Tyrannosaurus rex was the ultimate dinosaur. The charismatic carnivore was not only the last of its family, but also the largest and most powerful member of a lineage that originated over 80 million years earlier. We especially love the dinosaur for the latter aspects. Yet, despite the persistent fame of T. rex, paleontologists are really only just starting to understand where the tyrant came from. The majority of tyrannosaur history is still murky, a tale told through partial skeletons, teeth, and fragments scattered through Jurassic and Cretaceous time. A scrappy tyrannosaur found in Utah adds a few more clues to the T. rex backstory.
Found within 75 million year old rock among the Book Cliffs – near Green River, Utah – the tyrannosaur is only represented by part of a hindlimb. That’s not very much to go on, but as paleontologists Tracy Thomson, Randall Irmis, and Mark Loewen suggest in an in-press Cretaceous Research paper, the anatomy and location of those petrified pieces may provide important clues about a major evolutionary pattern among North America’s Cretaceous dinosaurs.
If you were able to take a road trip from New Mexico to Alberta, Canada around 75 million years ago, the communities of dinosaurs you’d pass along the way would change as you drove north. The dinosaurs in the ancient southwest would belong to different genera and species than those living in prehistoric Montana and Canada, even if the distant communities shared components such as tyrannosaurs, ceratopsids, hadrosaurs, and ankylosaurs in common.
Paleontologist Tom Lehman articulated this idea of dinosaur provincialism around the turn of the 21st century, and fossil finds have since supported the notion that there was a barrier which created an evolutionary split between northern and southern dinosaurs by about 75 million years ago. But nobody knows what this barrier was, or even where it was. The pattern of dinosaur evolution outlines an evolutionary divergence, but the physical feature that isolated dinosaur lineages has not yet been discovered. Yet the tyrannosaur Thomson and colleagues describe could help identify where evidence of that barrier might be.
At the time the Book Cliffs tyrannosaur was prowling around, east-central Utah was a coastal plain on the border of a warm, shallow sea that divided North America in half. This habitat was hundreds of miles north of the swamps stalked by the short-snouted tyrannosaur Teratophoneus, which lived at about the same time in what is now southern Utah, but far to the south of the floodplains ruled by northern tyrants such as Gorgosaurus and Daspletosaurus.
As yet, the Book Cliffs tyrannosaur doesn’t have an official scientific name. There’s too little of the dinosaur to tell whether the bones represent a new species or signify a previously-discovered tyrant. But the details of the recovered bones hint that the tyrannosaur was closely related to Daspletosaurus and Albertosaurus – Canada’s Cretaceous carnivores. The key feature is a trait called the plantar ridge that is present on the fourth metatarsal bone of the Book Cliffs tyrannosaur and other tyrannosaurs found to the north, yet was lost among southern tyrannosaurs.
Why should a tyrannosaur with a northern feature be found so far to the south, much closer to tyrannosaurs that lacked that trait? Thomson and coauthors outline several possibilities. The Book Cliffs tyrannosaur could represent the southernmost reach of northern tyrannosaur types and therefore might have lived close to the border that divided northern and southern dinosaur forms. Then again, the tyrannosaur might have been an immigrant that somehow got around the barrier after the evolutionary split, or maybe the tyrannosaur was a more archaic form that retained the plantar ridge with other southern species lost it.
Which scenario is correct isn’t clear yet. Paleontologists need to uncover more of the tyrannosaur, as well as additional dinosaurs from before the time of the 75 million year old split, in order to test these ideas. Nevertheless, paired with biogeographical evidence from other ancient organisms, the Book Cliffs tyrannosaur might help paleontologists delineate the border between the north-south Cretaceous provinces and narrow down their search for what caused such an evolutionary rift. And by doing that, researchers might be able outline just a bit more of the T. rex story.
Traditionally, T. rex was thought to have evolved from a northern tyrannosaur stock – Daspletosaurus, or a similar dinosaur. But as Thomson and coauthors tease in the paper, upcoming research hints that T. rex is actually a descendant of southern tyrannosaurs that later moved north. By figuring out the tempo and mode of the evolutionary dinosaur shuffle, paleontologists may be able to trace where our most cherished carnivorous dinosaur came from.
(Note: I know all three of the paper’s authors through my volunteer work at the Natural History Museum of Utah, but I have not been to the Book Cliffs field site nor have I had anything to do with this research.)
Thomson, T., Irmis, R., Loewen, M. 2013. First occurrence of a tyrannosaurid dinosaur from the Mesaverde Group (Neslen Formation) of Utah: Implications for upper Campanian Laramidian biogeography. Cretaceous Research. http://dx.doi.org/10.1016/j.cretres.2013.02.006