It was big. It was bad. And, surprisingly, it was a better hunter in water than on terra firma. Meet Spinosaurus aegyptiacus – the first theropod dinosaur found that sported a body better adapted to a life in the water than on land.
Even before considering its mix of features, unique among theropods, S. aegyptiacus clearly was a monster. The reconstruction of a nearly complete skeleton of the creature measured about 50 feet from the tip of its snout to the end of its tail and is thought to have weighed around 12 tons – the biggest top-of-the-food chain dinosaur yet described.
Its short arms were tipped with curved, sharp claws. Its snout was long and narrow, and its jaws lined with sharp teeth that seem better suited to grabbing and holding than at tearing. It sported a sail along its back supported by a row of vertical spines that reached seven feet tall at the sail's highest point.
But what captured the attention of the team describing the creature's features were its adaptations to water, making it a giant "crocoduck," if you will.
"You're looking, sort of, at a model of a duck, something that's paddling with its hind legs," says University of Chicago paleontologist Paul Sereno, a member of the science team reporting its analysis of the creature's fossil remains in Friday's issue of the journal Science.
"But then ... it is like a crocodile in having a propulsive tail. It's a chimera, half duck and half crocodile. We don't have anything alive like this animal that we can use alone as a model."
S. aegyptiacus roamed what is now North Africa some 97 million years ago. The first specimen was discovered in Egypt in 1911 by German paleontologist Ernst Stromer, who published a description of it four years later. He found strange backbones with spines, a long, unusual lower jaw that seemed well-suited for catching fish, and enough of the skull to suggest that the skull itself was elongated. The fossilized bones were large enough to suggest that the creature was bigger than Tyrannosaurus rex.
His specimens were destroyed during an Allied bombing raid in 1944, but Stromer also was an expert anatomist and had made exquisitely detailed drawings of the fossils, which survived the war.
"People were finding other bits and pieces of Spinosaurus in Africa – a vertebra, some more skulls, some arm bones, but we still didn't know what the whole creature looked like," says Thomas Holtz Jr., a paleontologist at the University of Maryland at College Park, who did not take part in the study.
In the meantime, more-complete fossils of more primitive Spinosaurs were cropping up in Britain and Africa. These gave researchers a clearer idea of what these creatures were like, he says. They had croc-like head but legs that weren't much different than a garden-variety theropod. Like other theropods, and their descendants, modern birds, they had hollow bones. They likely walked the countryside or waded into the shallows looking for something meaty to snack on.
S. aegyptiacus had a snout with some unusual features, compared with the more primitive relatives. For instance, its nostrils are located well up the length of its snout, much closer to its eyes than those of its older relatives. Still, "we thought of it as sort of a wading dinosaur," Dr. Holtz says, of S. aegyptiacus.
The team reporting these latest finds have the legs as well as one fairly complete skeleton of S. aegyptiacus, "and boy, were we wrong," Holtz says.
Unlike its ancestors, S. aegyptiacus bears a range of unique features that represent adaptations to spending much of its time in the water.
For instance, its long neck and trunk shift the creature's center of mass forward, which made swimming easier. Its small hip bones and short hind legs suggest muscular thighs that, if the feet were webbed, would have made it a powerful paddler. The nostrils high up the snout would have been the creature's snorkel as it tried to grab prey with its mouth. Like crocodiles, S. aegyptiacus appears to have had receptors at the end of its snout sensitive to change in pressure, a way to detect the presence of fish in murky water. Unlike other theropods, the creature had dense, solid bones with no space down the center for marrow, a feature that would have affected its buoyancy in water.
Those waters – typically large, broad rivers – were teeming with fish. But the region was no Riviera. In the water, car-length coelocanths swam alongside sawfish 23 feet long, huge lungfish, and from six to eight species of shark.
"It's not the kind of place you'd want to go for a swim," quips Nizar Ibrahim, a paleontologist at the University of Chicago and the lead author of the Science paper describing the adaptations.
The fossils were unearthed between 2008 and 2013. In addition, the team used partial fossils from museum collections to build its reconstruction. This has led some researchers to worry that the reconstruction is really a hybrid of different animals.
But the reconstruction "looks pretty solid," says the University of Maryland's Holtz. The foundation comes from "a good part of at least one skeleton, for the first time," he says. And while they are also including elements from other specimens, "we've recognized other, isolated elements of Spinosaurus in the past," so including them in the analysis "is not that unusual."
If there is a quibble, it's the team's attempt to use its reconstruction as the archetype of S. aegyptiacus, Holtz says. Typically, that honor goes to specimens whose fossils all come from the same spot where the original was found, in this case from near where Stromer found his specimens in Egypt. It might be possible that the animal from Morocco could be a closely related regional relative.
That said, "it's probably the same species, for all we can say," he adds.
"We have plenty of other fish-eating theropods," he says. But S. aegyptiacus is unique, even compared with its ancestors.
The earlier ancestors were striking their aquatic prey from above, he says. "This thing's coming at them from the side, or for all we know it's coming at them from below."
It might not be terribly agile, he says, "but still, I wouldn't want to be in the water with it."