Was There More Than One T. Rex? The Species Debate

For over a century, Tyrannosaurus rex has stood alone as the singular, undisputed king of dinosaurs. However, a controversial study published in the journal Evolutionary Biology shook the foundation of paleontology by suggesting the “King” might actually be three distinct species. This proposal sparked a fierce scientific debate regarding how we classify ancient life and what defines a species in the fossil record.

The Proposal: Three Monarchs Instead of One

In March 2022, a team of researchers led by independent paleontologist Gregory Paul, along with Scott Persons and Jay Van Raalte, published a paper arguing that the variations found in Tyrannosaurus skeletons were too significant to belong to a single species.

After analyzing skeletal remains from 37 specimens, the team proposed reclassifying the fossils into three separate groups based on their stratigraphy (the rock layer where they were found) and physical characteristics.

The Proposed Species Trio

The researchers suggested the following three species:

• Tyrannosaurus rex (King of the Tyrant Lizards): This would remain the standard classification for younger fossils found in the upper layers of rock. These specimens generally have robust femurs and one incisor-like tooth in the lower jaw.
• Tyrannosaurus imperator (Emperor of the Tyrant Lizards): These represent the oldest fossils found in the deepest layers of the Hell Creek Formation. These specimens possess robust femurs like T. rex but have two incisor-like teeth.
• Tyrannosaurus regina (Queen of the Tyrant Lizards): This species would encompass the more slender (gracile) specimens found in the middle and upper rock layers. They are characterized by thinner leg bones and one incisor-like tooth.

The Evidence: Femurs and Teeth

The primary argument for splitting the genus relies on two specific anatomical features: femur robustness and tooth count.

The Femur Ratio

Gregory Paul and his team measured the length and circumference of the femur (thigh bone) in various specimens. They found that some bones were thick and heavy (robust), while others were surprisingly slender (gracile).

According to their data, robust femurs outnumbered gracile ones by a ratio of two to one. They argued that if the difference were merely due to sex (male vs. female), the ratio should be closer to ⁵⁰⁄₅₀. Since the robust forms appeared earlier in the fossil record, they hypothesized that T. imperator eventually evolved and split into the robust T. rex and the slender T. regina.

Dental Differences

The second metric involved the “incisiform” teeth at the very front of the lower jaw. Some skulls featured two of these small teeth on each side, while others only had one. The 2022 study attempted to correlate these dental counts with the femur measurements to create distinct clusters of species.

The Rebuttal: Science Pushes Back

The scientific community responded quickly. In July 2022, a separate team of paleontologists, including Thomas Carr from Carthage College and Steve Brusatte from the University of Edinburgh, published a refutation in the same journal. Their analysis concluded that the evidence for splitting T. rex was insufficient.

Variation Within Species

The counter-study argued that the differences identified by Paul’s team were minor and fell well within the expected range of variation for a large predator. To prove this, they compared the Tyrannosaurus measurements to those of 112 distinct species of living birds (the closest modern relatives to dinosaurs) and four other theropod dinosaurs.

They found that modern species often show significant variation in bone structure without being different species. For example, looking at the skeletons of modern lions or wolves reveals differences in size and bone thickness based on age, nutrition, and environment, yet they remain the same species.

Issues with “Gracile” vs. “Robust”

Critics pointed out that using bone thickness is unreliable because “robustness” is a continuous spectrum rather than two distinct groups.

• Individual Growth: As dinosaurs aged, their bones naturally thickened to support increased weight. A “gracile” skeleton might simply be a younger adult, while a “robust” one is a fully mature individual.
• Distorted Fossils: Millions of years of pressure underground can warp or flatten bones, altering their measurements.
• Sample Size: While 37 specimens sound like a lot, many are fragmentary. Statistical clustering based on incomplete data sets can lead to false patterns.

The Verdict: Stan, Sue, and the Status Quo

If the three-species hypothesis were accepted, it would require renaming some of the world’s most famous fossils.

• Sue: The massive skeleton at the Field Museum in Chicago fits the criteria for Tyrannosaurus imperator.
• Stan: The famous specimen sold at auction for $31.8 million would likely be reclassified as Tyrannosaurus regina.

However, the current consensus among major paleontological institutions and researchers is that the evidence does not support the split. For now, Sue, Stan, and other specimens remain Tyrannosaurus rex. The variation is widely accepted as a mix of individual differences, sexual dimorphism (differences between sexes), and growth stages (ontogeny).

This debate highlights a fundamental challenge in paleontology. When working with animals that have been dead for 66 million years, scientists cannot observe behavior or DNA. They must rely on bones alone, making the line between a variation and a new species blurry. Until a “smoking gun” fossil appears—perhaps a distinct juvenile preserved with clear differences—T. rex retains its singular title.

Frequently Asked Questions

Why did researchers want to split T. rex into three species? Researchers noticed variations in the thickness of the femur (thigh bone) and the number of front teeth in the lower jaw. They believed these differences, combined with the rock layers where the fossils were found, indicated three distinct species evolving over time.

What is the difference between robust and gracile? “Robust” refers to bones that are thick, heavy, and built for strength. “Gracile” refers to bones that are more slender and lightweight. The debate centers on whether these are different species or just differences in age and gender.

Are T. imperator and T. regina officially recognized? No. While the names were proposed in a scientific paper, they have not been widely adopted by the international scientific community. Most paleontologists consider the study inconclusive and continue to classify all specimens as Tyrannosaurus rex.

Could the differences be male vs. female? Yes. This is called sexual dimorphism. Just as male and female birds or mammals often differ in size and shape, it is highly possible that female T. rex were larger and more robust than males (or vice versa), which would explain the skeletal variations without needing new species names.