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Ancient Alligator Ancestors Unearthed

4 March 2013 2:45 pm
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Original Artwork by Danielle Byerley © Florida Museum of Natural History

Ancient gator. The discovery of the skull of a new crocodilian ancestor, Culebrasuchus mesoamericanus, depicted here by an artist, gives researchers information on how caimans evolved from alligators.

As crews of workers drill and dig into the ground surrounding the Panama Canal, they're not only setting the stage for the country's economic future—they're also unearthing signs of its distant past. Two of the latest fossils discovered in the massive expansion project are skulls of crocodilians, ancient relatives of alligators. The skulls, the oldest of their type ever discovered in Central America, are giving scientists key information on how and when North American alligators evolved into caimans, alligator relatives that now thrive in South America.

"The authors have brought together some really intriguing fossils that shed a lot of light on a very poorly understood part of the crocodilian record," says Christopher Brochu, a paleontologist at the University of Iowa in Iowa City who was not involved in the new work.

Researchers studying the evolution of alligators and caimans have previously relied on a sparse patchwork of fossils to piece together the animals' history. North American fossils from 70 million years ago revealed a common ancestor of both alligators and caimans that lived during that time. And a plethora of more-recent caiman fossils have been discovered in South America, implying that their ancestors at some point crossed between the continents. Paleontologists have hypothesized that an alligator ancestor left North America and crossed into Central America before evolving into caimans in South America. But signs of crocodilians during this transition have eluded fossil hunters.

"We really didn't have any fossil record at all for any of Central America," says paleontologist Alexander Hastings of Georgia Southern University in Statesboro, who led the analysis of the new skulls while completing doctoral work at the University of Florida.

Hastings and his collaborators estimated the ages of the two new skulls, based on the rocks they were found in, at 19.83 million years and 19.12 million years, a time frame in which little is known about caimans in this part of the world. Then, they compared the physical features of the skulls with those of 32 previously characterized species of alligators and caimans from the past and present. The new skulls, the team concluded, belonged to some of the most primitive caimans after the lineage split from that of alligators.

"What really struck me was how the skulls have such a great blend of traits from alligators and caimans," Hastings says. In particular, he says, alligators and caimans have distinct jaw structures due to different diets. The fossils' jaws showed a mix of the structures.

The ages, locations, and physical characteristics of the new fossils—which have been given the scientific names Culebrasuchus mesoamericanus ("Central American Culebra crocodile," after the Culebra Cut in Panama) and Centenariosuchus gilmorei ("Gilmore's Centenario crocodile," after Charles W. Gilmore, a paleontologist who uncovered fossils during the original excavation of the Panama canal a hundred years ago)—suggest that caimans began to evolve in Central America, rather than not evolving until their alligator ancestors reached South America.

The new data, published online today in the Journal of Vertebrate Paleontology, also add weight to a hypothesis that the gap between South America and North America—before the Isthmus of Panama formed 3 million years ago—was relatively narrow. The location of the new fossils shows that crocodilians would have had to cross a waterway, and neither caimans nor alligators are very tolerant of salt water.

More work needs to be done to determine where the new fossils fit in the evolutionary tree of crocodilians, Brochu says. "We know so little about the caiman record that we really don't even know what the most primitive caimans looked like," he explains. "And so every time we get a fairly young fossil, we have to wonder how basal it really is."

Additional fossils, Brochu says, will ultimately help complete the evolutionary tree. Their past could also shed light on how caimans have adapted so well to South America, Hastings says. "Caimans have thrived in ecosystems all throughout the Amazon River basin, one of the most diverse areas of the world," he says. "Understanding how they've become such pillars of the ecosystems can help us understand what it takes for species to become established there."