What fossils tell us about life after mass extinction

If you were an animal living through a mass extinction, your best chance of survival might depend on finding a unique way to make a living.

New research into species that either survived or perished after the asteroid impact that wiped out the dinosaurs is reshaping what we thought we knew about mass extinctions.

A team of scientists set out to understand how life bounces back after such catastrophic events – and what they found was both unexpected and a little unsettling.

The study was conducted by experts from the University of Chicago, the Smithsonian Institution, and the Natural History Museum in London.

The research was focused on the aftermath of the end-Cretaceous extinction, 66 million years ago. This event killed off about three-quarters of all species, including iconic creatures like Tyranosaurus rex.

Yet, despite this enormous loss, the ecosystems in the ocean held on to their structure in surprising ways.

Life before and after extinction

The team built a detailed catalog of fossilized clams, mussels, oysters, and other marine mollusks. These animals, with their durable shells, left behind an abundant fossil record. That made it possible to reconstruct a vivid picture of marine life both before and after the extinction event.

“What we wanted to do was not just count species, but count ways of life,” explained paleobiologist Stewart Edie. “How do they make their living? For example, some cement themselves to rocks; others tunnel into sand or mud; some are even carnivorous.”

With this information, the researchers mapped out the ecological landscape, comparing the period before the disaster to what followed. The results were startling.

Even though a vast number of species vanished, nearly all the different ecological niches – the particular ways species survive and interact with their environment – remained intact.

“That’s extremely statistically unlikely,” said Katie Collins, co-author of the study. “If 75% of the species died out, you would expect at least some of the ways of life to be lost completely – some of those niches only had one or two species in them. But that’s not what we see.”

Rethinking how ecosystems recover

The findings challenge two major ideas about extinction recovery. For a long time, some scientists believed that mass extinctions simply sped up natural evolutionary processes.

Dinosaurs, for instance, might have eventually been outcompeted by mammals, and the asteroid just fast-tracked that outcome.

Others argued that mass extinctions were like a biological reset button, clearing the stage for new life to evolve and fill empty niches.

But neither explanation fits what the researchers observed. Instead, the ecosystem’s structure – the ways of life that species employed – seemed remarkably resilient despite the massive loss of individual species.

David Jablonski is the William R. Kenan Jr. Distinguished Service Professor of Geophysical Sciences at UChicago and one of the study’s authors.

“It’s a really interesting, and slightly disquieting finding,” he said. “How ecosystems recover from mass extinctions is a huge question for the field at the moment, given that we’re pushing towards one right now.”

“We don’t understand how loss of functional groups relates to loss of biological diversity,” Jablonski added.

Extinction’s surprising aftermath

The team also uncovered another surprising twist. They found that the survivors of the mass extinction event didn’t necessarily dictate the shape of the post-extinction ecosystem.

“We thought the survival pool would lay the foundation for the modern world, it would all just flow from who got through the extinction, but that wasn’t the case,” said Edie. “It gets scrambled. A genus that had many species survive the extinction doesn’t necessarily wind up on top later on.”

And just because a particular way of life had a lot of survivors didn’t mean it stayed dominant in the long run.

Many scientists have long assumed that mass extinctions level the playing field. Survivors should theoretically have an open field to diversify and dominate. This seemed to be the case for mammals on land. But in the marine ecosystems, the story was much more complex.

“That may have been what happened with mammals, but in the marine ecosystem, it didn’t work that way,” said Jablonski.

Why mass extinctions matter now

These findings carry important implications for today’s conservation efforts. Modern oceans face a host of threats – acidification, pollution, overfishing, and climate change among them.

Understanding how past ecosystems and life responded to mass extinction could provide valuable insights into how today’s oceans might fare.

“This is something we really want to understand if we’re thinking about modern extinction and rebound in oceans, and how to manage it,”  Jablonski explained.

“Billions of people depend on the ocean for food, and we can see that reserves and management policies need to take into account the larger ecological structure of the biota, rather than just the individual species.”

The full study was published in the journal Science Advances.

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