Tyrannosaurus rex may have been three distinct species: study

Stan, one of the largest and most complete Tyrannosaurus rex fossil discovered, is on display, Tuesday, Sept. 15, 2020, at Christie's in New York. (AP Photo/Mary Altaffer)

New research into the remains of the Tyrannosaurus Rex suggests that the species could be re-categorized into three distinct groups based on some skeletal differences.

According to a research article published in the journal Evolutionary Biology, physical differences in the femur and dental structures across 37 different Tyrannosaurus specimens suggest that previously accepted variations between the remains could mean that one of the world's most recognized dinosaurs is actually made up of three distinct groups or species.

Across the collected specimens, some of the Tyrannosaurus had more "gracile" femur bones – meaning shorter in length and circumference – while others had more "robust" femurs. Because the larger femurs occurred in about two times as many specimens as the more slender femurs, researchers said that sex is likely not the reason for the variation, but also said they couldn't completely rule it out. Robust femurs were also found in juvenile specimens, which researchers said made the age of the specimen unlikely to be the cause either.

Researchers also determined whether the specimens had one or two slender incisor teeth, which they said also resulted in variation across the Tyrannosaurus remains.

Where the remains were found also indicates that the species may have changed as time passed, researchers said. In the lower, older layers of sediment located in the Lance Formation in the western United States, only robust femurs were found. But researchers said in later, shallower layers of sediment, more "gracile" femurs were found, suggesting that the Tyrannosaurus physically developed as time went on.

"The differences in femur robustness across layers of sediment may be considered distinct enough that the specimens could potentially be considered separate species,” the study's lead author Gregory Paul said.

As a result of these variations, the study's authors said they recommended adding two new species of Tyrannosaurus.

The first new species nominated, the researchers said, is Tyrannosaurus imperator, meaning "tyrant lizard emperor." This species would be designated for specimens with robust femurs and two incisor teeth, found in the lower and middle layers of sediment.

The second new species nominated by the researchers is Tyrannosaurus regina, or the "tyrant lizard queen," which are characterized by their gracile femurs and single incisor tooth. Researchers said this species is mostly found in the middle and upper layers of sediment.

The currently recognized Tyrannosaurs rex species, meaning "tyrant lizard king," was found mostly in the upper layers of sediment, and are characterized as having robust femurs and one incisor.

Researchers said some specimens involved in the study could not be identified based on their remains and were not assigned to a species.