Rachel Sullivan
The 15-metre long carnivorous dinosaur Spinosaurus aegyptiacus lived 95 million years ago and is the only known example of a semi-aquatic dinosaur.
"There were lots of aquatic reptiles, such as ichthyosaurs and plesiosaurs, but until this discovery it was thought that dinosaurs hunted only on land," explains paleontologist and study lead author Dr Nizar Ibrahim from the University of Chicago.
"The discovery shows that dinosaurs were much more diverse and adaptable than previously thought."
The paper is published in Science today.
Spinosaurus was first discovered a century ago by German paleontologist Ernst Freiherr Stromer von Reichenbach in Egypt. However, the partial skeleton he found were destroyed during Allied bombing of Munich in 1944.
Recently, archeologists have discovered new Spinosaurus fossils along desert cliffs in Morocco.
The area was once home to an extensive river system, where coelacanths the size of cars lived alongside seven metre long sawfish, and three metre long lungfish. Freshwater sharks and crocodile-like predators, some as long as a bus, also lived in the river system.
"To hunt successfully in territory occupied by animals this size, Spinosaurus would need to have been big to have survived," Ibrahim says.
Fish-eater
Ibrahim and colleagues CT scanned the new fossil finds and digitally pieced the skeleton together. They filled in the blanks using information from museum collections and Stromer's notes, sketches and photos.They found that Spinosaurus had a range of adaptations for its aquatic lifestyle, including a long slender jaw and cone shaped teeth that were ideal for capturing slippery prey.
"Its nasal opening was also much further back allowing it to breathe when the skull is submerged. Pressure receptors on the tip of its snout were similar to those used by crocodiles today to detect prey movements," Ibrahim says.
"It has fish-eater written all over its skull."
The rest of its body had strange proportions too, he adds. "Its pelvis was small and it had short, muscular hind legs - adaptations that would have made it easier to paddle in the water and which are only seen in animals that returned to the sea, such as the ancestors of whales."
Spinosaurus also had broad feet and long flat claws to help it paddle through soft mud and a very flexible tail that it would have used in a side-to-side motion like a crocodile.
The final evidence for its aquatic lifestyle came from inside the bones, which lacked the hollow marrow cavity typically found in predatory dinosaurs.
"Instead the bone was very dense, something that we see in penguins and manatees and is thought to help with buoyancy control."
The function of the animal's sail, which would have likely protruded prominently from the water, is still under debate, but Ibrahim thinks it may have served as a warning to other predators not to enter a spinosaur's territory.
Life-size replica printed
Once the digital model was complete, the researchers 3D-printed a life-size replica of the skeleton."We knew from looking at it on the monitor that this animal was really big," Ibrahim says, "but it isn't until you are standing face to face with it that you get a true sense of its size."
The model will become the centerpiece of an international traveling exhibition, while the original bones will be returned to Morocco.
Finding other semi-aquatic predatory dinosaurs in the future isn't out of the question, says Ibrahim, but cautions would-be dinosaur hunters that they are rare.
"Most predatory dinosaurs looked a lot like T. rex. We see the same blueprint used over and over again. Spinosaurus was a major departure from that."
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