Mass Extinction: Small Fish Dominate Oceans Following Devastating Event, Researchers Say

Small fish dominated oceans following a mass extinction 359 million years ago, according to researchers from the University of Pennsylvania. This finding sheds light on how species may develop after the next mass extinction.

"Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine," Lauren Sallan, assistant professor in the University of Pennsylvania's Department of Earth and Environmental Science in the School of Arts and Sciences, said in a news release.

For at least 40 million years following the mass extinction, known as the Hangenberg event, oceans were teeming with smaller fish. This suggests that tiny, fast-reproducing fish may actually have an evolutionary advantage over larger animals. 

Scientists have developed several theories to explain changes in animal body size over time. One such theory is known as Cope's rule, which states a species' body size tends to increase over time to avoid predation and to become better hunters.

On the other hand, the Lilliput Effect states that there is a temporary trend toward small body size following mass extinctions. However, this belief is not widely supported because a true understanding of small body-size trends following mass extinctions is lacking.

To put an end to this debate, researchers examined records of 1,120 fish fossils spanning 419 to 323 million years old. When comparing ages and body size, their analysis confirmed Cope's rule – vertebrates gradually increased in size during the Devonian Period from 419 to 359 million years ago. By the end of the Devonian Period, for example a fish called arthrodire placoderms measured about the size of a school buse and had large slashing jaws. 

However, when tracing the evolutionary timeline a little farther researchers found fossil evidence also lined up with the Lilliput Effect. Following the Devonian extinction, more than 97 percent of vertebrate species were wiped out and body size steadily declined for at least 40 million years.

"Some large species hung on, but most eventually died out," Sallan added. "So the end result is an ocean in which most sharks are less than a meter and most fishes and tetrapods are less than 10 centimeters, which is extremely tiny. Yet these are the ancestors of everything that dominates from then on, including humans."

Current ecological predictions state the Earth is on the brink of a sixth major extinction event. If this is the case, the recent study sheds light on how long it might take some large species to recover.

"It doesn't matter what is eliminating the large fish or what is making ecosystems unstable," Sallan said. "These disturbances are shifting natural selection so that smaller, faster-reproducing fish are more likely to keep going, and it could take a really long time to get those bigger fish back in any sizable way."

Their study was recently published in the journal Science. A video of their findings can be found online, courtsey of YouTube. 

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