Guest Post: Love the Tyrant, Not the Hype

Tom and, shock!, *not* Tyrannosaurus....

Today Tom Holtz brings his piece to the table. Well, I say piece, Tom has promised me three (though he also originally promised to do a guest post about 2 1/2 years ago…) and this is a pretty big first one. Although he’s not a blogger, Tom is renowned for spending a lot of time online and handling questions from the public and getting involved in debates, so he’s very active on the outreach side of things. His recent dinosaur book featuring Luis Rey’s art really is an instant classic (and Tom maintains a great online species list for this). Anyone who knows Tom’s work knows about his fondness and affiliation for tyrannosaurs, so having just covered this little critter, it’s great timing for him to dive into tyrannosaurs for us:

PART I: Introduction, and Nuts and Bolts

Some time ago (2009-ish…) Dave asked if I could do a blog post for him. Well, I finally got around to doing that. So I decided to go out on a limb and write something about Tyrannosaurus rex.

For those who know me, there is no surprise that I am interested in tyrannosaurids. (Heck, I am in the midst of writing a kids book on T. rex for Random House.) I am hardly alone in this: tyrannosaurids in general, and T. rex in particular, are extraordinarily well-known to the public, and especially to the news media. In fact, as has been mentioned here and elsewhere in the blogosphere, T. rex has somehow become the measure of all things theropodan (or dinosaurian, or even just paleontological!) So when the alvarezsaurid Linhenykus was first reported, the press called it “a one-fingered T. rex relative”  or a “T. rex cousin”: yes, true, but far more closely related to (among other critters) Velociraptor. [Dave ads: I was of course associated with that research and did my best to excise this reference from our dealings with the meida – I failed]. Same for the basal theropod Eodromaeus, or pliosaurs, or even the stem-arthropod Hurdia.

Yes, Tyrannosaurus rex is a wonderful animal. And in some situations (for example, when discussing new tyrannosauroids like Zhuchentyrannus or Bistahieversor, or Xiongguanlong) it is very appropriate to tie these new discoveries in to their better known close relative as a way of helping the public understand them.

But if people keep on automatically defaulting to use the phrase “T. rex cousin” or “T. rex of the [fill in the blank]”, they will lose sight of why Tyrannosaurus is as interesting an organism to study as it is.

And that would be a shame, because there are many wonderful things about this species that make it a great subject of scientific research.

Tyrannosaurids are highly derived dinosaurs with really unusual morphologies

I’ve said for some time now that it is a shame that tyrannosaurids were known since the beginning of the 20^th Century, since their familiarity means that many people don’t appreciate how fricking weird they are compared to other large-bodied theropods. If the first complete tyrannosaurid skeletons were uncovered only in the 1980s (along with the first good spinosaurids, abelisaurids, and therizinosauroids), they would easily stand along side these forms as “bizarre new discoveries.”

Just to list a few features, from anterior to posterior:

Tyrannosaurids have incisor-like teeth: In nearly all other theropods the premaxillary teeth (the front of the upper jaw) are grossly similar to those in the maxillae and the dentaries. Yes, they may be smaller (although not necessarily) and are often more asymmetrical than the main teeth, but they are still roughly similar.

Not so with tyrannosaurids. Indeed, tyrannosaurid premaxillary teeth are so weird that for awhile O.C. Marsh considered some of these (under the genus name Aublysodon) to be those of Cretaceous mammals of unprecedented size! As many readers here know, tyrannosaurid maxillary teeth have a cross-section something like a capital U, with the rounded part facing forward, and the tips of the “U” (representing the carinae, or rows of serrations) facing tongueward.

Furthermore, tyrannosaurids are peculiar with their premaxillary teeth lined up in a row left-to-right. This is different than practically all other theropods, in which the premaxillary teeth for a diagonal curving backward, so that the last premaxillary tooth is well posterior to the first (middlemost) one.

This whole arrangement gives tyrannosaurids a “scraper” in the front of the jaws, very different from the arrangement of teeth behind. As shown by Hone and Watabe there is trace fossil evidence that tyrannosaurids were capable of several different sorts of feeding and food manipulation, for which the very different styles of dentition would be adapted.

The rest of their teeth aren’t “normal”, either: At least where “normal” is defined as “ziphodont”: flat-sided, blade-like teeth with serrations lined up the front and down the back, and a root which is just about the same height as crown. This sort of tooth is primitive for theropods, found in forms as diverse as Herrerasaurus, Ceratosaurus, Carcharodontosaurus, and Velociraptor.

In marked contrast, the anterior lateral teeth of tyrannosaurids are what I (and others)  have termed “knife-edged bananas”: mediolaterally thick, very deeply rooted, with asymmetrically-oriented carinae. Behind these teeth are smaller, blunter teeth that are about as close as tyrannosaurid dentition gets to the ancestral form (see Jamie Headden’s review here).

And tyrannosaurid teeth are freaking enormous, especially in Tyrannosaurus rex itself. Part of that is due to the deep roots, but even the crowns are quite long. As a consequence, any individual lateral tooth takes up more space in the gumline of Tyrannosaurus than a tooth in (for example) Gorgosaurus or Allosaurus.

Amusingly (well, at least to me), the fact that Tyrannosaurus is highly specialized in having giant sized teeth smashed into the media’s use of T. rex as the measure of all things in news reports on the naming of Bistahieversor. The headlines sometimes emphasized “New tyrannosaur had MORE TEETH than T. rex!”  Well, yes. So does Velociraptor. And Allosaurus. And Tawa. And, well, pretty much all theropods other than ornithomimids, caenagnathoids, and toothless birds.

Tyrannosaurids have broad snouts and wide backs of their heads, and T. rex more than most: Most big theropods have “hatchet-heads”: narrow mediolaterally, deep dorsoventrally, with relatively parallel sides of the skull. Technically termed “oreinirostral”, this skull form is fairly rare in the modern world, but has been much more common in the pre-Cenozoic: present not only in typical theropods, but also carnivorous terrestrial crurotarsans, predatory basal synapsids, and so forth.

Tyrannosaurids are, once again, weird, and Tyrannosaurus the most extreme of the clade. The snout itself has a more rounded cross-section (which is also present in abelisaurids, for instance), and is particularly broader across than in a comparable-sized allosauroid. And the posterior portion of the skull flares out mediolaterally, more so in T. rex than in the rest of the clade. Among other things, this increases the volume of the jaw-closing muscles; increases the size of the neck muscles (as shown by Snively and Russell); and provides for greater possibility of binocular vision than in other large-bodied theropods (as discussed by Kent Stevens).

Tyrannosaurids have big brains (for a large theropod), big olfactory bulbs, and a great sense of balance: Thanks to various teams of paleontologists armed with CT scans and visualization software (and extra special thanks to the Witmer Lab), we have a far better understanding of the insides of T. rex’s head now than just a few years ago. Among other things the scans of the brain cavity and associated structures of T. rex reveal are: that tyrannosaurids have larger brain size than other comparably-sized theropods, an aspect of their coelurosaurian ancestry retained after they evolved giant size; that the olfactory bulbs of tyrannosaurids are disproportionately larger than those of all other studied carnivorous dinosaurs of the Mesozoic, with the exception of the dromaeosaurids; and that the relative size of the semicircular canals (the organs of equilibrium) in tyrannosaurids indicate that they were highly sensitive to rapid movements of the head, again a trait inherited from their smaller ancestors.

Tyrannosaurids have really, really reduced arms: And not just short. After all, even carcharodontosaurids have short arms. Tyrannosaurid arms are short, slender, and rather short in the digit count.

Tyrannosaurids have HUGE hips: In contrast, the hind limbs of tyrannosaurids are very well developed. One of the lesser appreciated aspects of tyrant dinosaurs is how big (relatively thinking) their pelves are, and in particular their ilium and its associated muscles.

If you look at skeletons of other larger-bodied theropods, such as Acrocanthosaurus and Giganotosaurus and Carnotaurus and compare that to Tyrannosaurus you’ll find that the ilium occupies less space anteroposteriorly and dorsoventrally in the non-tyrants. Furthermore, unlike in most other large theropods the ilia of tyrannosaurids converge towards the midline, so that the muscles occupied an even greater volume (relative to body size) than the lateral view might suggest.

Tyrannosaurid feet are awesome: The other end of the hind limb is also very derived in tyrannosaurids. Tyrants differ from typical large-bodied theropods in the possession of an arctometatarsus (“compressed metatarsus”), where the middle long bone of the foot is compressed between the other two main weight-bearing bones. As described by me some time ago, and subsequently explored in greater detail by Eric Snively and colleagues, this adaptation seems to represent a functional adaptation towards enhanced cursorial (running) ability. While this does not mean that a fully grown 8 ton (or more) Tyrannosaurus rex was as fast as a racehorse, it does suggest that it was faster and more agile than other similar-sized animals (such as hadrosaurids and ceratopsids) that lack comparable speed adaptations. Furthermore, a juvenile T. rex might have been a very swift animal indeed.

This covers some of the physical attributes of the tyrant king that make it an interesting animal. However, there is more about T. rex than its anatomy that make it a wonderful study subject. More about that later.