Evidence for the swimming dinosaur--a new species that remains unnamed--comes from its footprints, which have been preserved in a Wyoming rock formation dating to about 165 million years ago. At that time, a warm and relatively shallow body of water called the Sundance Sea covered a large part of the western U.S. Although scientists had previously found proof of swimming dinosaurs in other parts of the world and from other time periods, they had never come across evidence for dinosaurs in Wyoming at this time, much less ones that could swim.
The fossilized footprints are a series of tracks capturing the act of a bipedal, coelosaur-like dinosaur wading into water. At the beginning of the series, the footprints are fully formed, but gradually they become half-footprints and finally just claw marks, suggesting the animal became more buoyant as it entered into deeper water.
"The tracks suggest it waded along the shoreline and swam offshore, perhaps to feed on fish or carrion," says paleontologist Debra Mickelson of Colorado University in Boulder, whose team identified the tracks. Judging by the size of the tracks and their path into the water, Mickelson and her team estimate that the dinosaur was an ostrich-size carnivore.
The tracks are not the only ones Mickelson's team has found in the rocky outcrops of nothern Wyoming. The area contains millions of dinosaur tracks of different sizes all deposited around the same time, which suggests the animals traveled in packs and intentionally swam out to sea, most likely to feed.
Mickelson is now making tracks herself to search for bones and will also be looking for a name for the dinosaur.