The biggest and the baddest among meat-eating dinosaurs, Spinosaurus may have also been the first dinosaur to take to the water, swimming in North Africa's rivers some 97 million years ago, researchers reported on Thursday.
Floating like a crocodile to stalk prey, the 15.2 metre predator bore a massive sail on its back that would have risen from the water like a shark's fin. The carnivore probably ate fish, ancient crocodiles, and anything else afloat.
"It was the biggest carnivorous dinosaur, but Spinosaurus wasn't a land animal," says University of Chicago paleontologist Nizar Ibrahim, a National Geographic Society Emerging Explorer who led the discovery of the new fossils. "This was a creature adapted to life in the water."
Up to then, dinosaurs had ruled only the land. After 150 million years of dinosaur evolution, "suddenly we see these adaptations in Spinosaurus where it is able to swim," says University of Chicago paleontologist Paul Sereno, a co-author of Thursday's report in the journal Science describing new fossils that reveal how the fearsome ancient predator lived.
The fossil bones of Spinosaurus aegyptiacus described in the study turned up in sandstone beds in the Moroccan Sahara. Complete with skull, claws, and bones that formed the sail on its back, the fossils reveal a crocodilian snout, paddle-like feet, and dense bones that aided buoyancy, adding up to a life aquatic for the giant predator.
"All in all, the discoveries by this team show that Spinosaurus is an extremely unusual and specialized carnivorous dinosaur," says dinosaur expert Thomas Holtz of the University of Maryland in College Park, who was not part of the discovery team.
The fossil finds are also featured in the October National Geographic magazine, and will appear in a National Geographic/NOVA special on PBS in November.
"Spinosaurus has almost no 'junk in the trunk,'" Holtz says, noting its narrow hips and short thighs. "This doesn't make much sense for a land animal that makes a living chasing other land animals. But if it is an animal that doesn't spend most of its time on land, but instead in the water, it doesn't need strong leg muscles."
German paleontologist Ernst Freiherr Stromer von Reichenbach first discovered Spinosaurus in the Egyptian desert in 1912. But a World War II bombing of of Munich, Germany, destroyed his fossil finds, clouding scholarly research into the creature.
Picking up the hunt for more fossils, Ibrahim tracked down Stromer's records from archives and his family's castle in Bavaria. On his search, he came across a fossil collector who had turned up the new Spinosaurus fossils along Saharan desert cliffs called the Kem Kem beds in 2009. He traveled to the fossils' entombment site last year, hunting down the fossil collector to confirm the fossils' origins and accurately date them.
In the era of Spinosaurus, a vast river and swamp system stretched across North Africa, filled with lungfish, sharks, and crocodilian creatures, as well as dinosaurs. Spinosaurus enjoyed a steady buffet from this range, the study suggests, and it was ideally suited to catching fish, the fossils reveal.
Some chemistry findings from other spinosaur fossils, published in 2010, had already pointed toward Spinosaurus preferring a fish diet, notes paleontologist Eric Buffetaut of France's National Center for Scientific Research in Paris.
"But at the end of the day, you need to see the story told by the bones," says Ibrahim.
From tip to tail, he says, a digital reconstruction of those bones (done with funding provided by the National Geographic Society) tells a story of semiaquatic adaptation. The creature's skull sported small nostrils midway up the snout, perfect for breathing with the jaws submerged like a crocodile. Also similar to crocodiles, the tip of the snout possessed nerve and blood vessel channels, sensitive to sudden pressure changes in the water from fleeing prey.
Once prey was located, Spinosaurus's large, backward-slanted and conical teeth made perfect rakes for catching fish. Long, powerful front arms brandished hooked claws to catch anything those teeth missed.
A long neck and extended trunk served perfectly for brandishing those weapons. But these features probably made Spinosaurus too front-heavy to ever walk upright on two legs on land. Powerful, short back legs ended in flat paddle feet, possibly webbed, that helped the dinosaur swim but didn't make it swift out of the water.
On top of that, "when we saw very dense leg bones, we were surprised," Ibrahim says. Today, penguins typically have such dense bones to help them maintain buoyancy in the water, and heavy thigh bones may have helped Spinosaurus do the same.
Solid limb bones are unheard of in other giant theropod dinosaurs, such as Tyrannosaurus and Giganotosaurus, Holtz says. "Tyrannosaurus and other typical giant meat-eaters are balanced over their hips, sort of like giant teeter-totters," he says. "But it now seems that Spinosaurus was more buoyed by water, and could afford to be 'front-heavy.'"
A watery life may also help explain the massive sail that ran along the Spinosaurus spine, Ibrahim suggests. These crests were the tallest such sails of any known dinosaur, at about 1.8 metres high.
Some experts have seen the sail as a device for shedding heat, while others saw it as a kind of camel's hump for storing fat. But the sail appears to have had few blood vessels for shedding heat and was wrapped "snugly in skin" rather than fat, according to the study.
Most likely, the sail was a form of display to other spinosaurids, either a warning to rivals or a come-hither to potential mates, visible when swimming but unseen by underwater prey, Ibrahim suggests. "It would have been very visible to anything with its eyes out of the water," he says.
Sadly for fight fans, when Spinosaurus emerged on land, it probably didn't brawl with other predators, Sereno says, and certainly not with Tyrannosaurus rex. The two species lived continents (and millions of years) apart from one another, making a fight scene possible only in the 2001 movie Jurassic Park III, in which both species had been resurrected from extinction.
But several T. rex-like predators did live in the vicinity of Spinosaurus, such as the slashing toothed Carcharodontosaurus. "Big predators would likely have stayed away from fighting each other," Ibrahim says. "Whichever one got in the first big bite would have probably won a fight."
Mostly living in the water, Spinosaurus may have been more likely to tussle with the 25-foot-long (7.6 meters) sawfish of the era. Those were armed with a massive snout lined with jagged, spiky teeth called denticles. "That might have been very dangerous," Sereno says. "It was a predator-rich environment."