Researchers from the University of Pennsylvania, led by Lauren Sallan, an assistant professor at the Department of Earth and Environmental Science of the School of Arts and Sciences, have established that the Hangenberg event, also known as the mass extinction which occurred 359 million years wiped out larger animals on Earth and induced remaining ones on land and sea to grow smaller in size.
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“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 Sallan.
To this extent, the researchers said fast-producing, smaller fish evolved after the mass extinction event and these have greater advantages over the pre-extinction period animals.
Why fish and animals became small following the mass extinction is not really know and scientists have advanced various theories to this. Using the Cope’s rule, some scientists said ancient animals were larger because they were not hunted by larger predators and had an advantage of body size over smaller animals, making them grow larger themselves.
Other scientists say animals grow larger when oxygen increases, and also in colder climates. Yet, some others believe that under the Lilliput Effect which held after mass extinctions, animals temporarily grew smaller – but no real proofs could be advanced to support this theory.
Sallan and co-author Andrew K. Galimberti, now a graduate student at the University of Maine collected 1,120 fish fossils that lived 419 to 323 million years ago; and they analyzed their body size using research papers already published, museum specimens, photos, fossil collections and others to determine the traits and facts of each species.
The researchers were able to know that according to Cope’s rule, vertebrates grew in size during the Devonian Period which existed 419 to 359 million years ago.
“There were fish called arthrodire placoderms with large slashing jaws that were the size of school buses, and there were relatives of living tetrapods, or land-dwelling vertebrates, that were almost as large,” Sallan said of the Devonian Period. “You had some vertebrates that are small, but the majority of residents in ecosystems, from bottom dweller to apex predator, were a meter or more long.”
Almost 97% of vertebrates were wiped out during the mass extinction and for 40 million years after that, body sizes of animals became smaller – Sallan and Galimberti said.
“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.”
The researchers found it difficult to show that oxygen or temperature had any effects on body size of animals following the mass extinctions, but they found a link to ecological factors.
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“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.”