A recent study published in the journal Evolutionary Biology has sparked a fierce debate among paleontologists about the diversity of Tyrannosaurus, the iconic predatory dinosaur that ruled western North America more than 66 million years ago. The study, led by paleoartist and independent researcher Gregory Paul, proposes that there were not one, but three species of Tyrannosaurus, based on differences in their bones and teeth.
The case for three species
Paul and his co-authors, Scott Persons from the College of Charleston and Philip Currie from the University of Alberta, analyzed 37 specimens of Tyrannosaurus and found that they varied in the robustness of their femurs (thigh bones) and the presence of certain chisel-like teeth. They also noticed that these variations correlated with the geological age and location of the fossils.
The researchers argue that these variations are too significant to be explained by individual variation, sexual dimorphism (differences between males and females), or ontogeny (changes during growth). Instead, they suggest that these variations reflect three distinct species that evolved from a common ancestor over time.
The oldest and most robust form, which they name Tyrannosaurus imperator (“tyrant lizard emperor”), lived about 70 to 68 million years ago and had two incisor teeth in each jaw. The younger and more gracile form, which they name Tyrannosaurus regina (“tyrant lizard queen”), lived about 67 to 66 million years ago and had one incisor tooth in each jaw. The intermediate form, which they retain as Tyrannosaurus rex, lived about 68 to 67 million years ago and had one or two incisor teeth in each jaw.
According to Paul and his co-authors, this scenario would reveal a more detailed picture of tyrannosaur evolution during the Late Cretaceous period. They also claim that their findings would reclassify some of the most famous Tyrannosaurus fossils on display in museums around the world, such as Sue and Stan.
The criticism from other experts
However, not everyone is convinced by the new study. Many paleontologists who specialize in tyrannosaurs have expressed skepticism or outright rejection of the idea that there were three species of Tyrannosaurus. They point out several flaws and limitations in the study’s methods and interpretations.
One of the main criticisms is that the study relies on a small sample size and a subjective assessment of morphological variation, without using rigorous statistical tests or phylogenetic analyses to support their conclusions. Thomas Carr, a paleontologist at Carthage College who has studied tyrannosaur ontogeny extensively, says that the study is “just shades of gray and shapes in clouds” and that “there’s no validity here at all.”
Another criticism is that the study ignores or dismisses alternative explanations for the variation observed in Tyrannosaurus fossils, such as individual variation, sexual dimorphism, or ontogeny. For example, Holly Woodward, a paleontologist at Oklahoma State University who has used histology (the study of bone microstructure) to estimate the age and growth rate of tyrannosaurs, says that some of the specimens used by Paul and his co-authors are clearly juveniles or subadults, not adults of different species.
A third criticism is that the study does not consider the ecological and biogeographical context of Tyrannosaurus fossils, which could also account for some of the variation seen in their bones and teeth. For instance, Stephen Brusatte, a paleontologist at the University of Edinburgh who has studied tyrannosaur diversity and evolution, says that Tyrannosaurus may have adapted to different environments or prey items across its vast range, without necessarily becoming separate species.
The implications for future research
Despite the controversy, the new study has stimulated a lively discussion among paleontologists and dinosaur enthusiasts about the nature and diversity of Tyrannosaurus. While most experts remain unconvinced by the evidence presented by Paul and his co-authors, they acknowledge that there is still much to learn about this iconic dinosaur and its relatives.
As more fossils are discovered and new techniques are applied to study them, such as histology, CT scanning, biomechanics, isotopes, and genomics, new insights may emerge about how Tyrannosaurus lived, grew, varied, and evolved. Whether these insights will support or refute the idea of three species remains to be seen.