Mass Extinction Killed Giant Marine Animals and Led to the Rise of Small Fish

First Posted: Nov 13, 2015 07:07 AM EST

It turns out that large animals suffer during mass extinctions. Scientists have looked at a mass extinction that occurred 359 million years ago and have found that it caused the die-off of large species while smaller species survived.

"Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine," said Lauren Sallan, one of the researchers, in a news release.

Scientists have long debated the reason behind changes in animal body sizes. One of the main theories is known as Cope's rule, which states that the body size of a particular group of species tends to increase over time because of the evolutionary advantages of being larger, which include avoiding predation and being better able to catch prey.

Other theories suggest that animals are larger in the presence of increased oxygen or in colder climates, and the Lilliput Effect states that after mass extinctions, there is a temporary trend toward small body size.

In order to learn a bit more about body size, the researchers looked at a dataset of 1,120 fish fossils spanning the period from 419 to 323 million years ago. The scientists gathered body-size information from papers, specimens, photographs, and bits of fossil.

So what did they find? With Cope's rule, vertebrates increased in size during the Devonian Period. In fact, at the end of that period there were fish the size of school buses. After the mass extinction, though, more than 97 percent of vertebrate species were wiped out. In addition, body size declined and continued downward for a long time after the event-for at least 40 million years.

"Before the extinction, the ecosystem is stable and thriving so that organisms can spend the time to grow to large sizes before they reproduce, for example," said Sallan. "But, in the aftermath of the extinction, that ends up being a bad strategy in the long term. So tiny, fast-reproducing fish take over the entire world."

The findings are published in the journal Science.