Fossils from Morocco gave an international team of paleontologists the opportunity to fill in huge gaps in their knowledge about Spinosaurus, one of history's scariest dinosaurs — and the result was a huge surprise: This dinosaur was adapted to swim in rivers and waddle on land.
"So far, Spinosaurus is the only dinosaur that shows these adaptations," said the University of Chicago's Nizar Ibrahim, lead author of a study detailing the find in the journal Science.
Spinosaurus' hind limbs were unusually small for a dinosaur — in fact, they were more like a duck's webbed feet. The long bones were particularly dense, more like a penguin's bones. Its long snout had prey-seeking pressure receptors to seek out prey in the water, like the receptors that crocodiles have. And the creature's center of gravity was so far forward that Spinosaurus was likely to have been most comfortable swimming.
"It's a chimera: half duck, half crocodile," said University of Chicago paleontologist Paul Sereno, a co-author of the paper.
With a length of 50 feet (15 meters), the sail-backed dinosaur ranks as the largest known land predator — even bigger than Tyrannosaurus rex. The first specimen of Spinosaurus aegypticus was discovered in Egypt a century ago by German paleontologist Ernst Stromer, but those fossils were destroyed in a World War II air raid.
Stromer figured out that Spinosaurus' long snout was well-suited for catching fish, but scientists assumed that the creature stomped around on land like T. rex. Spinosaurus lived in a different habitat and a different time — in North Africa instead of North America, and 95 million years ago instead of 65 million years ago. Nevertheless, Hollywood pitted the two monsters against each other in the movie "Jurassic Park III." (Spoiler alert: Spinosaurus won.)
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Even when the movie was made, paleontologists really had no idea what Spinosaurus' hind end looked like. Instead, they assumed that Spinosaurus aegypticus had hind limbs similar to those of earlier spinosaurs, such as Suchomimus.
The researchers' discovery had its roots in a chance meeting: While Ibrahim was visiting the Moroccan Sahara in 2008, an amateur fossil collector showed him a box of bones, including a blade-shaped bone with a peculiar reddish streak. A year or so, paleontologists in Milan showed Ibrahim a set of Spinosaurus bones with the same markings. That was a eureka moment — but it meant Ibrahim had to get back in touch with the Moroccan collector.
"That was like looking for a needle — not in a haystack, that would be easy — but in the Sahara," Ibrahim said. He finally caught up with the man in a Moroccan cafe.
The collector guided Ibrahim and his colleagues to a rich fossil site in a rock formation known as the Kem Kem beds. Back when Spinosaurus lived, that area was part of a large river system, populated by dinosaurs, crocodile-like creatures, sharks and giant coelacanths and lungfish.
"It's a real predator's paradise," Ibrahim said.
The team recovered scores of Spinosaurus bones, including parts of the skull and jaw, sail-like spines, forelimbs and hind limbs and the tail. They also pored over Stromer's detailed drawings of the Spinosaurus that was destroyed decades ago. They turned all those bones into a virtual 3-D model of the beast, and ended up convinced that Spinosaurus lived a semiaquatic life.
"Even the big sail of Spinosaurus may in part be an adaptation to life often spent in the water," Ibrahim said.
The paleontologists now suspect that Spinosaurus was following an evolutionary scenario similar to the one that played out for ancient whales: In both cases, the best explanation is that a type of land-based animal gradually adapted to life in the water.
"Little did we know that Spinosaurus had moved into a new ecological niche that resulted in some very peculiar adaptations," University of Maryland paleontologist Thomas Holtz, who was not involved in the latest research, told NBC News. "It shows that the world of dinosaurs still has some surprises for us."
Holtz said he wouldn't be surprised if scientists found out that other dinosaurs they assumed were strictly land-dwellers turned out to be semiaquatic, based on evidence yet to be found. And he said movie directors might have to revisit that "Jurassic Park" question: If Spinosaurus fought T. rex, who would win?
"Now the answer to the question depends," Holtz said. "Are they on water, or are they on land?"
If the battle took place in the water, Spinosaurus would win, hands down. But if they fought on land, Spinosaurus would just be waddling around, trying to get in a bite or two, "while Tyrannosaurus rex runs circles around it," Holtz said.