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Crocodiles Swam Atlantic?

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What can fossils of ancient animals tell us about the history of modern animals?

Quite a bit, according to Chris Brochu, associate professor of geoscience in the UI College of Liberal Arts and Sciences. Brochu is the co-author of an article published in the March 7 issue of the Royal Society Journal that tells how a fossil of a crocodile that lived millions of years ago in Puerto Rico helped reveal that related crocodiles likely came to live in present-day South America by swimming across the Atlantic Ocean from Africa.

The story began with a conversation Brochu had with a colleague about fossilized crocodile remains found in a fresh water river delta off the coast of northern Puerto Rico. At first he was merely hopeful, because crocodile fossils are common, but are often very incomplete. But his reaction changed after he got a look at the fossil.

"My jaw dropped," he says. "This was the back of a skull, which is the best part if you want to know what kind of crocodile you have. It belonged to a gharial, a group found today only in fresh water in India and neighboring countries. These were sea-going, coastal animals that didn't have the aversion to salt water that crocodiles do today."

Brochu explains that the Indian gharial is not found in salt water, but the geographic distribution of fossil relatives suggests it descended from ancestors that lived in, or were at least able to withstand, saline conditions. The fossil describes a new type of gharial (since named Aktiogavialis puertoricensis) that lived during the Oligocene epoch, a geologic period ranging from 34 to 23 million years ago. The fossil is from an animal related to a group of gharials otherwise restricted to South America.

When he looked at the geological time frame and the location of the find, together with known information about the relatedness of crocodile species, he was left with two facts: that a single trans-Atlantic crossing likely accounts for the South American crocodiles and that gharials, being coastal animals, developed their current affinity for freshwater comparatively recently.

"Prior to the find, the question had been, 'How did South American fresh water gharials get there?' Now we have a fossil from Puerto Rico that helps answer the question," he says. "This discovery tells us that South American crocodiles got there by crossing the Atlantic, probably from Africa.

"And it reinforces the notion that the ecology of modern animals can't always be understood by looking at modern relatives," he says. "It is important to include fossil information in a phylogenetic, or evolutionary development, context when assessing the ecological history of modern organisms."

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