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Fossil helps researchers decipher evolution of caimans, alligators

The Devil’s Graveyard Formation, a geologic formation in Far West Texas, contains fossils hailing from the latter half to the Eocene epoch. Photo by Chris Kirk
The Devil’s Graveyard Formation, a geologic formation in Far West Texas, contains fossils hailing from the latter half to the Eocene epoch. Photo by Chris Kirk

Jan. 15 (UPI) -- Alligators and caimans are closely related, but their evolutionary history is complicated.

And because the two groups look so similar, untangling their evolving relations across time and space has proven especially difficult for paleontologists.

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Thankfully, a new fossil, described Friday in the journal PeerJ, has offered scientists insights into the history of Alligatoridae family, the group of crocodylians that includes alligators and caimans.

The fossil, a partial skull, was originally unearthed in 2010 at a dig site in Far West Texas, a mountainous region of the Chihuahuan Desert.

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Far West Texas, or Trans-Pecos Texas, is home to what's known as the Devil's Graveyard Formation. It's a geologic formation preserving fossils from the latter half of the Eocene period, from approximately 48.9 million years ago to 33.9 million years ago.

"The Devil's Graveyard Formation provides a unique window into the evolution of North American vertebrates during the middle and late Eocene," study co-author Chris Kirk said in a news release.

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"There are a host of extinct species that are only known from the Devil's Graveyard, including several primates, rodents, lizards, and now this new fossil caiman," said Kirk, a paleontologist at the University of Texas at Austin.

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Upon excavation, scientists estimated the skull belonged to a relative most closely related to alligators. But their initial hunch quickly gave way to doubt.

"When you are at the early diversification of groups, their features aren't as differentiable," said lead author Michelle Stocker.

"It was harder to tell if this is more closely related to caimans or to alligators because those two are really closely related already. And the differences between them are subtle, especially early in their evolutionary history," said Stocker, an assistant professor of vertebrate paleontology at Virginia Tech.

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Back in the lab, researchers precisely measured the shape of the skull's braincase, the part of the skull that enwraps and protects the brain. Braincases are unique to each species, making them ideal for distinguishing between ancient lineages.

After analyzing the braincase, researchers determined the fossil actually belonged to an ancient caiman species.

Using radiometric dating, biochronology and biostratigraphy, researchers measured the age of the animals found in the layers above and below the ancient caiman. They determined the remains of the caiman ancestor were 42 million years old.

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The discovery of the caiman confirmed the extent to which North America's -- and the entire planet's -- climate was transformed during the Eocene.

"The presence of a fossil caiman in the Devil's Graveyard, about [745 miles] north of where caimans are found today, really says something about how different the climate of West Texas was in the middle Eocene," said Kirk.

Surveys suggest caimans made it as far as northern Canada during this period of dramatic global warming.

But between 33.9 million years ago and 23 million years ago, Earth's climate trends reverse and the planed began to cool. As a result, caimans retreated toward the equator.

Today, caimans are found in only Central and South America. Only alligators remain in North America.

"This caiman seems out of place. Caimans today are a South American radiation, and data from modern forms, including DNA, would suggest a very simple single origin from a North American ancestor," said co-author Christopher Brochu.

"This new form, along with some older North American fossil caimans, suggests a far more complex early history with multiple crossings of the seaway that separated North and South America until fairly recently," said Brochu, a researcher at the University of Iowa.

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Scientists hope future digs will turn up a more complete skeleton and more examples of Eocene caimans.

"If we can find another individual, we will get a better sense of its relationships, and it might be able to say something about what variation could be present in this taxon, or how they grow, or where else they might be found," said Stocker. "This is a one-and-done kind of fossil right now. Hopefully there are more out there."

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