Dinosaur Cannibal?Mystery in New Mexico
for National Geographic News
|December 19, 2002|
It's not easy rescuing a reputation that's been trashed, especially if the subject has been dead for more than 200 million years. But that's what Robert Gay, a paleontology student at Northern Arizona University, is trying to do for a dinosaur with a rep as a cannibal.
Coelophysis (SEE-loh-FIE-sis) was a small, fast, meat-eating predator that lived in western North America during the Late Triassic (roughly 228 to 208 million years ago). A huge bone bed at Ghost Ranch, New Mexico, has hundreds, if not thousands of Coelophysis skeletons, representing all age groups.
"There is no other dinosaur in the world for which we have as many skeletons," said Hans-Dieter Sues, curator of vertebrate paleontology and associate director for science and collections at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
Yet this spectacularly abundant fossil assemblage presents paleontologists with as many questions as answers.
Was Coelophysis a cannibal? Did they hunt in packs? Why were so many in one place at the same time? How did they die? Where did they die?
It's a Hard, Cruel World
These early dinosaurs had bodies the size of turkeys, but with long legs, long tails, and long necks. The neck and tail accounted for most of its 6-to-7-foot (2-meter) length. Coelophysis had hollow bones, was bipedal, and weighed 50 pounds (around 20 kilograms).
"It was extremely agile and nimble, and must have been a really fast little bugger, it was built so light," said Michael Williams, curator of vertebrate paleontology at the Cleveland Museum of Natural History.
The Ghost Ranch bone quarry was first excavated in 1947 under the direction of Edwin H. Colbert, a paleontologist with the American Museum of Natural History. Colbert published a scientific description of the animal in 1989. In it, he described two adults with the skeletons of young Coelophysis in their rib cages. The juvenile skeletons were too large and well developed to be unborn babies. This led him to conclude that Coelophysis was a cannibal.
Gay, in a presentation at the 62nd Annual Meeting of the Society of Vertebrate Paleontology this past October, argued that Coelophysis had been maligned. He contended that the larger animals were merely lying on top of the younger ones.
There is a precedent for revising a dinosaur's reputation. Oviraptor fossils were first found near nests, giving them their name, which literally means "egg stealer." Later finds revealed that they were sitting on their own nestshatching, not eating the eggs.
However, Gay may have an uphill battle redeeming Coelophysis.
"It's possible, but I'm skeptical," said Spencer Lucas, curator of paleontology at the New Mexico Museum of Natural History. "I've always liked Colbert's interpretation that juveniles inside the rib cage of the adults suggested cannibalism."
"It really wouldn't be all that surprising if they chowed down on the youngsters," said Williams. "It happens all the time with crocodiles, and lions are famous for eating the young of other males." The Cleveland museum has just mounted a permanent Coelophysis exhibit under Williams' direction. "The other question that arises," he points out, "is that if we assume the juveniles were eaten, we don't know if they were eaten dead or alive."
"It wouldn't surprise me either way," said Gregory Paul, paleontologist and author of a recently published book on the origins of birds and dinosaurs titled Dinosaurs of the Air (Johns Hopkins University Press).
Sues comes down solidly on the side of Coelophysis being cannibalistic.
"I looked at one of the specimens with Ned Colbert," said Sues. "The skeletons have bones of young Coelophysis in the rib cage. I'm pretty convinced of his interpretation."
Adding to his conviction is the fact that it simply wouldn't be surprising or aberrant behavior, said Sues.
"Cannibalism among predators is very common. If you look at modern crocodilians, or the komodo dragonkomodo dragons spend much of their youth in the trees because if they're on the ground they're going to be gobbled up. Young crocodiles move to areas without adult crocs for the same reason. This is well-documented. It's nothing unusual."
The Carnegie museum has one of the huge blocks of mudstone excavated from Ghost Ranch, and Sues and several colleagues are working to extract skeletons from the rock to conduct detailed studies of the Coelophysis.
Mysteries of the Bone Bed
The bone bed presents other mysteries. One is "the question of why there were so many predators in one place, flocking together," said Paul. "Predators tend not to get along well in large groups."
Did they hunt in packs? Most meat-eating predators today do not. However, evidence has been coming in to suggest that there was pack-hunting among meat-eating dinosaurs, said Sues, who draws the bird analogy. "Birds certainly flock together, it may be that this is a very ancient behavioral pattern," he said.
It is also unclear whether the site is an accumulation of bones that occurred over time, or evidence of a mass kill. If a mass kill, how did they die? All sorts of scenarios have been proposed.
Colbert suggested the animals might have been poisoned, and the bodies of the dead animals swept away by a flash flood, to congregate in a low spot and be quickly buried in mud.
Others have kept the flash-flood theory but suggested the animals died during a drought. Others say at this point, there is no way of knowing.
"They didn't get buried where they diedyou're not looking at the battlefield," said Lucas. "There's good evidence for the skeletons having been washed into a topographic low. The carcasses are all concentrated together, layer upon layer. Beyond that, flash flood, fire, droughtright now we have to say 'cause of death: unknown.'"
Another unanswered question concerns the skeletons themselves, which appear to represent two distinct types, gracile (slender) and robust (strongly formed).
"Are we seeing two different species, or were Coelophysis sexual dimorphs (one sex is larger than the other)," said Paul. "And if they're sexual dimorphs, which is the male and which the female?"
"Among mammals, such as ourselves, the male tends to be slightly bigger," said Lucas. "But that doesn't hold for all species."
"The likelihood is higher that the robust skeleton is the female," said Sues. "If you look at bird species, the female is frequently the more robust."
Rewriting the Story
Coelophysis is enjoying a renaissance of new interest by biologists. All agree it's past time to rewrite the official description.
"Even though we've had this stuff for half a century, there's still a lot we don't know," said Lucas. "It's a lot of work to get fossils out of the rock. Only in the last 20 years has there been a reawakening of interest in the biology of dinosaurs. People are asking new questions for a new millennium."
"Colbert's description is not written in the way species descriptions are written today," said Mark Norell, chairman of the Division of Paleontology at the American Museum of Natural History. "It's based on a lot of reconstructions to make an idealized type. Preparators today now have better technology for fossil preparation."
There is no question of their importance.
"Coelophysis is one of the best-known early dinosaurs and gives us a window into early dinosaur evolution and the origins of dinosaurs," said Lucas.
The connection between the origin of birds and the origin of dinosaurs suggested by Coelophysis is tantalizing.
"They were theropods (meat-eaters), the group that eventually became ancestors to birds," said Paul. "It had a smaller, more reptilian-like brain, and it was one of the first dinosaurs to show a truly tridactyl foot, where the third toe is completely separated from the foot, like birds today. We don't know if they were feathered. Like birds, they had a fairly large pelvis, and very early signs of air sacs."
"The quarry at Ghost Ranch is probably the single most important bone bed of the Triassic," said Lucas. "It's a remarkable windfall for paleontologists, one of the best-known fossil assemblages in the world."
"Once Coelophysis has been thoroughly studied I think it will provide an interesting window into the life and times of early dinosaurs," concurred Sues. "Because there are so many, we can look at growth rates, sex ratios, which have implications for behavior we're also getting other animals, including an early crocodile from Ghost Ranch," he said. "We currently have one bizarre new animal that I couldn't even guess what it is at this point."
So, was Coelophysis a cannibal? The consensus seems to be anything's possible, but the jury's still out. Should Robert Gay be feeling discouraged by the somewhat tepid reception his interpretation of the data received?
"Bright young minds coming in to look at the data in different ways that's what science is all about," said Williams.
Hans-Dieter Sues is a member of the National Geographic Society's Committee for Research and Exploration.
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