for National Geographic News
The nearly complete skeleton of an ancient aquatic mammal with legs has been unearthed in Jamaica.
The 50-million-year old skeleton is one of the best examples so far of the evolution from a land animal to an aquatic animal, said Daryl Domning, a paleontologist at Howard University in Washington, D.C., who reported the discovery.
The skeleton found in Seven Rivers, Jamaica, is a new genus and species of the order Sirenia, which encompasses the ancestors of modern-day manatees and dugongs.
Commonly known as sea cows, sirenians are plant-eating mammals that spend their entire lives in water. They started out as land animals, however. This new find fills a significant gap in the fossil record, helping scientists complete the picture of how land animals evolved to sea creatures.
"This is the most primitive fossil found so far," said Domning. "We've found others with legs that couldn't support the animal's body weight. But this is the first whole skeleton with legs that could support the animal's body weight out of water, yet has clear adaptations for aquatic life.
"We essentially have every stage now," he added, "from a terrestrial animal to one that is fully aquatic."
From Land to Sea
Domning thinks sea cows may have made the transition from land to aquatic creature to benefit from a resource that wasn't being fully exploitedin this case, sea grassand to avoid competition with other animals. Sea grass, he noted, has been present in coastal ecosystems since the Cretaceous era, 146 to 65 million years ago.
"Even today, sea turtles might use the resource a little," said Domning, "but there are still no large sea animals other than manatees and dugongs consuming sea grass."
The fossil skeleton Domning found is nearly 7 feet (2.1 meters) long. The animal probably weighed several hundred pounds when it was alive. It had four well-developed legs, which would have enabled it to move around on land.
But it also had anatomical features that obviously suited it for aquatic life. Its nostrils were enlarged and drawn backward into the skull. Its heavily boned skeleton would have acted as ballast, much as a weight belt serves the same purpose for a diver.
Like the hippo today, the fossilized animal could live on land or in water. "The hippo spends its days resting in the water and its nights on land searching for food," Domning said. "We think this animal spent much more of its time in the water."
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