Wednesday, September 1, 2010

Big, fast and vicious: a pair of meat-eating dinosaurs from Moroccocould force a rethinking of ancient geography.

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Digging up dinosaur bones is always a grueling business, but conditions in the Kem Kem wilderness of southwest Morocco last summer were especially bad. The temperature soared to 120 degrees F almost every day, and the fossils were hundreds of feet up, poking out of the dusty face of a sandstone cliff. "It was," says Paul Sereno of the University of Chicago, who led the joint U.S.-Moroccan expedition, "the most brutal fieldwork I've ever done." Worse yet, the team wasn't finding much--lots of moderately interesting bits and pieces but nothing even close to a major discovery.

Then, on the 41st day, a young American named Gabrielle Lyon--a writer, as it happened, not a professional scientist--came upon a few small bones embedded 500 ft. up in the 92 million-year-old stone. Within a few days, the explorers had unearthed the virtually complete skeleton of a previously unknown predator. It was a 27-ft.-long monster similar in size to the North American Allosaurus but far leaner and presumably quicker as well; the scientists named it Deltadromeus agilis, or "agile delta runner." A few days later, Sereno came up with an even more spectacular find: the skull of another meat eater whose head, at least, was a shade bigger than that of the 45-ft.-long Tyrannosaurus rex, traditionally considered the largest predator ever to have walked the earth.

Super size and speed make the finds perfect fodder for the evening news and for the imaginations of eight-year-old dino devotees. But scientists are more interested in what the discoveries, reported last week in the journal Science and in National Geographic, say about where dinosaurs lived and how they evolved. "We have a pretty good record from North America, Asia and South America," says paleontologist Mark Norell of the American Museum of Natural History in New York City. "But we're just now starting to get a picture of the dinosaur fauna of Africa."

Scientists are finding lots of surprises. The facial bones and teeth of the tyrannosaur-topping dinosaur, it turns out, are the same as those of a rare species, called Carcharodontosaurus saharicus ("shark-toothed reptile from the Sahara"), which were found in Egypt in the 1920s but destroyed during World War II. The new skull not only establishes the animal's size but also proves that the huge dinosaur had a comparably huge home range that spanned all of North Africa.

Even more surprising, while both of the newly discovered dinosaurs are unique to Africa, they seem to be related to species found in the Americas. When the dinosaurs began their reign 220 million years ago, all the planet's landmasses made up a single supercontinent known as Pangaea. Because land animals could move and mate at will, dinosaurs from that era look pretty similar all over the world. But by the end of the Jurassic period, about 150 million years ago, continental drift had torn Pangaea into a northern and a southern half, Laurasia and Gondwana. At that point, dinosaurs in each hemisphere should have started evolving along divergent paths. The African dinosaurs should thus most closely resemble those found in South America.

Instead, Carcharodontosaurus closely resembles certain North American carnivores. This implies that the species were exchanging genes well into the early Cretaceous period, which ended perhaps 100 million years ago. Did a bridge of land connect Laurasia and Gondwana after the rest of the landmasses had mostly separated? That's what the new evidence suggests. By 90 million years ago, the separation of continents was evidently complete; Deltadromeus and other African dinosaurs from that period are quite distinct.

Finally, while size records aren't supposed to be high on paleontologists' agenda, the immensity of C. saharicus does bear on dinosaur evolution, especially when it's put into context. Last fall paleontologists working in South America found a similarly huge carnivore, called Giganotosaurus, that was said to be a little larger than T. rex (in fact, the sizes of all three giants probably overlapped). "What's interesting," observes Norell, "is that everywhere you go in the world you have these truly enormous carnivorous dinosaurs that were much larger than any terrestrial carnivores since." One implication, says Sereno: "I think we're looking at the Olympic-size limit for this sort of animal. It's telling us that this is the largest they could grow and still survive." Or at least, that's where the evidence points today. Given that Africa's dinosaurs are just now coming to light, it's too early to draw conclusions. An even bigger predator could be lurking under the Moroccan sandstone, just waiting for the next field season.

Source Citation
Lemonick, Michael D. "Big, fast and vicious: a pair of meat-eating dinosaurs from Morocco could force a rethinking of ancient geography." Time 27 May 1996: 45. Academic OneFile. Web. 1 Sept. 2010.
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Gale Document Number:A18299971

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