Tyrannosaurus rex truly is the king of the dinosaurs. Reaching 40 feet in length, weighing in at over nine tons and boasting a bite force of over 12,000 pounds, it's no wonder we’ve celebrated this ancient carnivore since the time its bones were first put on display over a century ago.
But the imposing frame of an adult T. rex is only part of the animal’s story. At the annual Society of Vertebrate Paleontology meeting in Dallas, Texas earlier this month, Carthage College paleontologist Thomas Carr highlighted how dramatically the tyrant changed as it grew up—and what this means for the fate of a fossil that’s been surrounded by controversy for three decades.
Carr’s presentation centered on a pair of finds made in eastern Montana, separated by six decades. The first was a small tyrannosaur skull uncovered in 1942 that then rested in the Cleveland Museum of Natural History collections for four decades. The fossil was mostly forgotten until Robert Bakker and his colleagues made a sensational announcement in 1988.
According to the team, that fossil—nicknamed the Cleveland skull—represented a totally new genus of small-bodied tyrannosaur that prowled the same habitats as the beefier T. rex. Befitting its small stature, they named the tiny terror Nanotyrannus.
Not all paleontologists were sold on the proposal, though, including Carr. In 1999, he proposed that Nanotyrannus was really a juvenile T. rex, simultaneously rejecting Bakker’s hypothesis and providing a new look at what the “tyrant king” was like as an awkward youth. The differences between Nanotyrannus and Tyrannosaurus were signs of how these dinosaurs aged rather than the hallmarks of different species, Carr argued.
The debate over the proper identity of Nanotyrannus has been stomping on ever since, with Bakker and other supporters insisting that the differences in size, tooth count and other features mark the Cleveland skull as a distinct dinosaur.
But now Carr and his coauthors have completed a comprehensive analysis of a definitive juvenile T. rex, discovered in 2001 and nicknamed Jane by the Burpee Museum of Natural History in Illinois. The new evidence supports the notion that the Nanotyrannus fossils described so far are really the rare remains of young Tyrannosaurus.
At about 11 years old, Jane was a little bit more mature than the animal represented by the Cleveland skull, but it had not yet developed the classic, deep-skulled profile we usually associate with T. rex. According to Carr, Jane was just at the cusp of undergoing a major growth spurt, in which the dinosaur would have dramatically transformed from a lanky teen into an adult with a bone-crushing bite.
This puts Jane in a transitional stage between younger T. rex, like the Cleveland skull, and the classic form of the adults, Carr says.
While Nanotyrannus defenders claim that there are other fossils that bolster their hypothesis, these fossils have not been placed in accredited museums or other public institutions – a standard requirement in paleontology.
The owners of a proposed Nanotyrannus fossil found in Montana even tried, and failed, to sell the specimen at public auction, which could have hidden the bones away in a private collection and robbed scientists of the opportunity to study it. More than a dozen other T. rex fossils are already privately held, which puts up a barrier to learning more about the dinosaur.
Carr stresses that his conclusions about Jane don’t rest on a single fossil. “No specimen is an island unto itself”, he says, and the lessons learned from Jane hinge upon having a sample of younger and older animals that helped put the fossil in context.
Not that all the gaps are filled just yet. There are parts of the T. rex growth series that are still unknown. At the top of Carr's fossil wish list is a T. rex specimen that shows when the skull switched from Jane's long, narrow profile to the deep jaws of the adults. With luck, future discoveries will be placed into public museums and provide even more context showing how the great T. rex grew from an awkward teen into an imposing predator.