A team of fossil hunters led by the Field Museum announced Thursday that it has unearthed the oldest known dinosaur bones, a pair of toothy, 230-million-year-old jawbones that might help explain how dinosaurs came to reign over the ancient Earth.
Most big dinosaur fossil finds are just that–big. But these jaws are so old they date to an epoch when dinosaurs were still puny, their footsteps more pitter-pattering than thundering.
One jaw, the color of a bleached chicken bone washed up on a beach, fit in the palm of the team co-leader, John Flynn, chairman of the museum’s department of geology.
“These are fairly scrappy,” Flynn said, examining the cracked and chipped fossil in his hand. “But they are really beautifully preserved, considering their age.”
Although the bones were freed from red rock in Madagascar in 1997, it took two years to mount the argument that establishes their age at 230 million years–a result that surely will stir controversy among paleontologists. The research is published in Friday’s edition of the journal Science.
The two jawbones belonged to different prosauropod dinosaurs that were no bigger than small kangaroos and probably had strong back legs, allowing them to reach up and nibble plant leaves.
They lived in a time when dinosaurs had yet to elbow their way to the top of the biological heap and were competing with other reptiles just beginning to develop traits that would come to define mammals.
In fact, the remote Madagascar site where the two dinosaur jaws were found also yielded remarkably well-preserved fossils of these mammal-like reptiles.
“A wealth of bones is pouring out of our quarries,” said Andre Wyss, associate professor of geology at the University of California at Santa Barbara and another member of the team.
Usually, the problem with very old fossils is that history has piled so much more rock on top of them that they are crushed and twisted beneath the grindstone of the generations that follow.
These bones, though fractured and scattered, form a sort of splintered, real-world diorama that could illuminate life just as Pangea, the earth’s supercontinent, was beginning to break up and dinosaurs were coming to dominate life on the planet for more than 100 million years.
Eventually, a process paleontologists have come to call “giganticism” would make dinosaurs into the huge creatures that lorded over the Jurassic period, not to mention children’s imaginations and museum shops.
Some 80 million years after the kangaroo-size creatures found in Madagascar were alive, their apparent descendants included the behemoth brachiosaurus, Flynn said.
A 40-foot-tall skeleton of a brachiosaurus altithorax, a plant-munching monster that could have weighed up to 85 tons, stands in the museum’s Stanley Field Hall, just below the public display that will house the Madagascar dinosaur jaws through Jan. 2.
How the dinosaurs came to be so dominant has long been a mystery, in part because so few fossils have been found from their early years.
Somewhere out there might be a fossil, perhaps 250 million years old, that could be called the very first dinosaur.
“We are getting closer to that moment, because we are getting closer to the roots of the dinosaur branches,” Flynn said. “This starts to lift some of the fog from the fossil record.”
The precise estimate of the jawbones’ age, 230 million years, is sure to stir controversy because it narrowly edges out the record-holder for old dinosaur bones, a 227-million-year-old, cat-size fossil of an eoraptor found in Argentina by the University of Chicago’s Paul Sereno.
The Argentine bones were dated by measuring the decay of radioisotopes, a method that most scientists believe is very reliable.
But the Madagascar bones were not encased in rock that allowed radioisotopic dating, so the researchers had to use other means.
By looking at the fossilized remains of the animals that were living side by side with the dinosaurs, the Madagascar team deduced that their fossils had to be older than the Argentine bones.
“If you have a picture of the Chicago Bears and you don’t know what year it was taken, you would look for the veterans that had been around for a while, and you would look for the rookies, and by looking at who was in the picture and who wasn’t, you could establish when it was taken,” Flynn explained.
“This will be controversial,” he said. “People will say the argument is weak. But we feel it is a pretty persuasive argument.”
Sereno could not be reached Thursday for comment.
Neil Shubin, a paleobiologist at the University of Pennsylvania, said the dating “isn’t as good as (the researchers) would like.”
“It is at least equivalent in age to the Argentine stuff,” Shubin said. “It opens a new window into one of the most important intervals in evolution, a time of great upheaval.”
The Field Museum has formed a partnership with a Madagascar university that has produced four fossil-hunting expeditions.
The team set out to look at rocks it believed came from the murky middle and late Triassic periods, where fossils are rare.
Because they were digging in such old rock, the team members knew that any dinosaur fossils they found would be among the oldest.
The team, which included Northern Illinois University’s Michael Parrish and the Field Museum’s William Simpson, was digging in 1997 when someone peeled away a layer of reddish sandstone and exposed a tooth with a serrated edge, a field mark of a dinosaur.
“That is a eureka moment,” Flynn said. “We knew we had what we thought we would have.”
There might be more out there too. The discovery of the two dinosaurs, both part of a lineage of herbivores, suggests there already was a line of yet-undiscovered predatory dinosaurs.
“You know the meat-eaters must be around, because they have already split from the plant-eaters,” Flynn said.
There still are uncataloged fossils and unopened rocks taken from the Madagascar quarries. “There may be other specimens out there that will be more important, but we don’t know because they are encased in rock,” Flynn said.
Although it doesn’t grab the public attention that dinosaur research does, the discoveries of new mammal-like reptiles also excite paleontologists. Just as the history of dinosaurs has its mysteries, so does the emergence of the mammals.
The team unearthed a 6-inch-long skull of a new mammal-like reptile, also 230 million years old, that is about the size of a small dog.
The evolution of mammals was taking a different course than that taken by the ever-growing dinosaurs. These dog-sized reptiles were predecessors to the first true mammals, which were no bigger than mice.
“We haven’t been able to pin down what was going on because we didn’t have the fossil record,” Flynn said.
The agreement with the Madagascar university allows the Field Museum to prepare and display the fossils.
After Jan. 2, the dinosaur fossils will be returned to Madagascar, “their home country,” as Flynn put it.
