Days of the Deinos
Did predatory dinosaurs leave clues to
their pack-hunting habits at kill sites?
Ostrom was not familiar with either creature at the time, because neither had been formally described in the scientific literature. He later named the herbivore Tenontosaurus, or sinew reptile. His genus name for the theropod, Deinonychus, or terrible claw, was inspired by the large sickle-shaped claw it brandished on the second toe of each foot. The species name, antirrhopus, or counterbalancing, reflects the presence of bony projections that run alongside the vertebrae in the last two-thirds of the tail. This bony lattice kept the tail rigid and allowed it to project straight back to counteract the weight of the head and body.
Ostrom described Deinonychus's skeleton in marvelous detail, using its construction to portray the animal as an intelligent, active, possibly warm-blooded killera revolutionary view.
The notion of pack hunting was inspired by the assemblage, or concentration of bones, at the Shrine site. The sediments surrounding the fossils indicated that the carcasses had not been transported to the site by water or other natural forces but had been preserved on the spot where the animals died. Largely on the basis of this evidence, Ostrom concluded that the four Deinonychus were part of a pack of six or eight animals and that the lone Tenontosaurus was their prey. While the idea of some small theropods as ferocious pack hunters is now firmly planted in the minds of millions, thanks to the novels of Michael Crichton and the Steven Spielberg movies based on them, few paleontologists agree with Ostrom's interpretation of the Shrine site fossils.
Tenontosaurus was a medium-sized herbivore (similar to the hadrosaurs, or duck-billed dinosaurs, which were so abundant later in the Cretaceous) that walked on two sturdy hind limbs, balancing its weight over the hips. If Ostrom is correct, then about half the Deinonychus pack perished during the attack on the Tenontosaurus, which eventually succumbed to the others. This might seem like a curious interpretation: a predator is not doing very well if four of its kind are lost for every prey killed. But let's look a little more closely. Deinonychus, for all its adaptations for predation, stood maybe four feet high, was no more than ten feet long (the slender tail accounting for a substantial portion of that), and weighed 150 pounds at most. Tenontosaurus was a big, bulky prey animal. When fully grown, it was more than twenty feet long and probably tipped the scale at 2,000 to 2,500 pounds. Imagine the scenario: A Tenontosaurus is feeding on vegetation, thinking of nothing more than its next mouthful, when a group of Deinonychus appear and attack. Leaping on the herbivore's back, they bite, kick, and pierce its hide with their sicklelike claws. Does the Tenontosaurus submit to the onslaught, roll over, and die quietly without a struggle? More likely, the one-ton animal would fight for its life, rear up, twist and turn, swing its long tail back and forth, and stamp and flail as it tried to shake off its attackers. It seems plausible to me, as it did to Ostrom thirty years ago, that the considerably larger and heavier beast could have fatally injured four Deinonychus. Similar scenes take place today when mule deer kill timber wolves and when ostriches kill juvenile lions with powerful kicks. Any seemingly defenseless animal is capable of extraordinary feats when under severe attack.
Clearly, a more sensible course of action for Deinonychus would have been to attack old and weak or young and helpless animals, as wolves, cheetahs, lions, and many other mammalian carnivores do today. Perhaps the pack that lost so many members at the Shrine site was desperate for food and had no choice but to tackle a large, healthy individual. The evidence is circumstantial, as critics have pointed out. In his 1988 book Predatory Dinosaurs of the World, artist Gregory Paul declared it much more likely that the four Deinonychus died simultaneously while feeding on the carcass of the Tenontosaurus, a view reiterated by Museum of the Rockies paleontologist Jack Horner in his 1997 book Dinosaur Lives. But, you have to ask, died simultaneously as the result of what? Shock? Indigestion? Poison volcanic gas spreading across the floodplain? Horner likens the idea of a Tenontosaurus killing four Deinonychus to that of a lone wildebeest dispatching four lions.
Nobody knows for certain what took place at the Shrine site. We do know, however, that whether hunted down and killed by a pack or simply scavenged after death, Tenontosaurus was the preferred food of Deinonychus. Approximately eighty occurrences of Tenontosaurus remains have been discovered in the Cloverly formation to date, and thirty-five include Deinonychus teeth. While Deinonychus fossils are rarely found with other possible prey animals, three or four Deinonychus teeth typically turn up wherever there are Tenontosaurus remains. And at a site discovered in the Cloverly formation in 1992, there were even more.
Laid out in its death pose at this new site was a beautifully preserved, near-complete specimen of a young Tenontosaurus. Four Deinonychus teeth were found alongside the bones; later, in the laboratory, seven more teeth were uncovered. It's possible that a few more teeth were missed in the field or unwittingly discarded during preparation because they were concealed within small lumps of rock. So we have a subadult Tenontosaurus no more than fourteen feet long (compared with a length of about twenty feet for the adult at the Shrine site), preserved with at least eleven Deinonychus teeth.
But how can we distinguish between the remains of a victim hunted down and devoured by a pack and an animal that simply died and was scavenged by a few passing Deinonychus? As is the case at the Shrine site, this Tenontosaurus was preserved where it died. After death, the desiccation of the abundant supporting tendons that line the vertebrae of the neck and tail cause these parts to coil. The tail of Tenontosaurus, which accounts for about one-third of the animal's total length, is particularly heavy with supporting tendons. In this specimen, the pronounced curvature of the tail and the neck toward each other effectively counters any claim that the bones were carried to the site by water currents. The Deinonychus teeth were found in the region of the abdomen and pelvis, suggesting that the predators lost their teeth while feeding on the viscera. Most modern carnivores begin with the areas around the anus and abdomen when they feast on freshly killed prey, and it's likely that carnivorous dinosaurs did the same.
The number of teeth indicate that more than one Deinonychus was involved with the carcass. Like all other theropod dinosaurs, Deinonychus shed and replaced teeth throughout its life. The teeth would fall out upon the animal's reaching maturity but also could be wrenched out earlier by the stress associated with the biting and tearing of flesh. Because of this, theropod teeth are very common in sediments containing dinosaur fossils. The teeth from this site vary from recently erupted to fully mature ones. Given that Deinonychus had only sixty teeth in its jaws at any one time, it's unlikely that all eleven were wrenched from the mouth of just one feeding animal. This would leave the Deinonychus toothless after five similar meals. The possibility that Deinonychus was replacing shed teeth in a few weeks or months, and therefore had the ability to sustain such dramatic tooth loss, was quashed by Greg Erickson, who, as a master's degree student at the Museum of the Rockies, worked on replacement rates of teeth in various dinosaurs and living reptiles. After CT-scanning portions of the lower jaw of Deinonychus and studying individual teeth, he came up with an estimate of 300 days for the time it took Deinonychus to replace a shed tooth with a mature one.
We know that this Tenontosaurus was not yet an adult, so it didn't die of old age. Of course, this doesn't rule out death from disease or injury and doesn't confirm that it was cut down by a pack, but it's a start. Next, we have a concentration of teeth around the abdomen and pelvis. This may indicate that the pack fed on the abdominal contents while they were still warm and moist. If, after the viscera had been consumed, the remainder of the carcass was scavenged over time by many individuals, we would expect a much more disturbed carcass and a wider scattering of teeth.
Here again, we can never know for sure what happened with this particular plant eater. But we would do well to remember that no predator will refuse a free meal. If Deinonychus had a choice, it would certainly opt to scavenge a carcass rather than risk injury or death tackling a much heftier, living animal, as seems to have been the case at the Shrine site. The same holds true for Tyrannosaurus. If this phenomenally powerful, forty-foot-long carnivore with five dozen teeth (some of which were up to six inches long, not counting the roots) came across a carcass, it would devour it. But if it was hungry, Tyrannosaurus presumably would kill the first thing it could catch. Modern carnivores operate this way, and we have no reason to suppose carnivorous dinosaurs were any different.
Three pieces of evidence, then, suggest that Deinonychus hunted in packs: the unusual association of bones at the Shrine site; the telltale numbers of Deinonychus teeth at the sites of various Tenontosaurus remains; and the anatomical adaptations that equipped Deinonychus for a life of active predation in addition to opportunistic, passive scavenging. The only way to improve on this circumstantial evidence might be to find a specimen of Tenontosaurus locked together in a death struggle with two or more Deinonychusnot a likely possibility. So paleontologists continue to search for and analyze fossil assemblages and to look for clues regarding their origin.
While Tenontosaurus fossils had also turned up far from the Cloverly formation, in Oklahoma and Texas, Deinonychus remains had until recently been known only from Montana and Wyoming. In 1994 Rich Cifelli, of the Oklahoma Museum of Natural History, and his crew were exploring a new Tenontosaurus site on state land when they discovered teeth of a small theropod that looked suspiciously like those of Deinonychus. Within a small area, they found four discrete concentrations of fossils, each consisting of Tenontosaurus bones and theropod teeth. Eager to find some evidence of which theropod was feeding on the carcass, the crew was excavating the last of the four specimens when they ran into some unusual-looking, thin black bones near the tail. Further exploration revealed these bones to be portions of the most complete specimen of Deinonychus ever found.
The Oklahoma assemblage and others in the vicinity are in the Antlers formation. Like the Cloverly in Montana, this formation is of Early Cretaceous age and represents an ancient floodplain. Because this environment was very similar to that of the Cloverly 100 million years ago, it is not surprising that we find similar fossils here. For the moment, the discoveries in Oklahoma present no evidence that Deinonychus was doing anything there other than scavenging carcasses of Tenontosaurus that were washed into the area, but it does indicate that Deinonychus roamed-and fed on Tenontosaurusacross an enormous expanse of what is now the western United States. Teeth like those of Deinonychus have been found throughout the country, from Utah to Maryland, so further discoveries may come to light.
On display only since the renovation of the dinosaur halls in 1995, the American Museum of Natural History's Deinonychus is suspended in midair. A creature truly taking a flying leap, it is juxtaposed with images of an Archaeopteryx taking wing, to highlight the kinship of small, highly active dinosaurs and birds, both ancient and modern. Throughout the Hall of Saurischian Dinosaurs, Deinonychus's theropod relatives are depicted not as plodding sluggards but as intelligent, agile predators. In their stance and vigor, they bear witness to the unrivaled influence of the Deinonychus envisaged by John Ostrom.
A native of Belfast, Northern Ireland, Desmond Maxwell was primarily interested in geology when he was growing up. Ireland is a great place to study earth science, he notes, because you can find rocks from all past geologic ages. I started working on dinosaurs in Montana, so its only relatively recently that I have become interested in them. Maxwell moved to Montana in 1991 after completing his Ph.D. in geology at The Queens University of Belfast and writing his thesis on pareiasaurs, a group of reptiles that lived long before the time of dinosaurs. After two years at the Museum of the Rockies, where he got to know the newly reopened Shrine site firsthand, he became an assistant professor at the New York College of Osteopathic Medicine in Old Westbury, New York. Maxwell is now an assistant professor of biological sciences at the University of the Pacific in Stockton, California, and he continues to excavate and study dinosaur fossils in Montana, Oklahoma, and Utah.