The Tyrannosaur Controversy – Response!

Back in March, a [paper] was published that described evidence that T. rex may actually represent 3 species and not just 1. The authors used ratios of the length and circumference of the femur and also ratios of specific teeth diameters to define the differences between the 3 species. The authors of that paper named the new species Tyrannosaurus regina and Tyrannosaurus imperator

Specimens of T. rex showing the new species. According to the study, A and B are T. rex, C and D are T. regina, and E is T. imperator. Figure 1 from the original study.

That study ignited a ton of conversation and was widely published in different news outlets, almost as if the reign of the great Tyrannosaurus rex was at an end. However, from a scientific point of view, there were several questions that needed to be tested by other research groups in order to check if T. rex was actually 3 species or not. 

That study was officially [published] in July! The authors point out a few issues with the original work. They also re-did the analyses using a more thorough dataset and slightly different methods. Buckle up, this one is a doozy.

Firstly, the characteristics that the original study used to distinguish the 3 species do not actually separate all of the existing T. rex specimens – there is some overlap, making these characters difficult to use. 

Secondly, the original study did not use all of the information available for T. rex. There are 1850 traits that have been studied on Tyrannosaurus and its relatives that are available for use. Usually, new traits are added to the ones already available so that any new analyses use all of the data to get the most thorough answer. The original study based their species divisions on only the femur and tooth measurement traits.

Thirdly, the original study indicated that the amount of variation in the skeleton among the different Tyrannosaurus specimens was unusually large. When there’s a lot of variation, then it is more likely to be showing multiple species instead of just one. The new study compared the variation in Tyrannosaurus to that of 112 other species of theropods (including birds). They found a very typical amount of variation in Tyrannosaurus, not an unusually high amount.

Figure 4 from the new study shows that the variation in Tyrannosaurus (dashed line) is about average when compared to over 100 other therapods (yellow shading shows the range).

Fourthly, the tooth measurements from the original study were not consistently made. Some of the measurements were from teeth from the same side of the mouth, some from opposite sides, and sometimes no teeth were there so they measured the tooth socket instead. 

Lastly, the analysis the new study did showed that all of the information clustered all of the Tyrannosaurus specimens into 1 group, not 3. Part of the reason for that is the original study used a method in which you tell the analysis how many groups to look for, and then it usually shows you that many groups as a result. The new study did not define how many groups to look for, leaving it to the analysis to come up with the mathematically best answer (which was 1). 

Figure 3 a and c from the new study. A shows how the specimens clustered based on their femur measurements. The different colors correspond to the new species the original study categorized the specimens in (blue – T. rex, red – T. imperator, green – T. regina, purple – uncertain). The groups do not separate out the specimens by the species the original study indicated. C is the graph showing that the most likely number of groups is 1, not 3, for these measurements.

There were more discussion points in the new study than what I’ve listed here, but I hope that the case is pretty clear – Tyrannosaurus rex is a single species, not three.

An important point that the new study made is that half of the specimens in the original study came from private collections or commercial fossil companies. When specimens are sold to individuals or to companies, they become almost impossible to study. By doing this, the original study made it difficult to replicate their results. Replication is how we confirm conclusions in science, so if we cannot see the specimens, we cannot confirm a result. Specimens belong in museums or publicly accessible collections. 

You tell ’em, Indiana Jones!

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!

Summary:

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.