Researchers claim to have discovered a new species of dinosaur — Tyrannosaurus mcraeensis, closely related to Tyrannosaurus rex — in a study co-authored by a GW professor released earlier this month.
The study analyzed bones from New Mexico previously thought to have belonged to a T. rex and claims that differences in their structure and geographic location from other T. rex fossils indicate the fossils belong to a different species. Alexander Pyron, an associate professor of biology and a co-author of the study, said identifying the new species reveals there were more diverse families of dinosaurs than previously thought.
“Previously we only knew a few species, and we thought maybe there were only maybe four or five species of dinosaur walking around in that area of time,” Pyron said. “Then you start finding all of these new species, and they have these morphological differences.”
There are two differing hypotheses relating to tyrannosaurs’ origins — one claims that T. rex and T. mcraeensis came to North America via a land bridge connecting to Asia, while the other argues the tyrannosaurs evolved from animals in Southern Laramidia, a land formation that is modern-day Western North America. Because researchers found that the fossil is 6 million to 7 million years older than other T. rex fossils found in North America, the study supports the theory that the dinosaur originated in Western North America and was thus classified as a different species.
Pyron said the jaw of the T. mcraeensis is noticeably more slender than the T. rex and has a different ridge over its brow, which, along with the difference in age and location, led the researchers to conclude the fossil was out of the range of what they believe are normal variations found in T. rex bones. Debates over the identification of fossils are a common problem in paleontology due to disagreements about how species should be classified, incomplete skeletons and individual physical variations within species.
Pyron joined the study, which started in 2013, to analyze the phylogeny, or evolution, and geographic patterns of the bones. He said boaters discovered the fossil in New Mexico in 1983, while most T. rex bones were found in Asia or the Northern United States.
“What I analyzed was their evolutionary history, phylogeny and geographic distribution, their geographic range,” Pyron said. “The paleontology team, these bones were discovered 40 years ago and after they had created this evolutionary diagram of phylogeny, they asked me to kind of step in on the computational and statistical analysis side to try to understand some of these patterns.”
With the study’s findings of earlier tyrannosaurs living in New Mexico, Pyron said the research supports the theory that tyrannosaurs come from Western North America. He said it is feasible that the creatures could have migrated north from the New Mexico area, but they do not have enough information to confirm the reason for the difference in location.
Pyron said the study tells scientists how these related species were able to evolve and take on different physical traits in different geographic locations.
“What it tells us is that not only were they moving around over these vast different distances, but in these different areas, they were evolving these really subtle and distinct ecological differences as well,” Pyron said.
Experts in paleontology said there are differing opinions among scholars about whether the study’s findings have enough data to conclude the species is different from the T. rex.
Thomas Holtz, a principal lecturer of vertebrate paleontology at the University of Maryland, said the debate exists because there is no specified measure for what amount of difference constitutes a new species, and it is difficult to determine with such limited data.
“I know I have some colleagues who aren’t convinced, they think it really is T. rex, and they think it’s younger, like the old idea,” Holtz said. “Their arguments that it is old are good, and it does seem to be just outside the range of T. rex proper. So as different from T. rex as a tiger is from a lion, or leopard is from a lion.”
Holtz said the study raises questions about the differences in size between dinosaurs living in the Northern U.S. compared to the South because researchers do not know if the differences could be attributed to environmental factors. He said the new species is bigger than the T. rex, which aligns with other species from the South also being relatively big.
“No one has demonstrated what aspect of the environment it is, it’s some environmental difference,” Holtz said. “So is it the types of plants that are present? Is it overall productivity? Is it some other aspect of the habitat? Is it a temperature? We don’t have that narrowed down yet.”
Thomas Carr, an associate professor and director of the Institute of Paleontology at Carthage College, said the initial identification of the fossil as T. rex was correct. He said its bone structure and location were similar enough to other T. rex specimens that it could be explained as a different individual, instead of a different species.
“Variation in tyrannosaurs is tricky, and you have to really know a species well before identifying a new species,” Carr said. “It really has to be different, like day and night, but what they’ve presented is just shades of gray.”
Since there has been only one fossil found with these variations, the fossil is not enough evidence to call it a new species, Carr said.
“Part of the problem is that we have such a low sample size that a false positive like this is a relatively high probability,” Carr said.
Mark Loewen, a professor of geology and geophysics at the University of Utah and co-author of the study, said one of the defining features of the T. rex skull, the postorbital bone, was notably different in this fossil from every other fossil assumed to be a T. rex.
“The main difference is there’s a bony protuberance that comes right behind the eye, we call it a postorbital boss,” Loewen said. “And the shape and configuration of that is different when compared to those of the multiple specimens of T. rex that we have.”
Loewen said the origins of Tyrannosaurs in Laramidia were debated, but he didn’t think the Asian origin hypothesis was likely.
“The only way to cross is over the land bridge of Alaska, which is above the Arctic Circle. And it’s dark for 180 days out of the year,” Loewen said. “So it’s not a great place for interchange between Asia and North America.”
