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Dinosaur Expert Paul Sereno Describes Giant "Fern Mowers" and Tiny "Duck Crocs"
News of a fern-mowing dinosaur--45 feet long and sporting 1,000 needle-shaped teeth--took journalists by surprise at the 2002 AAAS Annual Meeting in Boston on Monday.
A plant-eating sauropod, Nigersaurus roamed the African desert some 90 million to 100 million years ago, dinosaur expert Paul C. Sereno reported. With its hammer-shaped skull and stacks of small, narrow teeth, Nigersaurus was a massive, Mesozoic fern mower, said Sereno, a professor of organismal biology and anatomy at the University of Chicago in Illinois.
Sereno’s latest discovery was unearthed in Niger, where his team also recently found a dinosaur-munching crocodile, Sarcosuchus imperator. According to Sereno, Sarcosuchus--the so-called "super-croc"--may have preyed upon Nigersaurus, a relatively slow-moving plant-eater, as it noshed along river banks.
Super-croc may have snacked on tiny "duck crocs," too, Sereno said. With a skull less than four inches long and a gap-toothed grin reminiscent of talk-show host David Letterman, the duck-like crocodile was no doubt a tasty snack for Sarcosuchus, around 110 million years ago.
Also during his AAAS appearance, Sereno displayed the "wishbone," or furcula from a primitive carnivore--a spinosaur called Suchomimus. Though the wishbone played a very different role in dinosaurs and birds, Sereno said, it establishes an evolutionary link between these creatures.
Sereno’s ongoing field work in Africa has yielded a menagerie of new dinosaurs. These discoveries have included the giant predator, Carcharodontosaurus, which rivaled Tyrannosaurus in size.
Another find was the fleet-footed meat-eater, Deltadromeus, that has no close counterpart on other continents; and Suchomimus, the famous spinosaur whose furcula made an appearance at the AAAS Annual Meeting. Sereno has also reported giant long-necked plant-eaters, found in a communal death site, included the 60-foot long Jobaria.
"Such finds are rapidly filling in Africa's dinosaur world during its phase of isolation during the Cretaceous," Sereno noted.
Discoveries of the past decade have transformed the fossil record of dinosaurs, he added--from their first appearance in the middle Triassic, to their final radiations at the end of the Cretaceous. "The fossiliferous sequence of middle to late Triassic beds in northwestern Argentina provides a very complete look at the earliest dinosaurs and the timing of the dinosaurian radiation," he said.
Primarily on the basis of his fossil evidence, which includes very complete skeletons of the early theropods, Eoraptor and Herrerasaurus, Sereno proposes that the initial radiation of dinosaurs significantly preceded their global dominance in diversity and abundance.
Fossil discoveries help establish a global picture of dinosaur evolution, and they underscore the role of large-scale extinction in shaping that world, according to Sereno. Transient land corridors provided intercontinental bridges that also strongly affected the evolution of dinosaurs. Dinosaurs, as exclusively land-dwelling animals with a global distribution, provide the best test-case for how evolution responds to the break-up of a supercontinent.
Other major evolutionary questions involving dinosaurian evolution are the focus of new work by Sereno:
"Why did it take 50 million years for dinosaurian predators and herbivores to reach their maximum body size but mammals only a handful?" he asked. "And, why is there so much empty ecospace during the Mesozoic, in comparison to mammals during the Cenozoic? Where are the burrowers, the climbers, the aquatic specialists?"
The answers, Sereno suggested, lie in the posture and body size of early dinosaurs and the constraints these imposed on all subsequent evolution. Computer simulation of the fragmenting dinosaur world, Sereno says, will help us unravel the large-scale rules at work.
-- Ginger Pinholster