More than 66 million years ago, a “tyrant lizard king” ruled western North America: the fearsome predatory dinosaur Tyrannosaurus rex. But how large was this monarch’s royal family? Could what we call T. rex today be composed of multiple species?
In a controversial new study in the journal Evolutionary Biology, three paleontologists argue that the fossils assigned to T. rex cluster into three different body types, which they further contend represent three separate species. In addition to T. rex, the researchers propose two new species names: T. regina and T. imperator, from the Latin for “queen” and “emperor.”
“For Tyrannosaurus, all the specimens from North America have been put into the same species, T. rex … and that’s become something of a problem because nobody was testing whether that was really true or not,” says study leader Gregory Paul, a paleoartist and independent researcher who wrote The Princeton Field Guide to Dinosaurs.
Paul and his co-authors contend that their findings, if validated, would sharpen our view of dinosaur evolution during the Cretaceous period, which lasted from 145 million to 66 million years ago. “We're interested in the possibility of trying to see very fine level species-to-species evolution,” says study co-author Scott Persons, a paleontologist at the College of Charleston in South Carolina.
If the results are valid, they would reclassify some of the best-known Tyrannosaurus fossils on display in museums around the world.
However, paleontologists who were not involved in the new research, including some of the world’s leading experts on T. rex, are skeptical of the study’s results. “It’s just shades of gray and shapes in clouds—there’s no validity here at all,” says tyrannosaur expert Thomas Carr, a paleontologist at Carthage College in Kenosha, Wisconsin.
Paul’s team argues that the known fossils of Tyrannosaurus vary more than the bones of other large predatory dinosaurs. The researchers also argue that the fossils fall into three distinct clusters, based in part on the stoutness of their skeletons and the presence of certain chisel-like teeth, traits long noted to vary among Tyrannosaurus skeletons. “I wanted to get [the new species] named partly to force people to face the issue,” Paul says.
The challenge is that the variation within Tyrannosaurus fossils could have stemmed from many factors that would not require new species names. Dinosaurs’ proportions could have changed dramatically as they matured. Individual Tyrannosaurus grew slightly differently, just as humans reach a range of heights. It’s also possible that T. rex took on slightly different builds depending on their food availability or the ecosystems in which they lived.
The outside experts say that the study didn’t go as far as it could have to vet these scenarios or weigh their combined effects.
“Most of us would predict that yeah, there probably should be multiple species of Tyrannosaurus rex … The real question is, does this paper do a really rigorous job of doing that?” says Lindsay Zanno, a paleontologist at the North Carolina Museum of Natural Sciences in Raleigh. “I would argue that the paper is relatively unconvincing.”
What’s in a species?
Paleontologists have batted around the notion of multiple Tyrannosaurus species for decades. The predator lived for more than a million years across a swath of ancient North America from western Canada to New Mexico. That’s a large area, and a long time—possibly enough for populations to drift apart and form multiple species. (Find out why paleontologists think billions of T. rex may have roamed the Earth.)
If anatomical traits in a given creature’s fossils clearly change over time, the creature may warrant multiple species names to distinguish earlier forms from later ones. In the last few decades, paleontologists have gained a better understanding of the way that such “chronospecies” may succeed one another.
Perhaps the most famous and well-established chronospecies come from western North America’s Hell Creek Formation, where Tyrannosaurus is found. Older sediments from the formation preserve one species of Triceratops, T. horridus, while younger rocks farther up the formation preserve a different species, T. prorsus. Some researchers have suggested a similar evolution for other Hell Creek dinosaurs.
Tyrannosaurus fossils can vary quite a bit from one another, notably in the dimensions of the femur, or upper leg bone. Some skeletons have stockier, more “robust” femurs, while some have narrower, more “gracile” bones. Years ago, paleontologists also noted that some Tyrannosaurus skulls had pairs of chisel-like incisors in the lower jaw, while others did not.
The current study is based on a dataset of measurements taken across 37 Tyrannosaurus skeletons, with a focus on the femurs and lower jaw teeth. Paul and his colleagues also plotted where 28 of these specimens were found within the Hell Creek Formation: lower, middle, or upper. The lower and deeper the rock layers, the older the fossils must have been.
For comparison, Paul’s team sought out datasets for other large predatory dinosaurs, including several other tyrannosaurs and the older non-tyrannosaur Allosaurus. According to their analysis, Tyrannosaurus showed more variability in its bone measurements than any of the other dinosaurs examined.
The team found that the group from the Hell Creek Formation’s lowest, oldest rock layers had robust skeletons and two pairs of chisel-like teeth. However, the fossils from the younger layers broke into two clusters. The dinosaurs in each cluster had a single pair of chisel-like teeth, but one group had a more robust skeleton and one had spindlier, more gracile bones.
Paul’s team argues that sex differences between male and female Tyrannosaurus can’t explain this variability through time. They also ruled out individual differences and the animals’ growth stages. The researchers instead say that each cluster could be interpreted as its own species.
The classic Tyrannosaurus rex, defined from a skeleton now housed in the Carnegie Museum of Natural History, fell into the robust and younger group, and it remains T. rex in the proposed naming scheme.
Paul’s team named the most ancient group T. imperator and suggested Sue, the famousTyrannosaurus at Chicago’s Field Museum of Natural History, as the defining holotype of the species. The researchers defined T. regina, the gracile contemporary of T. rex, using a skeleton at the Smithsonian’s National Museum of Natural History in Washington, D.C.
The fogs of prehistory
Whether these proposed Tyrannosaurus species hold up hinges on the validity of the data clusters—which outside experts say are not as well supported as they would like.
Other recent studies haven’t seen this same clustering, notably a massive analysis of T. rex’s different life stages that Carr published in 2020. As part of his study, Carr measured and analyzed 1,850 individual skeletal traits. He found no evidence that Tyrannosaurus came in distinct male or female forms, let alone clear clusters that would be explained by multiple species. “If these taxa were real, I would have recovered them,” Carr says.
Zanno adds that the study did not use the bones’ cross-sections to constrain how old each Tyrannosaurus individual was when it died, which means that the study may not have sufficiently dealt with the many changes that Tyrannosaurus underwent as it matured.
For Tom Holtz, a paleontologist at the University of Maryland, another concern is that the study does not precisely reconstruct where each fossil was found within the Hell Creek Formation, which proved crucial in defining chronospecies within Triceratops. Finer-grained location data for each fossil would sharpen the fossils’ relative ages, which would provide a more stringent test of whether Tyrannosaurus consisted of multiple species over time.
“It’s damn difficult. Many of these are historic specimens that were collected before people paid as much attention,” Holtz says. But “it doesn’t mean you don’t do it.”
Part of the challenge, Paul counters, is that some of this geologic information is now impossible to retrieve. The quarry that bore AMNH 5027—a well-preserved Tyrannosaurus skull now housed at New York’s American Museum of Natural History—was flooded by the Fort Peck Dam when it opened in 1940, for example, making the rock layers very difficult to study.
Scientists also raised concerns over some of the skeletons that were included in the new study. In part, the study relies on fossils housed in the Black Hills Institute of Geological Research, a paleontology business in Hill City, South Dakota, that sells fossils and fossil replicas to museums and collectors.
The study also includes “Stan,” a well-preserved Tyrannosaurus skeleton that the company was court-ordered to sell at auction in October 2020. To this day, the fossil’s location and buyer remain unknown. The scientists interviewed by National Geographic expressed concern about the ethics of relying on Stan and other privately held fossils of Tyrannosaurus.
Persons says that the study first entered scientific peer review before Stan’s 2020 sale, and he and Paul add that the study’s analyses rely on the largest possible sample sizes, which cannot be achieved without including the privately held fossils. “If we don’t use them the sample size is too small, and we can’t do anything; we’d have to wait decades,” Paul says.
Paul also argues that the new paper may throw a wrench into the fossil market, since the new study argues that not all Tyrannosaurus bones can be assigned confidently to the iconic name of T. rex. If Paul’s study is right, Stan is no longer T. rex; it’s T. regina.
The new study’s authors know that they are making a contentious argument for how Tyrannosaurus should be studied and described. To Persons, the naming of a species is ultimately a hypothesis—one that later data can either refute or confirm. “All it takes is one or two specimens that break those rules, right? … [Then] it’s back to the taxonomic drawing board,” he says.
“It would not surprise me at all—I’m realistic about my statistics—that it does turn out that [the new species definitions] are not right,” Persons adds. “What I am confident about is that there’s got to be more than one species of Tyrannosaurus.”
For Zanno, the study marks a first step toward a more comprehensive charting of the tyrant lizard king’s family tree. “No one hypothesis—whether it’s individual variation, whether it’s growth, or whether it’s the sexual status of your individuals—is going to explain all the variation that you see,” she says. “You need to take a multi-dimensional look at the problem.”