Tyrant reptile King? Which king?

Yes, my last post was a decently long time ago, so I thought I’d pick up the slack by writing about everyones “favourite” theropod, Tyrannosaurus rex. It is by far the most completely known of any dinosaur, with several famous and complete skeletons such as Sue, Stan, Trix, Tristan, AMNH 5027, and many more. The entire skeleton is known and represented, except, very funnily, for the feet *. But, oddly enough, such “completeness” may not be present, as the great Tyrannosaurus rex may be multiple species.


Tyrannosaurus “rex” specimens AMNH 5027 (above) and CM 9380 (below; holotype). Both images copyright Scott Hartman, used here simply to illustrate an idea.

Look at the image above. As per priority, the name Tyrannosaurus rex must stay with the holotype specimen, CM 9380. CM 9380 is relatively complete, as far as dinosaur specimens go, even though it lacks a tail and neck. However, there are a few minor differences between the specimens shown above, but simply not enough to change the consensus that both are T. rex. This may change with the next specimens I’ll show though.


Many Tyrannosaurus skeletons, including most of the ones mentioned here. Top to bottom: “Dynamosaurus imperosus“, CM 9380, AMNH 5027, Stan and Sue. Skeletals by and copyright GetAwayTrike

The oddest of all specimens is Stan. With much larger legs, a much smaller skull, and very different features that the other specimens, Stan is a definite candidate for a new species. Unlike T. rex, Stan has a very odd pelvis, particularly the ilium, and a very thin femur. The skull has larger fenestrae of different shapes. While the mandible is relatively similar, the cranium has some major differences.


The AMNH 5027 skull, as illustrated by, and copyright Tracy Ford. From Paleofile.


To keep the sources consistent, these Stan skulls are also by and copyright Tracy Ford.

The proportions of Stan’s skull are surprisingly different. While it would be expected that individuals of the same species are nearly identical, there are several glaring issues with that. Stan has a higher skull overall, with vertical orbits, a less rectangular antorbital fenestra, and a larger maxillary fenestra. The infratemporal fenestra has a much larger opening ventral to the constriction, the orbit doesn’t have a posterior protrusion, the lachrimal is more robust, the naris is more anterior-facing, the quadrate is more gracile, the dorsal maxilla expansion isn’t as long, the postorbital is smaller, the squamosals don’t extend as far behind the skull, the jugal fenestra beneath the antorbital fenestra is smaller, the lachrimal fossa is larger, and there is no ventral expansion at the posterior end of the maxilla. Thats around 15 features just in a moderate examination of the skull, which may even go up to 30 if I examine the skull more in-depth. With very different proportions, and many discrete characters, Stan is odd enough for me to consider it a separate species from AMNH 5027 and CM 9380, which are T. rex by default.


The dentary of Dynamosaurus, from the original description by Osborn in 1905.


The dentary of Sue in multiple views. From the Everythingdinosaurs blog, pictures by John Weinstein (Copyrighter uncertain).

Now for the third distinct section, the “Dynamosaurus” morphs. In fact, since the holotype of Dynamosaurus clearly fits into this group, I am tempted to classify them as Tyrannosaurus imperosus. These specimens, Dynamosaurus and Sue, are alike in multiple ways. From their very large size to their robust dentaries and other bones, Tyrannosaurus imperosus, while not as odd as Stan, should be a separate species in my opinion. Dynamosaurus and Sue both have a very robust dentary, with a more flattened ventral edge and a more prominent anterior dorsal expansion. The dentaries are nearly identical in shape, with a similar foramen near the anterior ventral edge, a very large and similarly shaped internal mandibular fenestra, and sutures and ridges identical in every detail as far as I can tell. From this, it can just be assumed, probably accurately, that they represent the same taxon, and that taxon is as unique as Sue, a large, robust dinosaur with odd features and a large size. Sue themself (I am not going into gender arguments) seems to have a few shared features, with the overall skull being more similar to AMNH 5027 than Stan, and the pelvis having some odd lumps and depressions like Stan, but still quite different. Trix is the only questionable specimen that may result in Sue being the same species as Stan, but if so the binomial Tyrannosaurus imperosus will remain.

* As a side note, every single specimen of Tyrannosaurus either has very fragmentary and intermediate pedal remains, or is missing 1 foot, with the other near completely preserved.


About ijreid

I am an amateur palaeontologist thats hobbies include studying extinct amniotes, specifically dinosaurs, birds, and mammals. Occasionally, I focus my time on detailed and accurate illustrations of dinosaurs, and I have completed drawings of Dysalotosaurus, Micropachycephalosaurus, Zhuchengtyrannus, Troodon, Eotyrannus, Europelta, and Achillobator. I do not believe in copyrights, and think that the world would be better if everything was open access.
This entry was posted in Anatomy, Phylogeny, Theropoda. Bookmark the permalink.

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