Tyrannosaurus rex, meaning “tyrant lizard king”, is possibly the most famous species of dinosaur. The image of a towering beast with short arms and crushing jaws (and perhaps feathers as well – scientists are still investigating!) pervades the public imagination. 

But a new and controversial paper threatens to replace that image of T. rex. Specifically, it suggests that the famed king of lizards was not actually one species, but three. The study, published in Evolutionary Biology, let one of the species keep the name T. rex, but it named the others T. regina (tyrant lizard queen) and T. imperator (tyrant lizard emperor). 

The authors of the paper studied 37 specimens of the genus Tyrannosaurus, all of which were attributed to Tyrannosaurus rex. They noted that variations in the skeletons of the specimens had been observed before, but they stated that “the possibility of sibling species within the genus Tyrannosaurus has never been tested in depth in both anatomical and stratigraphic terms.” Their analysis found that there were patterns in the differences between specimens that were consistent with the notion of multiple species. For example, they examined the incisors in some of the specimens, and found that an older specimen had two small incisors in each part of the lower jaw, while others had only one. From this evidence, they concluded that one species with stocky bones, T. imperator, had split into the small-boned T. regina and the stocky-boned T. rex.

However, many scientists disagree with this assessment. Steve Brusatt of the University of Edinburgh stated that “this variation is very minor and not indicative of meaningful biological separation of distinct species that can be defined based on clear, explicit, consistent differences.” The differences between the specimens may not be enough to justify splitting the species apart. An associate curator at the Field Museum of Natural History in Chicago called the hypothesis “very poorly justified”.

Ultimately, the taxonomic grouping of dinosaurs is more subjective than one might expect. There is no universal standard among paleontologists that delineates the borders between species. Nevertheless, it seems that until more evidence can be found to clearly differentiate between these species, Tyrannosaurus rex will stay as just one species – the one we all know and love.

References:

1. Paul, G.S., Persons, W.S. & Van Raalte, J. The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation Within the North American Genus Tyrannosaurus. Evol Biol (2022). https://doi.org/10.1007/s11692-022-09561-5
2. Elbein, Asher. “They Want to Break T. Rex Into 3 Species. Other Paleontologists Aren’t Pleased.” The New York Times, 1 March 2022, https://www.nytimes.com/2022/02/28/science/tyrannosaurus-rex-species.html. Accessed 6 March 2022.
3. Osborne, Margaret. “Why a New T. Rex Study Is Riling Up the Paleontology World.” Smithsonian Magazine, 3 March 2022, https://www.smithsonianmag.com/smart-news/scientists-propose-that-the-t-rex-was-actually-three-different-species-180979663/. Accessed 6 March 2022.