PHILADELPHIA, Nov. 12 (UPI) — In looking at life in the ocean before and after the Hangenberg event, a mass extinction 359 million years ago, scientists have revealed new patterns in the relationships between evolution, body size and catastrophe.
Before the Hangenberg event, large species dominated the marine world. But the event wiped out 97 percent of all vertebrate species. In the aftermath, larger species struggled, and smaller ones flourished.
“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, an assistant professor at the University of Pennsylvania, said in a press release.
Cope’s rule, the idea that species evolve larger and larger body types over time, is accepted by most scientists as one of evolution’s guiding principles. But extinction events offer an exception to this rule. And researchers have long debated why.
Sallan and her colleagues decided to look at whether trends in body size might have been affected by climatic conditions — as many scientists have contended.
But when researchers mapped body size patterns across climate trends, they failed to find a compelling correlation.
“There was no association with either temperature or oxygen, which overturns everything that has been assumed in vertebrates both today and in the past,” Sallan said. “Instead it tells us that these trends must be based entirely on ecological factors.”
The findings suggest it is the catastrophic event itself and the instability that follows that is the determining factor. Ecological stability allows the Cope rule to take effect. But instability encourages an opposite effect, the Lilliput Effect.
Being big was no longer an advantage in the wake of the Hangenberg event. Instead, smaller species had the upper hand.
“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.”
This downward trend in body size lasted at least 40 million years, researchers concluded.
How long a Lilliput Effect lasts or how large its effect is on the ecosystem varies, but it’s an effect that can be seen on a large scale, as well as a more intimate one. The pattern of plant life in the wake of the fire align neatly with the concept. The ecosystem, suddenly interrupted by fire, is first dominated by smaller, faster-growing grasses and shrubs.
Scientists say the concept could help conservationists better understand today’s ocean environments as larger fish species are threatened by climate change and overfishing.
“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.”
The new research was published online this week in the journal Science.
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