New research suggests a mass extinction 359 million years ago triggered the transition from giant marine creatures being the norm to much smaller fish.
The extinction, known as the Hangenberg event, is believed to have changed the world's oceans forever, the University of Pennsylvania reported. For at least 40 million years following the event the oceans were teeming with small fish instead of the behemoths it had previously contained.
"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 University of Pennsylvania's Lauren Sallan.
The findings suggest small fast-reproducing fish had an evolutionary advantage over larger animals. The findings could have implications for the future of the oceans, which are being devastatingly overfished today.
To make their findings, the researchers looked at a dataset of 1,120 fish fossils spanning the period from 419 to 323 million years ago. They found the patterns in the changes they saw in the fossils were in link with Cope's rule, which states the "the body size of a particular group of species tends to increase over time because of the evolutionary advantages of being larger, which including avoiding predation and being better able to catch prey," according to the researchers.
The fossils revealed vertebrates gradually increased in body size during the Devonian Period, from 419 to 359 million years ago. By the end of this period there were horrifying fish called arthrodire placoderms that had huge lethal jaws and were the size of school buses. Some small vertebrates did exist in the sea, but were the minority. Once the mass extinction occurred, more than 97 percent of vertebrate species were wiped out. The species that did survived saw a sharp decline in body size and the downward trend continued for 40 million years.
"Some large species hung on, but most eventually died out," Sallan said. "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."
There was no association between the decline in body size and temperature or abundance of oxygen, which is a widely held theory that has been proposed in the past. The findings suggest the changes were instead based on ecological factors.
"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," Sallan said. "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."
Some scientists have suggested the Earth is on the brink of a sixth mass extinction that is being driven by human activity. The findings could help predict how modern species will recover from such an event.
"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."
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