The Tyrannosaur Controversy

Maybe you heard, but earlier this year a [study] came out that suggested splitting the iconic Tyrannosaurus rex into 3 species. Usually my posts just describe the study that is done and put it into context. In this case, I will start with the normal summary, but at the end I’ll add my take on it. I try to only discuss open access research (you don’t have to pay to access the paper), because I believe science should be accessible to everyone. This paper is not open access, but I do have a pdf – please just ask if you’d like a copy. You can always email the author on the paper for a copy as well. Scientists are happy to share the work they do!


Tyrannosaurus rex was a giant, meat-eating, theropod dinosaur from western North America during the Late Cretaceous.The authors noted that Tyrannosaurus rex existed in the fossil record for possibly 2 million years. This is a *long* time, long enough for that species to have split into multiple species. Other studies have shown that T. rex has 2 main body types: a robust form and a more gracile (thinner and longer bones) form. Some paleontologists have tried to investigate if these body types could be showing female vs. male forms, without much success. Others have wondered if they are tied to different ages, but that also didn’t work out.

Robust and gracile forms of Tyrannosaurus. From Pinterest.

These authors measured the circumference and length of the thigh bone. They used the measurements to run a few different statistical tests and compared the results to other large, closely related theropods. They found that Tyrannosaurus had much more variation than other theropods.

The authors also looked for other characteristics that differed between the body types and found that the gracile forms have 1 incisor-type tooth on their lower jaw and that the robust form could have 1 or 2 of these teeth.

Figure 3 from the paper, showing the single small incisor-like tooth on the right (labeled “1”), and the spaces for 2 small incisor-like teeth on the left (labeled “1” and “2”).

Using these and other types of evidence, the authors split Tyrannosaurus into 3 species: Tyrannosaurus rex (“Tyrant Lizard King,” robust with 1 incisor-type tooth), Tyrannosaurus imperator (“Tyrant Lizard Emperor,” robust with 2 incisor-type teeth), and Tyrannosaurus regina (“Tyrant Lizard Queen,” gracile with 1 incisor-type tooth). I find their choice of names very fitting.

My Take:

Recently, other dinosaurs have been split into multiple species based on differences in their bodies and on living in different places. It makes sense that a species would evolve when given millions of years of time, since that’s how evolution works. However, the data need to support the division. In this case, I am not sure the evidence they report is enough to justify dividing the species. I have more specific thoughts, but this blog is not the space to express them. I look forward to the debates about this.