back.
“We compiled the ranges of the fossils to time intervals of less than a million years,” Wignall said. Tracking fossils at such a fine temporal scale, he explained, had never been done for the PT event, and the new results are like making a blurry photograph sharper.
This pattern of recovery matches well with how a lack of oxygen in the oceans would have affected life, explains David Bottjer, a paleoecologist at the University of Southern California in Los Angeles who was not involved in the new research. “Because the surface waters are subjected to a lot of mixing of oxygen due to wave action, the low-oxygen water is typically confined to the seafloor,” he said.
This means that animals at the bottom of the food chain like corals and sponges on the seafloor would have suffered the most in a low-oxygen ocean, more so than more mobile animals like the ichthyosaurs that evolved soon after.
How, then, did these top-level predators stay alive while the foundations of their food webs lay in shambles? For Benton, this is an open question, one that future research could address. “What were they eating?” he asks.
The Sixth Extinction
The PT extinction is the biggest of the “big five” mass extinctions that brought life to its knees over the past half-billion years. By comparison, Earth’s second biggest mass extinction—triggered by an ice age about 445 million years ago at the end of the Ordovician period—saw about 85% of all marine species go extinct.
Nowadays, as humans continue to pump greenhouse gases like carbon dioxide into the atmosphere, there is evidence that the oceans are changing in a way that is similar to the changes that followed in the wake of the volcanism that ushered in the PT extinction, explains Bottjer.
“This study could be telling us a lot on how ocean ecosystems will respond as we proceed into the increased environmental stress that may well lead to a sixth mass extinction,” he said.
—Lucas Joel (email: [email protected]), Freelance Journalist