Scientist's Finds Spur New Thinking on Dino Evolution
National Geographic News
|February 19, 2002|
One after another, the new dinosaur discoveries that Paul Sereno and his
colleagues have made over the past decade are remarkable. But thrilling
as they are individually, the fossils also excite Sereno because they
are helping to fill in the big picture of dinosaur
What are such discoveries telling him?
For one thing, that the breakup of the ancient supercontinent Pangaea was not rapid, as widely believed, but slow enough to drastically limit dinosaur evolution in the region.
The pace of the continental breakup, he argues, had distinct evolutionary impacts on dinosaur characteristics and lineages.
Fossils, when viewed together, hold clues to broad patterns of evolution. Since the start of his career in the 1980s, Sereno has been interested in reconstructing the dinosaur family treethe "phylogeny" of dinosaurs. He hopes to map dinosaur descent by tracing the many evolutionary changes recorded in dinosaur skeletons.
Calling dinsosaurs "a greatly underserved group," Sereno predicted at a lecture this week in Boston that soon "we will see answers to major evolutionary questions that have long been unaddressed." The University of Chicago paleontologist, who is a National Geographic Society explorer-in-residence, was speaking at the annual meeting of the American Association for the Advancement of Science.
Among the questions Sereno is working to understand is the role of biogeography in dinosaur evolutioncontinental drift and possible migration over temporary land bridges. "This was a global group, entirely land-based," he said. "How did something as drastic as the breakup of the supercontinent affect evolution?"
Another line of inquiry: what accounts for the high levels of diversity that occurred among dinosaurs well before they globally dispersed and became the dominant animals on Earth until their extinction 65 million years ago.
"What exists is a very different course from [that of] mammals and other groups," Sereno said. "It's almost like a Cambrian radiation in terms of tremendous diversity."
Wide Spectrum of Dinosaur Life
Scientists' understanding of dinosaurs has advanced by leaps in recent years, Sereno noted, because of new discoveries that shed light on dinosaurs from when they first appeared, in the middle Triassic, to their final radiations at the end of the Cretaceous.
Sereno and his colleagues have contributed significantly to those findings, especially through their excavations in Africa, where harsh conditions had limited dinosaur hunting until recent years.
Soon after beginning field work in Argentnia in the late 1980s, Sereno and his teams discovered highy complete skeletons of two of the oldest dinosaurs ever found. The fossils of the two theropods, known as Herrerasaurus (named for a member of the expedition) and Eoraptor ("dawn raptor"), dated from 228 million years agothe era when dinosaurs first arose.
Throughout the 1990s, expeditions to Niger and Morocco bought more stunning discoveries, which included:
Afrovenator abakensis ("African hunter"), the most complete skeleton of the Cretaceous era ever found in Africa, 130 million years old
Carcharodontosaurus, a 90 million-year-old shark-toothed predator that was at least as big as Tyrannosaurus but had a brain only half as large
Deltadromeus ("agile delta runner"), a 90 million-year-old predator with very delicate long limbs
Suchomimus tenerensis ("crocodile mimic from the Ténéré"), a 100 million-year-old sail-backed dinosaur with a long crocodile-like snout that it used to catch fish
Jobaria, a 135 million-year-old "graceful giant" sauropod with spoon-shaped teeth
Nigersaurus taqueti, an odd sauropod 110 million years old with 600 teeth and a mouth described as being "like a Hoover vacuum"
Recently, Sereno and his colleagues reported the discovery of other new and unsual dinsosaurs in Africa, along with several new crocodilians, ranging from 40-foot Sarcosuchus, or "Supercroc," to a dwarf croc the size of a cookie.
"Such finds are rapidly filling in Africa's dinosaur world during its phase of isolation during the Cretaceous," Sereno said.
Sereno is especially intrigued by some of the Africa fossils because many of the dinosaurs lived at a time when it's widely believed that the supercontinent Pangea was breaking apart to form the continents we know today. One land mass, which eventually separated into Africa and South America, drifted south, while another moved north.
According to most estimates, the continental breakup occurred about 150 million years ago. But Sereno's findings are at odds with that assumption. His evidence shows that some of the African dinosaurs dwelled tens of millions of years after the start of the continental drift, yet they're more closely related to North American dinosaurs than to dinosaurs from the southern land mass.
The results, he said, suggest that the break was not complete and intercontinental land bridges may have existed much longer than thought.
In a 1999 report in the journal Science, Sereno said: "I think there was some kind of a tenuous land bridge [linking Europe and Africa] for several million years" after initial breakup of Pangaea. "That land mass prevented the evolution, in isolation, of a unique southern dinosaur fauna."
Another question that arises is why distinct families of dinosaurs lived side by side during the Lower Cretaceous but apparently evolved at dramatically different speeds. Jobaria, for example, seemed not to have changed its anatomy much at all through millions of years, resembing its ancestors from much farther back, in the Triassic.
Why was Jobaria so well adapted that it didn't need to change? It's one of many evolutionary mysteries that remain to be answered.
New Tools of Analysis
Fossils are critical evidence in piecing together the evolution of dinosaurs because the anatomical features reveal relationships between different groups. What kinds of features do they share, and what defining characteristics differentiate various groups?
Paleontologists use a different kind of family tree, known as cladistics, to aid such research. The system classifies animals into like groups based on shared characteristics, showing possible evolutionary steps between different groups.
Another valuable new tool is the relatively new science of systematics, which emerged about the same time Sereno was starting his career in the 1980s. Systematics makes it possible to assemble a huge database of anatomical and other details about organisms and analyze them to look for patterns, similarities, and differences that point to large-scale rules at work.
With more comprehensive data and computer simulations, the current fragmented picture of dinosaur evolution will gradually give way to a more coherent and global understanding, Sereno noted.
As that happens, it should help Sereno answer many major evolutionary questions about dinosaurs that are the focus of new work.
"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 by comparison to mammals during the Cenozoicwhere are the burrowers, the climbers, the aquatic specialists?"
The answers, he suggested, lie in the posture and body size of early dinosaurs and the constraints these imposed on all subsequent evolution.
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