May 25, 2007
WASHINGTON (Reuters) -- Fossilized foot marks left by a big meat-eater on a lake bed in northern Spain 125 million years ago provide strong evidence that at least some dinosaurs were good swimmers, scientists said on Thursday.
Dinosaurs ruled the land from about 230 million years ago to 65 million years ago. But how they did in the water has been less clear.
There were numerous huge, fully marine reptiles living at the same time, including the plesiosaurs and ichthyosaurs, but they were not dinosaurs and in fact were only very distantly related to them.
Writing in the journal Geology, researchers led by Loic Costeur of the Universite de Nantes in France described tracks fossilized in sandstone that were left as a dinosaur swam in water roughly 10.5 feet deep, scratching the lake bottom with clawed feet.
"The animal used a pelvic paddle motion, much like living aquatic birds," Costeur said by e-mail.
Twelve "swim tracks" over a stretch of about 50 feet included long and slender sets of grooves. Fossilized ripple marks at the site suggested the dinosaur was swimming against a current whiling trying to maintain a straight path, the researchers said.
The researchers said the shape and nature of the tracks indicated they were left by a large bipedal theropod dinosaur and not a big crocodile even though these were around at the time. Theropods are the familiar big carnivores like the North American dinosaurs Tyrannosaurus and Allosaurus.
Scientists have been seeking evidence that dinosaurs -- like today's large mammals such as elephants and tigers -- were capable of swimming when circumstances demanded, like hunting in wet ecosystems, crossing rivers or escaping floods.
Previously discovered fossils showed swimming tracks apparently left by other dinosaurs such as sauropods -- long-necked animals like Diplodocus -- and duckbilled dinosaurs. But some of these have been disputed and were not as revealing as the new ones.
The new tracks provided the first definitive evidence of an active swimming behavior in dinosaurs and are the best record of swimming by theropods, researchers said.
The finding also extends the range of ideas about dinosaur behavior, Costeur added, including whether some thrived in aquatic environments.
The tracks were discovered three years ago in the Spanish province of La Rioja, Costeur said.
"The excellent preservation of these tracks provides an invaluable opportunity for biomechanical modeling in order to improve our understanding of dinosaur swimming ability and physiology," the researchers wrote.
Some birds, which scientists believe descended from small feathered dinosaurs roughly 150 million years ago, also were highly adept at swimming during the age of dinosaurs, including the diving bird Hesperornis.