Fossils Shed Light on Dinosaur Evolution
Remains of two large carnivorous dinosaurs--including a shark-toothed skull more than five feet long--have been unearthed from the Sahara, scientists announced Thursday.
Experts say the specimens, extracted from the red sandstone of an ancient river delta in Morocco, are the most complete to be discovered in Africa.
The finds offer a rare glimpse into the last, long chapter of dinosaur evolution, when the huge animals were forced to adapt after the continents broke apart eons ago and, like life rafts, drifted apart. Coupled with other recent discoveries in South America, they offer evidence that the continents may have remained linked for millions of years longer than many scientists had believed.
The discoveries also suggest that when dinosaurs in Africa finally were isolated by continental drift from sister species in North America, Europe and Asia, they abruptly took a “previously unrecognized” evolutionary path, the researchers said.
Phillip Currie, an authority on meat-eating dinosaurs at the Tyrrell Museum of Paleontology in Alberta, called the remains of the two 90-million-year-old beasts of prey “pretty amazing.”
The fossils were discovered last year by a University of Chicago team led by paleontologist Paul C. Sereno and Moroccan paleontologist Mohammed Iarochene. Along with the dinosaur specimens, the researchers found hundreds of bones, crocodile teeth, crayfish, turtle shells and other fossil remains.
The find was made public Thursday and is published today as a research paper in Science.
“What we wanted to do--and succeeded in doing--is to get a snapshot of dinosaur life on this isolated continent and how these floating [continental] plates affected dinosaur evolution,” Sereno said.
“I think it is a landmark in African paleontology,” he added.
The fossil remains were discovered in the Kem Kem region of Morocco’s Sahara. The expedition was sponsored by the David and Lucille Packard Foundation, the Eppley Foundation for Research and National Geographic magazine.
Dinosaurs roamed the region at a time when today’s rocky desert floor was a lush, forested flood plain with a river delta threading through its muddy flats. There, generations of dinosaurs, searching perhaps for water, left their tracks and their remains for modern researchers like Sereno to uncover.
Today, conditions around the site are so arid that in the first weeks of digging, the researchers sweated off as much as 25 pounds each, while the razor-sharp limestone rubble from the cliffs towering above cut their shoes to shreds.
The researchers combed the area for almost a month last spring, sifting tons of siltstone and sandstone fragments before discovering the first significant skeletal remains. Team member Gabrielle Lyon spotted a set of slender toe bones--each about 6 inches long--embedded in the rocky hillside.
“We were stunned,” Sereno said.
Digging carefully into the cliff, the team uncovered the fossil bones of a lean, sleek predator at least 25 feet long, with grasping hands and hind limbs that suggested it could accelerate quickly to chase down its fleeing prey.
The researchers quickly determined that it represented an entirely new species and named it Deltadromeus agilis, which means “agile delta runner.” It is an early offshoot from the line of theropod dinosaurs that in the Northern Hemisphere gave rise to Tyrannosaurus rex.
Less than a week later, the scientists found the second, larger dinosaur when Sereno stumbled across the back of its fossil skull protruding from the face of the cliff.
“My heart was pounding. I was overjoyed,” he said. “We knew we had something big, but we did not realize how big until we got back to the lab and started to clean the bones.”
Scientists took two months to piece together the skull from a jigsaw of more than 400 pieces. When they were finished, they had before them a skull 5 feet, 4 inches long, with distinctive, wrinkled teeth resembling those belonging to a modern great white shark.
The large skull appears to belong to an animal about 45 feet long. The Tyrannosaurus rex is thought to have ranged in size from about 40 feet to 50 feet.
By its teeth, researchers identified the huge beast as belonging to a species that had been discovered in Egypt at the turn of the century called Carcharodontosaurus saharicus, or “shark-toothed reptile from the Sahara.”
The Sahara predator may be closely related to what is now believed to be the largest meat-eating dinosaur known--a South American species called Gigantosaurus, which lived about 30 million years before Tyrannosaurus rex appeared.
Remains from the only specimen of Gigantosaurus discovered appear to have come from an animal about 43 feet long that weighed several tons more than Tyrannosaurus.
The researchers were surprised to discover that the Sahara animal’s brain was less than half the size of that of Tyrannosaurus--smaller than a clenched fist and no more than one-fifteenth the size of a modern human brain.
Researchers cite the disparity in brain size, among other features, to suggest that the African and South American carnivores evolved quite independently from the infamous Tyrannosaurus, Currie said.
Most scientists have believed that the continents drifted apart 150 million years ago, but these dinosaur family ties argue otherwise. The continents could have stayed together longer than scientists had thought, or land bridges may have acted as highways between them until wiped out by rising sea levels.
Some researchers suggest that these specimens show that the animals were able to travel on land bridges between Africa and South America until quite late in their evolution.
“We think we have an animal showing us . . . that there was a whole other stage of predators that preceded the Tyrannosaur stage that we did not know about,” Sereno said. “In the last 25 million years of dinosaur evolution, when those bridges broke down, things changed very quickly.”
