Tyrannosaurus rex, whose name translates to the tyrant lizard king, has long charmed the public as the star dinosaur in the Jurassic Park series. But the scene-stealing dino is stealing the spotlight again—this time, in a drama over how it should be classified. A controversial new study published in the journal Evolutionary Biology suggests that there may not just be one species in the Tyrannosaurus monarchy, but three, with T. regina and T. imperator as the long-lost cousins of T. rex.
All hail the king (rex), queen (regina), and emperor (imperator) of the prehistoric kingdom? Not so fast, say other researchers, who argue would-be differences in the fossil specimens are too minor to support such a dramatic rift.
To divide an extinct organism into species A, B, and maybe even C, there needs to be “enough separation” between the groups in the fossil record, says Ashley Poust, a paleontologist at the San Diego Natural History Museum who wasn’t involved in the study. He calls it “one of the biggest problems” of species identifications that only rely on what the eyes can discern.
Tyrannosaurus rex dominated the food chain in Northern America from 68 million to 66 million years ago. Over its two-million-year reign, members of the Tyrannosaurus genus could have spun off into several species, says Paul Sereno, a paleontologist at the University of Chicago who also wasn’t involved in the study. Just like the assortment of today’s predators roaming the African Serengeti, from lions to cheetahs to leopards, the top carnivores of the late Cretaceous period could have similarly diverged.
“It’s difficult to believe that one species could [have lasted] millions of years across that expansive territory, with the amazing amount of herbivores that were out there to be eaten,” says Sereno.
The study authors use two skeletal features, the stockiness of the femur and the number of teeth, to argue that T. rex should be redefined as three species. They recorded the length and diameter of thigh bones from 37 specimens. With their data, they gleaned that some Tyrannosaurs could be of a chunkier variety with a more robust femur. Or, the dinosaurs could have slender builds, as suggested by slimmer bones.
Moreover, the researchers propose that different Tyrannosaurus species could either have one or two incisors per skull—the sharp tooth adapted for ripping into flesh. The collaborators named the stockier, double-incisor carnivore T. imperator. Another hunky species with one incisor remained T. rex. Finally, they called the single-incisored, svelte dinosaur T. regina.
“This is a fairly subtle example of evolution [and] speciation,” says study author Gregory Paul, a freelance paleontologist. He thinks that as new Tyrannosaurus fossils are discovered, the larger sample size might allow researchers to run statistical analyses to unearth fresher findings about the tyrannical beasts. “Science is not dogmatic,” he adds, and what the world knows about the prehistoric lizard monarchy “is not set in stone.”
But two bodily features aren’t enough to tell different species apart, says Thomas Carr, a vertebrate paleontologist at Carthage College in Kenosha, Wisconsin, who didn’t participate in the study. Carr previously analyzed 1,850 attributes of Tyrannosaurus fossils, and concluded that the dinosaur should remain under one species. There was no meaningful clustering among the attributes to bifurcate T. rex into multiple species. If a checklist of nearly 2,000 traits can’t justify the existence of T. rex‘s long-lost cousins, two indefinite patterns just won’t cut it, says Carr.
“The features that identify species are utterly unique, smack-in-the-face-with-a-frying-pan obvious,” he notes. He thinks that femur size and incisor number shouldn’t qualify, given that the study couldn’t identify a quarter of the Tyrannosaurus specimens using the same metrics, despite the nearly perfect conditions of their skulls.
Any differences perceived in this study can be chalked up to variation among individuals within a species, Carr adds, like how Homo sapiens can come in different shapes, sizes, and skin tones.
[Related: How do we know what dinosaurs looked like?]
Other experts agree that the two features selected in the study aren’t distinct enough to diagnose different species. Jingmai O’Connor, an associate curator of fossil reptiles at the Field Museum of Natural History, Chicago, has a bone to pick with the vaguely descriptive terms peppered throughout the paper, including qualifiers such as “generally” and “usually.” She says such analysis might be “arbitrarily drawing the line in all the variation” when the disparities between the three supposed groups aren’t clear-cut at all.
The Field Museum houses Sue, the world’s most complete T. rex skeleton and possibly one of the biggest. For now, Sue will keep its designation of king, despite the study suggesting its reclassification to the rank of emperor.
It’s plausible that there were multiple species of Tyrannosaurus during their heyday, says Poust from the San Diego Natural History Museum. But he also thinks that the study’s fossil evidence might be insufficient to back the claim up and warrant the naming of new dinosaurs. “[The authors] look at species in a way that’s a little unclear,” he says. “If I went to the field and I dug up a Tyrannosaurus skeleton and looked at it, could I really easily tell which of these species it’s in?”
Results aside, Carr of Carthage College is also concerned that half of the specimens in the new research are privately owned Tyrannosaurus fossils, which is a violation of the ethical standards of the Society of Vertebrate Paleontology. Relics from private collections aren’t necessarily accessible to all those who want to analyze them, so studies that use them might not be reproducible and verified by other experts.
Among the specimens in the Evolutionary Biology study is the near-complete fossil Stan, which was auctioned off to an anonymous bidder for a record-breaking $31.8 million last October. Since then, paleontologists have feared that the T. rex specimen, whose whereabouts are now unknown, might be lost to science forever. “I wouldn’t touch that stuff with a 10-foot pole,” says Carr. “We have to stick with museum and university collections that are there to provide fossils for study for all time.”