This lineage was widespread and abundant in the Late Cretaceous, but just a few species survive today off the coasts of Australia. Credit: Image courtesy Smithsonian National Museum of Natural History.

An international team of scientists has found surprising answers to how life rebounds from mass extinctions.

The study by the University of Chicago, Smithsonian Institution and the UK’s Natural History Museum challenges assumptions about survival after global catastrophe.

If you’re an animal living through a mass extinction, it’s best to be one that’s found a unique way to make a living. 

A new analysis of the species that lived or died out in the wake of the asteroid that killed the dinosaurs has revealed unexpected patterns that counter our prevailing theories of survival in the wake of mass extinctions. 

A team of scientists from the University of Chicago, the Smithsonian Institution and the National History Museum of London carefully catalogued fossilised clams and mussels, assembling a picture of the ocean ecosystems just before and after the mass extinction 66 million years ago.

They found that though three-quarters of all species were lost, each ecological niche remained occupied – a statistically unlikely outcome.

One of the authors on the study, David Jablonski, who is the William R Kenan, Jr, Distinguished Service Professor of Geophysical Sciences at UChicago, said: “It’s a really interesting, and slightly disquieting finding.

“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.”

Extremely statistically unlikely

In the history of Earth, we have documented five major extinctions – cataclysmic events in which the majority of species die out due to some worldwide change – and are currently edging towards the sixth mass extinction.

Scientists are, therefore, very interested in understanding how biodiversity and ecosystems recover from these massive events. 

Jablonski, along with paleobiologists Stewart Edie at the Smithsonian and Katie Collins at the Natural History Museum in London, decided to examine the most recent past extinction.

Known as the end-Cretaceous, the event resulted in more than three-quarters of all known species dying out, including T-rexes and most of the dinosaurs. 

The team focused on clams, oysters, cockles and other ocean-dwelling molluscs.

Their hard shells are abundant and fossilise easily, which was important because the team wanted to document as complete a picture as possible of the ecosystem – both before and after the extinction. 

Edie said: “What we wanted to do was not just count species, but count ways of life. How do they make their living?

“For example, some cement themselves to rocks; others tunnel into sand or mud; some are even carnivorous.” 

The team painstakingly built a picture of the global ecological landscape just before the extinction, “before the roof came in,” Jablonski said, and compared it to the species found afterwards.

And they got a surprise; though huge numbers of species died out, virtually none of the ecological niches were lost. 

Co-author Collins said: “That’s extremely statistically unlikely.

“If 75% of all 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.” 

Prevailing models

The finding doesn’t fit with either of the prevailing models for how biodiversity recovers from extinction, the authors said.

Decades ago, scientists thought that major extinctions simply ‘hastened the inevitable’ – ie, dinosaurs were always going to lose out to mammals, and a meteor hitting Earth just happened to hurry it along.

More recently, the pendulum of thinking swung back the other way, and others proposed that mass extinctions are a defining biological event – whoever manages to survive in the new landscape then evolves to fill different niches.

But neither model fully explains this picture. 

Jablonski described the finding as “a bit of a wake-up call”. He said: “We don’t understand how loss of functional groups relates to loss of biological diversity.”

A scrambled effect

The team also found that the way species recovered was counter to expectation. 

Edie said: “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; it gets scrambled.

“A genus that had many species survive the extinction doesn’t necessarily wind up on top later on.”

And, Jablonski added, a mode of life that was packed with survivors hasn’t necessarily stayed that way.

Jablonski explained that many scientists assumed that if you flatten the playing field, as in a mass extinction, the survivors should all take advantage of the opportunity and diversify rapidly. 

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

This is important information for conservation efforts for the modern ocean, for example, which is under threat from acidification, pollution and overfishing. 

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

“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 study is published in Science Advances.