Tracks Reveal Dinosaurs Swam
Sketch of a swimming theropod dinosaur on the shores of the Cretaceous lake Cameros in Spain.
Credit: Guillaume Suan, University Lyon1, France

Newly discovered fossilized claw marks are painting a picture of a carnivorous dinosaur pedaling its hind legs as it swam against a strong current and struggled to maintain a straight path.

The fossils, part of a 125 million-year-old trackway, are the most compelling evidence to date that some non-avian theropod dinosaurs could swim, scientists say in the June issue of the journal Geology.

A team led by Ruben Ezquerra of the Foundation for Paleontological Patrimony spotted the 50-foot-long trackway of 12 consecutive S-shaped prints in La Virgen del Campo in La Rioja, Spain—an ancient lake basin and a site known for its abundance of terrestrial dinosaur prints.

Preserved in a layer of sandstone, the paleontologists found six asymmetrical pairs of two to three scratch marks measuring 20 inches in length. The spacing indicated a likely underwater stride of 95 to 107 inches. The sinusoidal shape and variable spacing of the prints suggest the animal’s weight was mostly supported by the water, so that as the dinosaur swam, its claws or toe-tips grazed over the sediment.

"The dinosaur swam with alternating movements of the two hind limbs, a pelvic paddle swimming motion," said study team member Loic Costeur of the Laboratory of Paleontology and Geodynamics in France. "It is a swimming style of amplified walking with movements similar to those used by modern bipeds, including aquatic birds."

They also analyzed water ripples etched into the sediment, and using a so-called “ripple index,” they determined there was a drift current flowing from the northeast and reaching a depth of about 10 feet.

They combined the current-direction information with the interpreted orientation of the animal. They think the current-direction finding helps explain why the prints showed more movement of the dinosaur’s right hind limb as well as a twisted orientation of its body. The “unbalanced” motion helped the theropod to fight against the current and maintain balance and direction.

Scientists have long wondered whether dinosaurs could swim. Two years ago, scientists announced at a meeting of geologists that they had discovered the tracks of a two-legged swimming dinosaur on the shores of an ancient sea in Wyoming. Until now, however, hard evidence to support that dinosaurs had swim fins was lacking. Past evidence that pointed to aquatic dinos was later found to be prints produced on dry ground.

"The trackway at La Virgen del Campo opens the door to several new areas of research," Costeur said. "New biomechanical modeling will increase our understanding of dinosaur physiology and physical capabilities, as well as our view of the ecological niches in which they lived."