Just one Tyrannosaurus, or three?

This is Big Mike, a bronze statue replica of USMN 555000 (formerly MOR 555) at the Museum of the Rockies. Most people would call this a Tyrannosaurus rex, but a new article suggests that this is actually a Tyrannosaurus regina.

Tyrannosaurus rex is one of the few dinosaurs that is known by its species name. When an organism is given a scientific name, its name consists of two parts. The first part, which is capitalized, is the genus name and the second part, which is not capitalized, is the species name. Most dinosaurs are known by just their genus names. For example, Diplodocus is one of the more familiar sauropod dinosaurs. There are actually two species of Diplodocus: Diplodocus carnegii and Diplodocus hallorum. The first one, Diplodocus carnegii, is the “typical” Diplodocus. The second species used to be classified in a different genus. Seismosaurus hallorum was originally described as a new species and a new genus, until it was decided that Seismosaurus is similar enough to Diplodocus that they are actually the same genus. Then, Seimosaurus hallorum became Diplodocus hallorum. The point is, Diplodocus is a genus name which actually describes two different species of animal, yet we are content to simply used the name Diplodocus without specifying the species.

When there are multiple species for a single genus, it is convenient to use an abbreviated form of the scientific name. For example, Diplodocus carnegii becomes D. carnegii and Diplodocus hallorum becomes D. hallorum. Going back to Tyrannosaurus rex, its name is often shortened to T. rex. Since there is only one species of Tyrannosaurus, there has only ever been one Tyrannosaurus.

Until now.


A recent study suggests that the species T. rex is too broad. The authors of the article suggest that the range of variation within T. rex is broader than the variation seen within other theropod dinosaurs. Thus, they suggest that T. rex should be split into three species: Tyrannosaurus imperator, Tyrannosaurus regina, and, of course, Tyrannosaurus rex. Admittedly, these names are pretty clever and cool: Tyrannosaurus imperator means “tyrant lizard emperor,” Tyrannosaurus regina means “tyrant lizard queen,” and, of course, Tyrannosaurus rex means “tyrant lizard king.”[1] 

In case you are wondering what are the differences between T. imperator, T. regina, and T. rex, the differences are very subtle. T. imperator can be identified because it has a robust skeleton (that is, a skeleton with heavy, sturdy bones), two slender incisiform teeth in the front of the lower jaw (just to be clear, there are many more teeth in the lower jaw, but the first two are slender), and it is found in lower strata that bear Tyrannosaurus fossils. T. rex also has a robust skeleton, has one slender incisiform tooth, and is found in the higher strata that bear Tyrannosaurus fossils. Finally, T. regina has a gracile body (the bones are more slender and light), one slender incisiform tooth, and is found in the higher strata that bear Tyrannosaurus fossils. If you are familiar with some of the Tyrannosaurus specimens, Sue in the Chicago Field Museum is a T. imperator, Stan, who used to be at the Black Hills Institute, and Black Beauty at the Royal Tyrrell Museum are T. regina, and Wyrex, housed at the Black Hills Institute, is a T. rex.

I want to run down a rabbit trail for a moment. I just named several Tyrannosaurus specimens, each of which has a name. They are not alone: other named Tyrannosaurus specimens include Wankel Rex, Scotty, the Nation’s T. Rex, Ivan, Peck’s Rex, Samson, and others. Why are so many Tyrannosaurus specimens given names? To tell them apart, of course. However, they do not need names for that. Specimens in museums are given catalog numbers and those are used to distinguish individual specimens. Sue, for example, is FMNH PR2081 and Black Beauty is TMP 81.6.1 and Wyrex is BHI 6230. The first set of letters designates the museum (FMNH is the Field Museum of Natural History in Chicago, TMP is the Tyrrell Museum of Paleontology in Drumheller, and BHI is the Black Hills Institute). The second set of letters and numbers is determined by each individual museum or institution. Usually, they reflect the year the specimen was found and order when the specimen was collected (the first found in that year is 1, the second is 2, and so forth). If a specimen changes institutions, the numbers will change, but there is still a record of previous listing of the specimen so that it can still be properly found and referenced. For example, the Tyrannosaurus currently labelled as USNM 555000 used to be cataloged as MOR 555. Why the change? Because the specimen used to be in the Museum of the Rockies (MOR) but is now at the Smithsonian (United States Natural History Museum [USNM]). However, the records of USNM 555000 will include the information that it used to be MOR 555. That way, if anybody reads an old paper that describes MOR 555, there will still be a paper trail that can be followed to find its current location.

By the way, the names don’t travel so easily. When USMN 555000 was at the Museum of the Rockies, it was called Wankel Rex. When it was moved to the Smithsonian, some people began calling it the Nation’s T. Rex. However, unlike museum records, nobody is keeping track of the names of Tyrannosaurus specimens, so re-naming a specimen has the potential to create confusion. Additionally, some people continue to call it Wankel Rex, meaning that a single specimen now has two names. That is why I think that the naming of Tyrannosaurus specimens is silly: it is redundant (they already have catalog numbers) and it is unofficial (the names can change without any written record).

Besides, the naming of dinosaur specimens is almost exclusively a Tyrannosaurus thing. There are a few exceptions: there is an Allosaurus named Big Al and an Edmontosaurus named Dakota, for example. However, the majority of named dinosaur specimens are Tyrannosaurus. Why does Tyrannosaurus get special treatment?

Because Tyrannosaurus is a superstar. I am not being facetious or mocking here: I am serious. Tyrannosaurus has transcended mere paleontology and has become a cultural icon. Yes, Tyrannosaurus is the name of a genus of dinosaur, but it is also a movie star (in Jurassic Park and franchise, for example), a cartoon star (Buddy from Dinosaur Train, for example), a costume (think of that inflatable T. rex costume seen all over the place), and more. It appears on billboards, in advertising, as a symbol for museums, as toys, as video game villains, and on and on. Sure, other dinosaurs do some of these things, but none of them are as prevalent as Tyrannosaurus.

A fun Tyrannosaurus skeleton at the Heart of America Science Resource Center. Sure, the reconstruction might be terribly inaccurate (Tyrannosaurus bodies are supposed to be horizontal, not inclined), but it is a fun skeleton for children (in this picture, three of my children) to see.

How did Tyrannosaurus get to be so popular? I heard it explained that “teeth sells.” People like to see big animals with sharp teeth. Whether it is to stand in awe of them or to be terrified by them, we seem to be attracted to big, scary predators. I agree that certainly helps with the image of Tyrannosaurus. However, Tyrannosaurus is not the largest theropod dinosaur. Carcharodontosaurus and Gigantosaurus are the same size as or marginally larger than Tyrannosaurus. And of course, there is Spinosaurus, which is widely regarded as the largest theropod of all time. If “teeth sells” and “bigger is better,” why don’t we see Carcharodontosaurus, Gigantosaurus, and Spinosaurus as often as we see Tyrannosaurus? The truth is, we see Carcharodontosaurus, Gigantosaurus, and Spinosaurus in popular media. All three appear as toys and models, in documentaries, and in video games, so they certainly have attention given to them. However, none of them approach the popularity of Tyrannosaurus. Of Carcharodontosaurus, Gigantosaurus, and Spinosaurus, only one has really broken into the movie industry, and that is the appearance of Spinosaurus in Jurassic Park III. Sprinosaurus has been riding that popularity ever since. Notice something there: it is not the size of a dinosaur that determines its popularity, but rather its appearance in popular media. People know about Spinosaurus because they saw it on movie screens. Carcharodontosaurus and Gigantosaurus are not as popular because they haven’t had a break-out role yet.

It is probably its appearance in movies that helped propel Tyrannosaurus to stardom. As far as dinosaurs discovered in the Golden Age of American paleontology goes, it was discovered rather late. Tyrannosaurus was named in 1905 while other classic dinosaurs were named earlier (Stegosaurus in 1887, Brontosaurus in 1879, Allosaurus in 1877, and Triceratops in 1889). Tyrannosaurus kind of snuck in at the last minute (again, relative to the Golden Age of American paleontology) and showed up all previous theropod dinosaurs by being the biggest. It wouldn’t be for another ten years before Spinosaurus would be found, and even then, the fragmentary nature of the first remains of Spinosaurus masked its true size for decades afterwards. In short, Tyrannosaurus was the undisputed “big guy” for a long time. Which means it was in a prime position to take center stage once dinosaurs began appearing in movies.

One of the first appearances of Tyrannosaurus in the movies, in this case, in The Lost World (1925).

What I believe is the first full length movie featuring dinosaurs is the 1925 silent film The Lost World. It is based on the 1912 novel of the same name written by Sir Arthur Conan Doyle (yes, the same person who created Sherlock Holmes). The 1925 movie is actually pretty good for its time. The dinosaurs were presented using stop motion animation, and rather than showing them as mere monsters, they were portrayed as animals. For example, the dinosaurs stampeded from a forest fire. Triceratops parents defended an infant from a predatory attack. They were shown doing actual animal things. Sure, the fights between the dinosaurs were some of the highlights of the film, but there was some care to portray the dinosaurs as animals in between the fights. However, it is during the fights where Tyrannosaurus makes his grand appearance.

Tyrannosaurus and Agathaumas square off in The Lost World.

There is a sequence in the 1925 The Lost World where an Allosaurus menaces a Triceratops. The Allosaurus ends up killing the Triceratops. Later, the Allosaurus encounters an Agathaumas (basically, a bigger, badder Triceratops: Agathaumas is based on fragments and is probably just a Triceratops) and this time, the Agathaumas defeats the Allosaurus. Then Tyrannosaurus shows ups and kills the Agathaumas. That is Tyrannosaurus‘s introduction in movies: being the biggest, baddest dinosaur around. It has kept that title ever since. In King Kong, which came out in 1933, the titular ape demonstrates his prowess by defeating a Tyrannosaurus in a wrestling match. Tyrannosaurus has been featured in almost every dinosaur movie since that time, usually showing up as the big bad of the film. That includes Jurassic Park, which came out in 1993. Yes, everybody remembers Velociraptor from that movie, but it is the Tyrannosaurus that (inadvertently) rescued the people from the Velociraptors in the climax of the movie. It is as if the audience needed to be reminded that sure, Velociraptor is cool but Tyrannosaurus is still the King (or the Queen, considering that the Tyrannosaurus in the movie was a female). In fact, that very same Tyrannosaurus returned for Jurassic World, Jurassic World: Fallen Kingdom, and will appear again in Jurassic World: Dominion. And by “that very same Tyrannosaurus,” I mean that individual from the first movie is the very same animal seen in the Jurassic World trilogy. Clearly, the legacy of Tyrannosaurus as a movie star is going strong. So sure, teeth sells, but Tyrannosaurus just happened to be sold to the public when dinosaurs in movies first became a thing, and so it transitioned from biggest theropod to movie star and its fame has been fixed ever since.

Perhaps one of Tyrannosaurus’s most famous early movie appearances, as King Kong’s opponent.

The fame of Tyrannosaurus has produced some ridiculous side effects, such as the endless naming of Tyrannosaurus specimens, but it has made it a popular dinosaur, both in the public and among paleontologists. In fact, Tyrannosaurus is probably the best known dinosaur, thanks to the attention it gets. We know more about its diet, the morphology of its skeleton, its habits, its bite force, and its life history than we do any other dinosaur (that is my opinion, but I think any paleontologist would agree that Tyrannosaurus is very well known compared to most dinosaurs). All of this attention has led to paleontologists noting details about Tyrannosaurus.

One such detail is that Tyrannosaurus can generally be divided into gracile and robust forms. A gracile form is one that is more slender and lanky compared to the heavier, larger robust form. Note that the separation of gracile and robust forms is arbitrary and controversial. While some people talk about the gracile and robust forms as if they are two distinct body types, others believe that they are just two ends of a spectrum of Tyrannosaurus diversity. There are robust and gracile people today, but there are also people who lie in the middle. It just represents variation within a species, nothing more. However, the “gracile/robust” divide has sparked speculation. Why are there two forms? Could it be that one is a male and the other is a female? The latter suggestion became quite popular after the discovery of FMNH PR2081 (or “Sue” as she is called). Since PR2081 was the largest and most robust specimen of Tyrannosaurus at the time, people began to speculate, “Since the specimen is called sue, could it be that ‘Sue’ actually is a girl? Are the robust Tyrannosaurus specimens the females and the gracile forms the males?” There was little evidence to actually show that PR2081 was a female, but it was noted that in predatory birds, the female is larger than the male, so since dinosaurs are related to birds (according to the theory of evolution), perhaps they had a similar sexual dimorphism. “Sexual dimorphism” refers to when the two sexes have distinct physical characteristics. I always thought that they argument that the robust form was female was odd, because in other birds, such as chickens, the sizes are the opposite: males are bigger than females. Why use predator birds specifically as a point of comparison? My guess is that it was because that allowed “Sue” to be a girl.

The idea that we can easily tell male and female Tyannosaurus‘s apart had died down: it is generally agreed among paleontologists that there is no easy way to tell them apart. There is a way, but it takes microscopic examination of the bones, and even then, it can only tell if it is a female that has been laying eggs (eggs require calcium, and a female will use the calcium in its own bones as a source for the egg shells: removing calcium from the bones for egg laying leaves tell-tale marks[2]). However, minor differences among Tyrannosaurus skeletons have led some paleontologists to suggest that there are multiple species of Tyrannosaurus, long before the recent paper came out.

I had a person experience with the suggestion that there are multiple Tyrannosaurus species. I used to work at the Museum of World Treasures in Wichita, Kansas. One of the museum’s centerpieces is a Tyrannosaurus named Ivan. Naturally, since there was only one named Tyrannosaurus species, we labelled it as a Tyrannosaurus rex and took pride in the fact that no other museum in Kansas had a T. rex on display. Imagine our surprise when the Natural History Museum at the University of Kansas had a press release claiming that they had just uncovered a Tyrannosaurus skeleton and that it will be the first T. rex to be on display in Kansas. Everybody at the Museum of World Treasures collectively said, “Hold up, what about Ivan?” After contacting the staff at the museum in KU, we got an answer, “Yes, we know about Ivan, but we are not sure that it is actually a T. rex.” See, the head paleontologist at KU was of the opinion that there were two separate species of Tyannosaurus. The second species had not been named (I believe he called it Tyrannosaurus x), but he was sure that there were two, and the only way to tell them apart was by the number of teeth. Since Ivan had no skull (the skull on display is a replica), he didn’t know for sure whether Ivan was a T. rex or a T. x, so he claimed that his museum’s Tyrannosaurus was the first known T. rex on display in Kansas.

The KU museum eventually toned down their claim, but I wanted to point out that suggesting that there are multiple species of Tyrannosaurus is not a new thing: the controversy I described occurred in 2016. Even before that, the suggestion was made that there might be two species of Tyrannosaurus. Of particular note is a book titled Predatory Dinosaurs of the World. In its description of Tyrannosaurus rex, it notes that it is possible that this one species could be classified as two, but there isn’t enough information to tell yet.[3] Why is this notable? For two reasons. The first is that the book was published in 1988, so the idea of T. rex being multiple species goes back decades. Second, the author of the book is Gregory Paul, the lead author of the recent article claiming that there are three species of Tyrannosaurus. It took 32 years, but Greg Paul finally got the evidence to show that there are multiple species of Tyrannosaurus.

Except, even now, his idea is controversial. Not everyone is accepting the paper’s conclusion.[4] I don’t. For one, the authors of the paper do not seem to be completely convinced themselves. They say things like, “The data do not meet the ideal proof of any of the four hypotheses, but they do significantly favor one over the other three.”[5] The “four hypotheses” mentionec are four ideas to explain the variatoin seen in Tyrannosaurus specimens. These four ideas are, individual variation, ontogentic variation (changes as the animal grows up), sexual dimorphism, and multiple species. So they cannot claim conclusive proof, only that the multiple species hypothesis is the best in their opinion. They also say, “The question is, therefore, whether the three morphotypes [the three forms of Tyrannosaurus] should be merely noted and otherwise not formally recognized, or if separation at the species level is advisable. During extensive discussions amongst authors of this paper all agreed there is sufficient evidence to show that there were morphological changes over time, and that the degree is sufficient to at least justify and perhaps compel taxonomic recognition.” To me, “at least justify and perhaps compel” sounds a bit like waffling. “We want to say that there are three species, but we can’t prove to you that there are, but we are going to go ahead and name two new species anyway.” However, that is a subjective interpretation of their words on my part.

A more important reason for questioning their identification of three Tyrannosaurus species is that their definitions of the three species overlap far too much. Here is their definition of Tyrannosaurus rex:

[G]enerally robust with an adult femur-length/circumference ratio of about 2.4 or less; usually one slender anterior incisiform dentary tooth.

Here is their definition of Tyrannosaurus imperator:

Generally robust with an adult femur-length/circumference ratio of 2.4 or less; usually two slender anterior incisiform dentary teeth.

Here is their definition of Tyrannosaurus regina:

Generally gracile with an adult femur-length/circumference ratio over 2.4, usually one slender anterior incisiform dentary tooth.

Now, the authors also separate the three Tyrannosaurus species by distribution: T. imperator is found in the lower and possibly middle Hell Creek Formation (and other formations) while T. rex and T. regina are found in the upper and possibly middle Hell Creek Formation (and other formations).

Now, here is my question: what do we call a Tyrannosaurus found in the middle of the Hell Creek Formation with two slender incisiform dentary teeth and a femur-length/circumference ratio of 2.3? Since it has a “robust” ratio, it must be either T. imperator or T. rex. Being found in the middle Hell Creek doesn’t help since both of these species may be found there. Thus, we turn to the incisiform teeth: T. rex has one while T. imperator has two, so the specimen described must be a T. imperator. Except that the definition of T. rex says that it is “usually” has one slender incisiform tooth. What if this is a T. rex that has an unusual number of slender incisiform teeth, namely two? Thus, we would guess that the specimen described is a T. imperator but we cannot rule out the possibility that it is a T. rex.

Similarly, what if we found a Tyrannosaurus in the upper Hell Creek with one incisiform tooth and a ratio of 2.42? Since it is from the upper Hell Creek and has one incisiform tooth is that is must be T. rex or T. regina. A ratio of 2.42 is greater than 2.4, so it would appear that this is a T. regina. However, the definition of T. rex is that is has a ratio  that is “about 2.4 or less.” The ratio of 2.42 is about 2.4, so we cannot rule out the possibility that this specimen is a T. rex.

See the problem? There are only two differences between the three species (teeth number and circumference) but there is an overlap of these two traits between the three species. T. rex kind of straddles the middle, and a single specimen can either be a T. imperator or a T. rex or a single specimen can be a T. regina or a T. rex. That can make it impossible to actually distinguish a given Tyrannosaurus as any particular species. In fact, the authors list 16 specimens of Tyrannosaurus that have insufficient evidence to distinguish which species there are. They can only classify 8 specimens as T. rex, 12 as T. imperator, and 7 as T. regina. That is, out of 43 specimens, only 27 can be classified as specific species: 16 remain unclassifiable. That doesn’t simplify our classification of Tyrannosaurus at all: it only complicates it.

Since having three species complicates the classification of Tyrannosaurus and considering the overlap between the three species, I think it is safe to say that T. imperator and T. regina have a tough road ahead of them to prove that they are separate species. Most likely, paleontologists will continue to treat Tyrannosaurus as a single species, Tyrannosaurus rex. You will occasionally find a paleontologist who will reference this article and claim that such and such a specimen is actually a T. regina or is actually a T. imperator, but they will be an exception to the rule.

Thoughts from Steven

[1]Paul, Gregory; W. Scott Persons; and Jay Raalte (2022) “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 TyrannosaurusEvolutionary Biology https://doi.org/10.1007/s11692-022-09561-5

[2]Schweitzer, Mary; Wenxia Zheng; Lindsay Zanno; Sarah Werning; and Toshie Sugiyama (2016) “Chemistry supports the identification of gender-specific reproductive tissue in Tyrannosaurus rexScientific Reports 6: 23099

[3]Paul, Gregory (1988) Predatory Dinosaurus of the World: A Complete Illustrated Guide, Touchstone, New York, pg. 345

[4]Silk, John (2022) “Scientists Find Tyrannosaurus had two more species other than ‘rex'” DW retrieved from https://www.dw.com/en/scientists-find-tyrannosaurus-had-two-more-species-other-than-rex/a-60954222 on March 9, 2022

[5]Paul, Gregory; W. Scott Persons; and Jay Raalte (2022) “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 TyrannosaurusEvolutionary Biology https://doi.org/10.1007/s11692-022-09561-5

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