One of the unique exhibits at the LA County Natural History Museum is the “T rex series”—a display of three skeletons of Tyrannosaurus rex ranging from a youngster to a “teenager” to a fullsize adult. The trio of skeletons illustrates a debate which has been raging since the 1990s, when paleontologist Philip Currie of the Royal Tyrell Museum in Canada made a series of discoveries which set off a still-running controversy about the social and family life of Tyrannosaurus rex.
"Museum Pieces" is a diary series that explores the history behind some of the most interesting museum exhibits and historical places.
In 1910, Barnum Brown, from the American Museum of Natural History in New York City, was canoeing down a river in Alberta, Canada, when he came upon some fossils eroding from the banks. Brown had just become world-famous several years previously for his discovery of a nearly-complete T rex skeleton in Montana, and now he identified these new finds as Albertosaurus, a close but slightly-smaller relative of the Tyrannosaurus. Although he noted that there appeared to be a large number of skeletons, he was only able to collect a few bones and bring them back to New York. There the fossils stayed in the Museum’s vaults, mostly forgotten, until 1996 when they were examined by Currie.
Intrigued by Brown’s field notes that described the large number of skeletons that he had left behind, Currie thought that the site may still hold some scientific interest. By painstakingly studying a few photographs taken by Brown, he was finally able to identify the spot where the bones had been found—in what was now the Dry Island Buffalo Jump Provincial Park. He began excavating the site.
It quickly became apparent that Brown had been right—there were a lot of skeletons here. Over the next ten years, in an area just fifty yards wide, Currie uncovered over a thousand bones from at least 26 identifiable individual Albertosaurus, ranging in size from large to small. “The indications are very clear in this bonebed,” he concluded, “that the tyrannosaurs were here because they died together at the same time and almost certainly were living together at the time of their death.” In other words, they were a pack.
This was a controversial assertion. Since most tyrannosaurids (a group of large meat-eaters that included Tyrannosaurus, Albertosaurus and Tarbosaurus) had been found as individual skeletons, it had always been assumed that they were solitary hunters, like bears or jaguars, coming together only occasionally for mating. So the idea that Albertosaurus—and by extension the other tyrannosaurids too—were pack hunters that lived in social groups, like wolves or lions, was a contentious one.
More evidence arrived. In 2006, Currie began five years of field work in the Gobi Desert as part of the Korea-Mongolia International Dinosaur Project. Scattered over several dozen sites, the team found almost 100 skeletons and bones of Tarbosaurus, an Asian tyrannosaurid. But one of these sites was particularly important—it contained the skeletons of six individuals ranging from juvenile to adult. Once again, Currie concluded that this was a family group that had died together.
Then, in October 2011, a hunting guide in British Columbia, Canada, found some dinosaur footprints. When Richard McCrea from the Peace Region Palaeontology Center investigated, he found three sets of tracks that, from the size and shape, came from one of the tyrannosaurids of the time (Albertosaurus, Gorgosaurus or Daspletosaurus). The trackways ran for about 200 feet and consisted of three individuals. The footprints all faced in the same direction, were parallel to each other, and were all pressed to the same depth in the mud—indicating that they had crossed together at the same time, as a group. One of the individuals had suffered a previously-healed injury and had lost the tip of one of the toes on its left foot.
Based on all this, Currie now offered a more detailed hypothesis which speculated on the possible family life of the tyrannosaurids. Comparing the skeletons of the juvenile Albertosaurus and Tarbosaurus, he noticed that they changed significantly as they got older. The “teenage” tyrannosaurids were slenderly built with longer legs and lighter skulls. The adults, on the other hand, were heavier and bulkier, with huge robust skulls that were capable of much more powerful bites. So Currie speculated that perhaps they hunted together in a pack that took advantage of each other’s strengths: the faster and agile youngsters may have driven prey animals into an ambush where the adults were waiting to deliver the killing bites. The entire pack could then feed on the downed prey.
Debate followed, with several different options offered as models for tyrannosaurid pack behavior. The basic problem is that we have no way of fossilizing “behavior”, so the only thing we can do is look at the behavior of modern animals as a model. The closest living relatives to the predatory dinosaurs are birds, but, with the sole exception of the Harris Hawk, no birds are known to hunt together cooperatively (and the Harris Hawks fly). The best models we have are pack animals like wolves and lions, but these are mammals, and their physiology and biology are completely different. In wolves, all of the pack members (except the cubs) participate in the hunt. In lions, however, it is only the adult females who do all of the hunting, but the adult males always get first pick at the resulting kill. Did T rex follow one of these models, or did it live in a different manner entirely? Perhaps, for example, the teens did all the hunting and both the adult males and females used their bone-crunching jaws on prey that had already been killed. We have no way of knowing.
All of this, in turn, fed into another controversy that had also been going on for many years. In 1946, a bone-hunter named Charles Gilmore found a skull in Montana which he identified as a young Gorgosaurus. But when paleontologist Bob Bakker examined it in 1988 at the Cleveland Museum of Natural History, he re-identified it as a Tyrannosaurus rex. But then he noticed something odd—some of the bones appeared to be fused together, a normal process which occurs during growth and appears in individuals which are fully-grown and “osteologically mature”. Strangely, although this skull appeared to belong to an adult, it was less than half the size of a typical T rex. It also had a different number of teeth, and some small differences in the skull structure. After much thought, Bakker decided that it was not a young T rex after all, but was an adult of a new species, a sort of miniature version of T rex that he named Nanotyrannus. When others examined the same skull, however, they concluded that it was really a juvenile tyrannosaurid, most likely a T rex.
Enter “Jane”. In 2001, an expedition from the Burpee Museum of Natural History, in Rockford IL, uncovered a nearly-complete skeleton at the Hell Creek Formation in Montana, not far from where the Cleveland skull had been found. Named “Jane” after a financial benefactor, this find was almost identical to the Cleveland skull but was much more complete, allowing a much more detailed analysis. Although most of the bones in the skeleton were unfused, indicating that it was still a juvenile, Jane was already over 20 feet long, about half the size of the adult T rex specimens. Some of the bones in Jane’s skull also seemed to be closer to those of adult specimens than the Cleveland skull. And the rest of the skeleton demonstrated the light build and long legs that had been seen in the juvenile Tarbosaurus and Albertosaurus. Although Bakker and a few others continue to hold out, the majority view today is that the species Nanotyranus is invalid, and that these actually are juvenile T rex specimens.
NOTE: As some of you already know, all of my diaries here are draft chapters for a number of books I am working on. So I welcome any corrections you may have, whether it's typos or places that are unclear or factual errors. I think of y'all as my pre-publication editors and proofreaders. ;)