"Getting the right conditions is quite rare," said Peter Galton, a retired paleontologist from the University of Bridgeport in Connecticut, who was part of the research team.
Part of what makes the new find interesting, Galton said, is that the footprints are very irregular.
The upstream leg appears to be moving straight ahead, but the other was pushing sideways in an effort to keep the animal from drifting downstream.
Not only do the tracks show that the dinosaur was nearly afloat, but they also indicate the type of swimming motion a T. rex or other theropod might have used.
"It is a style of amplified walking with movements similar to those used by modern bipeds, including aquatic birds," study co-author Loic Costeur of the University of Nantes in France said in a statement.
The study appears in the June issue of the journal Geology.
The discovery didn't come as a surprise to some.
"This is certainly an interesting find, but dinosaur experts have long accepted that theropods could swim (as can most animals when put in water)," Hans-Dieter Sues, a paleontologist at the Smithsonian Institution, said by email. He was not involved in the study.
Sues noted that in 1980 scientists found tracks in New England's Connecticut River Valley that appeared to be made by a swimming dinosaur.
But study co-author Galton said the New England tracks aren't definitive. Subsequent experiments revealed that lesser rheas, a species of ostrichlike bird, leave very similar footprints when running over nearly dry plaster of Paris. Therapods of the Cretaceous and modern-day rheas have similar physical builds.
Galton acknowledges that nobody really doubted that dinosaurs could swim.
"But this is the first scientific evidence that they could do what you think they could do," he said.
Taking to Water
The Spanish find may help dispel a decades-old myth that large web-footed dinosaurs, called hadrosaurs could escape their predators by taking to the water or swimming to an island.
Herbivorous dinosaurs may still have fled to such places. But, like modern bears, predators could simply swim out after them if they wished, according to scientists.
More information may come from additional swimming dinosaur tracks that have been discovered in Utah, Galton said.
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