Researchers led by Joshua B. Smith of the University of Pennsylvania found fossilized remains of the gargantuan animal near an Egyptian desert oasis, scene of other notable dinosaur finds.
Smith and his colleagues identified the monster, which they say weighed more than 60 tons, as a Sauropod, a type of plant-eater, and named it Paralititan stromeri. Paralititan means "tidal giant." Stromeri refers to Ernst Stromer, a geologist who found dinosaur fossils in the area in the 1930s and took them to Germany where they were destroyed by Allied bombing in World War II.
The new discovery was to be published Friday in the journal Science.
"This tells us we had really, really big (plant-eating dinosaurs) hanging out on the coast of North Africa back then," said Smith.
He said it was known that Africa had at least three very large predators living some 90 million years ago, but until now scientists were puzzled where those massive meat-eaters got their food.
"Now we've found a 60-ton steak that they were eating," said Smith.
Only a few fossilized bones were found at the site, but Smith said they were enough to calculate the size of Paralititan. The key was finding the right humerus, a bone in the four-legged animal that corresponds to the upper arm bone in humans. The bone was 5-1/2 feet long.
By comparing that humerus to the more complete skeletons of other Sauropods, Smith said his team could estimate the size of Paralititan.
The animal was 80 to 100 feet long, from its nose to the tip of its long and powerful tail, and it weighed 60 to 70 tons, he said.
That size puts Paralititan second only to Argentinosaurus, a Sauropod discovered in South America that has been estimated to have been 90 feet long and to have weighed about 90 tons.
"Paralititan is in the same family as Argentinosaurus," said Smith. "They are closely related."
At the time the two giants lived, Africa and South America had only recently split apart, moving under the forces of continental drift. For this reason, Smith said it is likely that the two animals had a common ancestor that once roamed across the combined land mass of the two continents.
Although the discovery site in Egypt is now desert, Smith said once it was a dinosaur paradise, with ferns and mangrove-like trees growing beside a warm tropical sea. There were many other dinosaurs, along with fish, turtles and sharks, he said.
"It was probably like the southwestern coast of Florida, kind like the Everglades," said Smith. "It was a really productive ecosystem that was probably a perfect place for these guys."
Something about the area, he said, allowed animals to thrive and grow to immense size.
Later, the sea level fell and the continents shifted more. Now the discovery site is almost 200 miles from the Medterranean, on a desert plateau.
It may have been "dinosaur heaven," Smith said, but it was also a dangerous place for Paralititan.
Living at the same time and place were three of the biggest known dinosaur predators fast, toothy hunters that at 50 feet were bigger than the more famous Tyrannosaurus rex.
Smith said that while recovering the fossils of Paralititan, the researchers found the tooth of a Carcharodontosaur, one of the fiercest of the predators.
"The skeleton was torn apart," said Smith, suggesting that meat-eaters feasted on Paralititan after it fell on a beach, lapped by the tides. Just what killed the animal was not clear, he said.
Michael Parrish, a dinosaur expert at Northern Illinois University, said Paralititan is an exciting find because it fills in details about a poorly understood era.
"This helps our view of that part of dinosaur history, which is not well understood," said Parrish.
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