Ancient Fossils Reveal Evolution of Marine Crocodilians Depends on Climate

First Posted: Aug 19, 2014 11:59 AM EDT
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The ancient ancestors of today's crocodiles once lived in our world's warm seas. Yet they became extinct when things cooled off. Now, scientists have taken a closer look at crocodilian diversity, and have found a link between climate and the variety of species.

Today, crocodiles are cold-blooded creatures that live in fresh water. There are only two notable exceptions that only occasionally venture into the sea. Because of their cold-blooded nature, crocodiles occur in tropical climates and, in the fossil record, are frequently used as markers for warm conditions.

These days, there are only 23 species of crocodiles. Yet there were hundreds of these species in the past. In addition, in the past 200 million years on four occasions, major crocodile groups entered the seas. Over time, though, these groups became extinct. Now, scientists have taken a closer look at the environment that drove these species to extinction.

"We thought each of these evolutionary events might have had a different cause," said Jeremy Martin, lead author of the new report, in a news release. "However, there seems to be a common pattern."

The researchers compared the evolution of a number of marine crocodilian species to the sea temperature curve during the past 200 million years. This temperature curve, which was established using an isotopic thermometer, has been widely applied for reconstruction of past environmental conditions.

So what did they find? It turns out that the colonization of the marine environment about 180 million years ago was accompanied by a period of global warming of the oceans. The first marine crocodilians became extinct about 25 million years later during a period of global freezing. Then, the same thing happened again.

"This work illustrates a case of the impact of climate change on the evolution of animal biodiversity, and shows that for crocodilians, warming phases of our earth's history constitute ideal opportunities to colonize new environments," said Michael Benton, co-author of the new study.

The findings are published in the journal Nature Communications.

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