Ancient Little Fishies Clue Us In to Why Tiny Animals Survive Extinction Events

In yet another instance where size does matter, it looks like the biggest fish in the pond are indeed at the top of the food chain... until they're not. Following a mass extinction, it's the small that survive and thrive.

A recent University of Pennsylvania study, also explained in the video above, found that following the Hangenberg extinction event (360 million years ago, at the end of what's known as the Devonian period) larger animals had a much harder time recovering and diversifying, while smaller fishes thrived.

"Smaller things are more able to survive and to diversify."
Lauren Sallan, University of Pennsylvania

But, we're getting ahead of the story (by millions of years, in fact). During the Devonian period, from 419 to 359 million years ago, animals generally grew along what's known as Cope's Rule, which states that evolution increases body size of the same animals over time. You can see examples of it all around. Humans, horses and marine animals have all grown larger over time. It's thought to occur because in a stable ecology animals have time to grow larger and become predators rather than prey. What about when an ecology is unstable, such as after the Hangenberg extinction?

"This loss seems to have killed everything in every setting no matter what its ecology was," says study lead author Lauren Sallan, a paleontologist and assistant professor at the University of Pennsylvania. "So it's almost like a reset button. That opens up possibilities to test other macro-evolutionary trends, because you have these two separate experiments – what happened before the extinction with this cast of characters, and what happens after the extinction with a completely different cast of characters in a devastated global ecosystem."

An artist's rendering of what small fish may have looked like following the Hangenberg extinction event.
Bob Nicholls

To look at the changes that occurred, Sallan and co-author Andrew K. Galimberti, now a graduate student at the University of Maine, developed a database of 1,120 fish fossils from the Devonian. They found that the species' evolution did indeed follow Cope's Rule, with fish growing larger throughout the period. (The largest evolved to be about the size of a school bus!) But then came the Hangenberg extinction, thought to be caused by a change in sea levels, and for 40 million years following the mass extinction, large fish didn't come back. Tiny fish and tetrapods of less than 4 inches (10 centimeters) swarmed the waters, while the largest shark was only about 1 meter (3.3 feet) long – a far cry from the school-bus days.

"What I noticed with ray-finned fishes and with the sharks is they're really small for a really long time," Sallan says. "You do have larger ones, but the vast majority of specimens are sort of like minnows or sardines."

She says that following the extinction there were some "very large predators, but they don't diversify and eventually they too die out and become lagging victims of the extinction. And they almost mask what's really going on and what's really being selected for, which is smaller sizes, faster growth, higher reproductive rates, larger populations of smaller sizes that have the ability to subsist on few resources. All those factors are related to smaller body size. So it really is that smaller things are more able to survive and to diversify."

In fact, she adds, "basically modern biodiversity as we know it is because they responded correctly, as it turned out, to mass extinction by becoming smaller afterwards."

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Fossils of fish in a fighting pose. The U-Penn team analyzed hundreds of fish fossils in the course of their research.
Carnegie Museum

This is all well and good for science, but does it matter today? Absolutely, says Sallan.

She points out that it doesn't take a mass extinction to alter an ecosystem, and something similar can happen as a much more localized event. In fact, overfishing of species such as Atlantic bluefin tuna and grouper is altering the oceans today. Her research suggests that species that humans love to dine on won't come back for millions of years if they go extinct now.

"You shouldn't expect large things to come back anytime soon when you eliminate them," she says. "It's not like minnows are going to suddenly become larger and take over that space. Instead it may be that you get more diverse minnows or sardines, or smaller cod, or whatever is left. But they're not going to necessarily replace the things that you've eliminated."