Skeletal reconstruction. New bones (red), other bones isolated in Kem Kem (yellow), 1912 bones (orange), other spinosaurids (green), inferred from adjacent bones (blue) / T. Keillor, L. Conroy, & E. Fitzgerald, Ibrahim et al., Science/AAAS
Janet Fang 11 Sep 2014, 22:38

Finally, meet Spinosaurus aegyptiacus, a fantastic swimmer and the biggest meat-eating dinosaur ever discovered. This 95-million-year-old Cretaceous predator had a massive, spiny sail on its back, and it paddled in the river, eating sharks. Whole sharks. The findings were published in Science this week. 

The very first S. aegyptiacus fossils were discovered about a century ago in the Egyptian Sahara, and it was named “spine reptile.” Those first fossils were housed at the Bavarian State Collection, but were destroyed in World War II during an April 1944 Royal Air Force raid of Munich. “We had to wait close to 100 years for a new skeleton,” Nizar Ibrahim from the University of Chicago says in a telebriefing statement. “The animal we are resurrecting is so bizarre, it is going to force dinosaur experts to rethink many things they thought they knew.”

Paleontologists working on desert cliffs called the Kem Kem beds in eastern Morocco recently unearthed a more complete set of fossils -- including portions of a skull, axial column, pelvic girdle, and limbs -- that suggests the dinosaur was semi-aquatic. This is a first: Dinosaurs were thought to be terrestrial, having never colonized aquatic environments like marine reptiles. 

Combining CT scans of new fossils with museum specimens and old sketches, Ibrahim and colleagues created a digital model of an adult specimen. Hunting down all these fossils (new and seemingly lost), along with century-old notes “was like searching for a needle in a desert,” Ibrahim says in a news release. The team also filled in the blanks by inferring from adjacent bones and using surrogates modeled after other spinosaurids like Suchomimus, Baryonyx, Irritator, and Ichthyovenator. Here's the full-length digital reconstruction with transparent flesh outline: 

The team revealed that S. aegyptiacus was more than 15 meters long -- three meters longer than T. rex -- and had a whole suite of aquatic adaptations that very likely allowed it to hunt prehistoric sharks, sawfish, and lungfish in an once-large river system stretching across northern Africa. 

Its flat feet, which were possibly webbed, propelled it through water. Dense bones without marrow cavities (like penguins) enabled buoyancy control. Its center of gravity was shifted to the rear to facilitate swimming, along with its long neck and the loosely-connected bones of the tail. Compared to closely-related species, it had a small pelvic girdle and short, muscular hind limbs for paddling in water, like the earliest whales. On land, it walked on all fours. 

The carnivore could also retract its small, fleshy nostrils to the top of its head to breath when partially submerged, and it had pressure-sensing openings at the end of its snout, like crocodiles and alligators. It had slanted, conical teeth that interlocked at the front and blade-like claws; both features are well-suited for catching and hooking slippery prey. To the right is a skull reconstruction (new bones in blue). 

Finally, its enormous dorsal spines were likely covered by skin -- creating a huge sail that probably helped anchor muscles and was definitely visible when the dinosaur was in the water. "Every time I see the sail on Spinosaurus,” Ibrahim tells Popular Mechanics, “I'm going to hear the Jaws theme in my head.”

Images: Tyler Keillor, Lauren Conroy, and Erin Fitzgerald, Ibrahim et al., Science/AAAS (skeleton), Davide Bonadonna, Ibrahim et al., Science/AAAS (skull)

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