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First ever evidence of a swimming, shark-eating dinosaur

University of Chicago paleontologist Paul Sereno describes Spinosaurus aegyptiacus, a predator of the Cretaceous Period that had adapted to life in water 95 million years ago. Sereno, his UChicago colleague Nizar Ibrahim, and an international team of associates announced their discovery in the journal Science online, at the Science Express website, on Thursday. (Video: UChicago Creative)
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When it wasn't putting T. rex to shame, the dinosaur Spinosaurus spent its time swimming -- and chowing down on sharks.

Until now, scientists didn't have any proof that there were swimming dinosaurs. There were some marine reptiles prowling the seas, to be sure, but paleontologists couldn't find fossils that put dinosaurs in the water.

New fossil evidence published Thursday in Science changes that, and the  Spinosaurus aegyptiacus is breaking records left and right. It's now the largest predatory dinosaur to have ever roamed the planet — nearly 10 feet longer than the largest T. rex specimen — although the carnivore was still dwarfed by some of its plant-eating contemporaries. But more importantly, Spinosaurus has the distinction of providing our first ever evidence for a semi-aquatic dinosaur.

Spinosaurus was discovered in the Sahara more than a century ago by German paleontologist Ernst Freiherr Stromer von Reichenbach, but all of his fossils were destroyed during World War II.

When a partial skeleton was uncovered in the Moroccan Sahara -- in a place once home to a massive system of rivers full of all sorts of sharks and other predators  -- scientists had a new clue that there was something fishy about the massive dino.

In addition to revealing a record-breaking length, digital modeling of the skeleton suggested a whole fleet of aquatic adaptations. Tiny nostrils, placed far back on the middle of the dinosaur's skull, presumably allowed it to breathe as it swam at the surface. It also had openings at the end of its snout that are reminiscent of ones in crocodiles and alligators. In the modern animals, these openings house receptors that let them sense movement in the water.

Huge, slanted, interlocking teeth seem perfectly shaped to catch fish, and hook-like claws would have been ideal for catching hold of slippery prey under the water. Big, flat feet (perhaps even webbed) would have been well-suited to paddling water or stomping through mud, and some unusually dense limb bones (more like those seen in penguins than those found in other dinosaurs, the researchers report) would have allowed it to keep itself under the water, instead of floating.

The dinosaur's skeletal shape indicates that it would have been a strange sight to us on land. The Spinosaurus's center of gravity was pushed forward by its long neck, so it was almost certainly impossible for it to walk on two legs. In fact, the Spinosaurus's legs and pelvis are quite like those seen in early whales -- much better for paddling than for walking. Like whales, these dinosaurs probably evolved from land-dwelling ancestors to become semi-aquatic.

Scientists aren't quite sure how Spinosaurus moved when it left the water -- which it must have done, at the very least, to lay and nest eggs. Spinosaurus didn't have the kind of limbs that scientists would expect in a four-legged animal, but it also couldn't have balanced on its hind legs for very long.

"I think that we have to face the fact that the Jurassic Park folks have to go back to the drawing board on Spinosaurus," co-author and University of Chicago paleontologist Paul Sereno said in a teleconference held by Science on Wednesday. "It was not a balancing, two-legged animal on land. It would have been something very peculiar."

This isn't to say that Spinosaurus wouldn't have been an impressive sight on land. "It would have been a fearsome animal. There's no question about it, you would not want to meet this animal on land," Sereno said. "But it was not gallivanting across the landscape."

While paleontologists continue to puzzle over how the Spinosaurus managed to walk, you can visit a life-size skeletal replica of the creature at the National Geographic Museum in Washington. The exhibit will run Sept. 12th through April 12.

Read more: The hunt for Spinosaurus