As a northeasterly wind whips against the shore, a meters-long dinosaur plunges into the shallow lake. Working hard, the predator takes strong strides with its hind limbs through the shoulder-deep water. The current is so strong that the beast must constantly fight to stay on course, but it succeeds, heading straight across the water. That's the story told by a remarkable set of fossilized footprints, described in the June issue of Geology, that provide the first hard evidence of predatory dinosaurs traveling in water.
The 125-million year-old trackway was discovered in 2004 during excavations at a famous fossil site in Northern Spain, called the La Virgen del Campo track site. The site had yielded many tracks of dinosaurs walking on land, so a team led by paleontologists Rubén Ezquerra of the Fundación Patrimonio Paleontológico de La Rioja, Spain, and Loïc Costeur of the Université de Nantes came looking for more in an untapped layer of rock. To their surprise, they found a set of footprints unlike any they had seen before.
With three telltale toemarks on each print, the tracks clearly belonged to a major group of bipedal, carnivorous dinosaurs called theropods. But the tracks themselves were different. When theropods walk on land, they typically leave claw marks and an imprint of the foot itself. The lack of the footprint suggested that this animal was not supporting its weight. A sedimentologist on the team confirmed that ripple marks in the stone had been created by currents in water 3.2 meters deep.
Another unusual feature of the new tracks was that the feet were placed 44 centimeters apart. Moreover, the dinosaur seems to have spread its legs, so that the feet were somewhat pigeon-toed. The orientation of the footprints with respect to the ripples shows the animal was efficiently battling its way against the current. "That suggests that it was a very good swimmer," Costeur says.
The dinosaur wasn't swimming, paleontologist David Fastovsky of the University of Rhode Island points out, since its toes were touching the ground, but Fastovsky thinks that it would have been fine in deeper water, too. "We suspected that some theropods could wade into the water and navigate around,” he says, “but finding an example has been difficult."
Don Henderson, curator of dinosaurs at the Royal Tyrell Museum of Paleontology in Drumheller, Alberta, says the track marks will be useful for understanding dinosaur biomechanics. "Trackways are fossilized behavior. They show what they're capable of and open up questions," he says. These footprints are consistent with a computer model Henderson has made of the general theropod body plan, which shows that the animals would float in water. With its toes just touching bottom, the animal probably had its head and neck exposed as well as its rigid tail. Because theropod tails were stiffened with ligaments, the animals could not have used them for propulsion, as crocodiles do. "This thing was doing a sort of dog paddle only using its hind limbs," Henderson concludes.
It's not clear which particular theropod made the tracks, but it was clearly big. The toe marks are typically 15 centimeters wide and 50 centimeters long, and the underwater stride was about 2.5 meters long. One candidate is Neovenator, which was 7-8 meters long and is known from the United Kingdom.