It ate whole sharks, and came equipped with feet like paddles and sealable nostrils that allowed it to swim under water. Spinosaurus, the largest known predatory dinosaur, now turns out to be the only known dinosaur that spent most of its time swimming. While palaeontologists envisaged it as a sort of gigantic heron, it was actually an enormous version of a crocodile.
“This is the first and only dinosaur we know of that has clear and unmistakable adaptations for a semi-aquatic lifestyle,” says Nizar Ibrahim of the University of Chicago. “It’s possible that we’ll find other dinosaurs with similar adaptations in the future, but right now, Spino is the only one.”
Spinosaurus was around 15 metres long, 3 metres longer than the more famous Tyrannosaurus rex. Fossils of Spinosaurus were first discovered in north Africa a century ago by German palaeontologist Ernst Stromer. But they were displayed in a Munich museum, and were destroyed in an Allied air raid in 1944. Since then only fragmentary remains have turned up.
Over the past decade, Ibrahim and his colleagues found more Spinosaurus bones in the Kem Kem beds of Morocco, a rich trove of fossils. The new fossils include parts of Spinosaurus that have not been seen before. “Altogether, we have over half the skeleton,” says Ibrahim.
Almost like a crocodile
“The leg bones were a crucial piece of evidence,” says Ibrahim. “Spino had paddle-like feet,” similar to those of water birds, “and dense leg bones.”
That is typical of a swimming animal. Dense leg bones would have served as ballast, weighing down Spinosaurus‘s rear. That would have kept its body horizontal and allowed the animal to manoeuvre its arms, neck and head while hunting prey. Whales, manatees and penguins have similar adaptations.
Spinosaurus‘s legs were also very short relative to its body, particularly the calf bones, and its hips were small. That meant its centre of balance was shifted so far forward that Spinosaurus could only walk on land on all fours.
Finally, Spinosaurus‘s nostrils were set far back in its skull, allowing it to breathe while almost submerged. Even better, the nostrils could close, keeping out water when it swam beneath the surface.
“We’ve known about Spinosaurus for more than a century, yet we weren’t able to fill in the details showing how much of an aquatic dinosaur it was,” says Thomas Holtz of the University of Maryland in College Park. “It has abandoned life on land to become an aquatic predator instead, which was very much unexpected.”
Not like a heron
The new fossils change our picture of Spinosaurus. Palaeontologists had thought it lived on land and hunted by standing in shallow water waiting for prey to pass.
“Previous interpretations have looked at Spinosaurus as something akin to a 13-metre-long great blue heron with a bad attitude,” says Ken Lacovara of Drexel University in Philadelphia, Pennsylvania. Now it seems it was more like a crocodile.
A semi-aquatic lifestyle would make sense, says Lacovara, as at the time north Africa was a huge tropical swamp. “Spinosaurus inhabited a low-lying mangrove forest.”
There was plenty of prey, including large sharks. There is no direct evidence that Spinosaurus ate the sharks, but Ibrahim says it probably did. “The skull anatomy tells us that Spinosaurus preyed on large aquatic vertebrates in the Kem Kem river system, and that includes sharks,” he says.
Spinosaurus is not the only extinct reptile that proves to have had an affinity for water. A newly discovered species of pterosaur, or flying reptile, had a pouch on its lower jaw like those of modern pelicans. It lived 120 million years ago in what is now north-east China (Nature Scientific Reports, DOI: 10.1038/srep06329).
(Image: Chuang Zhao)
The researchers who uncovered the fossil found a unique hook on the lower jaw that probably served as an anchor for a throat sac. They suspect the animal used the pouch to scoop up fish as it skimmed the water surface.
They named the pterosaur Ikrandraco avatar after the flying “ikran” creatures in the movie Avatar.
Journal reference: Science, DOI: 10.1126/science.1258750