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!

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