One of the great things about conferences is that you do get to see new work coming though. As a researcher you are generally familiar with major projects and the research themes of your colleagues and collaborators, but people entering the field for the first time, especially PhD students, can spring a surprise. At the Beijing Flugsaurier, I was delighted to meet W. Scott Persons IV who was there talking about pterosaur tails, but is looking at dinosaur tails primarily for his thesis under Phil Currie. Scott recently had a great paper published about the structure of tyrannosaur tail musculature and was kind enough to pen this for me to explain more. The photos are Scott’s property with the exception of art generously on loan from Scott Hartman. Right onto the tails:
The holiday season is here, and for me that means making a yearly migration from grad school in Alberta back to my home in sunny North Carolina. It will also mean slogging through predictable yuletide conversations with my visiting relatives: “So, you’re still going to school in Canada? It’s cold up there, aye?” Followed by: “Still into dinosaurs, huh? What about them are you studying?” I’ll say that I’m doing a project on the evolution and functional morphology of carnivorous dinosaur tails. After that, there’ll be polite smiles, an awkward silence, and finally: “Well . . . isn’t it interesting how specialized you can be, nowadays?”
I think my extended family members are always expecting and hoping that I’ll say my research is on a sexy topic, like mass extinction, the origin of birds, or at least something related to jaws or claws, and I’m always left feeling a little bitter by their disappointment. Of course, my relatives will be right. My research project is extremely specialized and several aspects of it would not have been possible ten years ago. But, investigating the posteriors of theropods (the group that includes the bipedal carnivores, like Tyrannosaurus and Velociraptor) is nontrivial, and the results have broad implications.
We mammals suffer from inherent tail prejudice. Big modern terrestrial mammals, like elephants and rhinos (which we often use as analogs, when thinking about dinosaurs), have short, scrawny, flyswatter tails, and our own species (with nothing but a vestigial nubbin) is at the furthest extreme. As a result, when considering dinosaur behavior and locomotion, there has been a tendency to not give tails their due.
Pick up a kids book on Tyrannosaurus (of which there are many), and, if the function of T. rex’s tail gets mentioned at all, it will probably be in the context of what I call the “Seesaw Explanation”. That goes something like this: T. rex was built like a seesaw, with its two legs acting like the central fulcrum (about which the two teeters totter). Sitting on one end of the seesaw was the weight of T. rex’s big head, tiny arms, and torso. On the other half was its big tail. According to the explanation, T. rex needed a big tail to keep it balanced, because, without it, the seesaw would have too much weight on one end, and the ‘tyrant lizard king’ would fall flat on its face!
But consider this saurian seesaw from the perspective of Darwinian economics. The tail of T. rex really was big. It accounted for more than half of the animal’s total length, and it was made of muscle and bone, which are metabolically expensive materials for an animal to produce. It isn’t that the Seesaw Explanation is wrong — the horizontal body position of Tyrannosaurus and most other theropods did give them a seesaw-like build, and the tail of T. rex was probably important for balance. But the explanation is incomplete and implies that carnivorous dinosaurs were so biomechanically maladapted that they needed to carry around an enormous counter weight, just to keep from toppling over. In fact, the tails of theropods were much more than dead weight, and (as the popular press has become found of hearing me say) there was more than junk in T. rex’s trunk.
Unlike modern mammals, most modern lizards and crocodiles have, undoubtedly, a superior appreciation for the benefits of a robust tail. The first step in my theropod tail study was to perform tail dissections on a series of modern reptiles. I began with a caiman (a South American crocodilian), and before even putting it under the knife, I was surprised by its tail morphology. The caiman, like all crocodilians and most other reptiles, has a tail that is really girthy just past the base of the hips. The tail literally buldges out and is wider than the hips themselves. I was expecting a tail shape similar to how the tails of theropod dinosaurs are commonly depicted – that is laterally compressed and tapering in girth smoothly from the hips.
As I cut into the caiman, I discovered what was responsible for this tail base budge: a single muscle, called the M. caudofemoralis. (The basal bulge of many crocodilians is often further exaggerated by a layer of fat, but my caiman only had a thin fatty layer). The reptilian M. caudofemoralis is unlike any tail muscle we mammals have. [Although, some mammals, including cats, do have a tail muscle that goes by the same name, the identically-named muscles are altogether unrelated (muscle terminology is confusing that way).] The reptilian M. caudofemoralis intimately ties the tail to locomotion, and it is better to think of it as an out-of-place limb muscle, rather than an axial or spinal-column muscle. The M. caudofemoralis usually extends less than halfway down the tail. Along its run, the muscle is attached to the tail vertebrae, and it is anchored, via a tendon, to the femur (the upper leg bone). When the M. caudofemoralis contracts, it swings the hind limb backwards.
If you think for a moment about how you run, you’ll realize (if it hadn’t already occurred to you) that it’s the force of the backwards strokes of your legs that propel you forward. The same is true of most reptiles, so the M. caudofemoralis makes a direct contribution to the locomotive power stroke. And not a small contribution ether! The M. caudofemoralis is usually the largest and the single most important femoral retractor. It really is fair to say that most reptiles are primarily tail propelled.
For those of you thinking phylogenetically, many modern birds also have an M. caudofemoralis, but, along with the rest of the tail, the muscle has become greatly reduced (probably relating to the evolution of forelimb powered flight and the need to minimize weight).
On to Dinosaurs
This raises the question: “Did dinosaurs have an M. caudofemoralis?”, followed immediately by: “How could you tell?” Fortunately, the M. caudofemoralis leaves some telltale signs. Most obvious is the fourth trochanter on the femur. The fourth trochanter is a usually prominent crest of bone that served as the femoral attachment site of the M. caudofemoralis. Being mammals, you and I don’t have femoral fourth trochanters, but most modern reptiles do. As it turns out, so did most dinosaurs.
Just noting that most dinosaurs had an M. caudofemoralis is nothing new. The big fourth trochanters are so obvious that the great anatomist Louis Dollo pointed out the presence of a M. caudofemoralis in Iguanodon way back in 1833. More recently, biomechanic Steve Gatesy published several papers discussing the muscle in theropods. What wasn’t known is just how big the muscle got. Unfortunately, the size and shape of the fourth trochanter is not an indicator of the size and shape of muscle that attached to it. Did T. rex have a tiny M. caudofemoralis, like modern birds? Or did it have a beefy budging M. caudofemoralis like crocodiles?
To come up with a method for estimating tail muscle mass, I took measurements of the skeletons and muscles of my dissection specimens. Next, I began work on a computer, using the digital modeling software Rhinoceros®. I created 3-D models of the reptile tail skeletons, and, using the muscle attachment sites as a guide, I digitally modeled each tail muscle over top of the skeleton. The computer was able to give me a volume estimation for each muscle, and, because muscle has a known and fairly consistent density, I could use the muscle volume to calculate the muscle mass. Then, I could check the accuracy of these estimated masses with the true masses measured during the dissections. Fortunately, once you’re familiar with the muscle arrangements and know what you’re looking for, tail muscles have a consistent size relative to specific portions of the skeleton. The modeling techniques developed by the project could estimate (with +/- 6% accuracy) the mass of the M. caudofemoralis and other tail muscles based only on the skeleton for a variety of tail morphologies, from crocodilians to chameleons.
The next steps were straightforward enough: visit museums, measure some dino tails, digitally model the tail skeletons, and estimate the muscle masses, but the end results took me by surprise!
Just from having done the preliminary modeling and the dissections, it was readily apparent to me that the tail of Tyrannosaurus was proportionately no less beefy than a modern crocodile’s, but, when I calculated the final modeling results, T. rex’s tail turned out to be significantly beefier. In a crocodile, the M. caudofemoralis accounts for roughly 2-3 % of the total body mass (obviously that number varies with the individual, age, and species). In T. rex the M. caudofemoralis was estimated to comprise somewhere in the neighborhood of 9-10 % of the total body mass (although Tyrannosaurus total body mass estimations are a tricky subject). Looking just at the tail muscles, the M. caudofemoralis of a croc makes up roughly 40 % of the total tail musculature. In a T. rex, it made up nearly 60 %. I also estimated exceptionally large M. caudofemoralis masses for other theropods.
Such abnormal mass estimations merit explanation. Naturally, the first thing I checked was that I hadn’t made some measurement or modeling error, but everything was in order. Upon close inspection, there are a number of theropod tail adaptations that facilitate an enlarged M. caudofemoralis, but the primary one, and the one most important in the case of T. rex, has to do with the position of the caudal ribs. In modern reptiles, the anterior tail vertebrae support small ribs that stick out transversally. The M. caudofemoralis is positioned directly below these ribs. The same arrangement was true of theropod dinosaur tail vertebrae. Except, in most theropods, the ribs were attached much higher on the vertebrae (anterior caudal ribs of most theropods are attached to the neural arch, rather than to the centrum). This left more room below the ribs to be filled by muscle, and the only muscle in this position was the M. caudofemoralis. Elevated caudal ribs are present on even primitive theropods, like Coelophysis and Herrerasaurus, so it seems theropods raised their ribs to allow the M. caudofemoralis to expand early in their evolutionary history.
Thin-tailed illustrations of most theropods couldn’t be more wrong. T. rex and company had powerful rear ends!
So, we have taken a little kid off one end of the seesaw and replaced him/her with a big kid. So, what? Well, remember, the new big kid is not some pudgy couch-potato, the kid is a junior Swarchenegger – he’s all muscle. Recognizing the true mass of T. rex’s tail muscles is important for considering questions relating to the animal’s athleticism. Again, the M. caudofemoralis is the biggest contributor to the locomotive power stroke . . . and it just got a lot bigger.
Ironically, I think the reason theropods have been drawn with thin laterally-compressed tails is because they look more aerodynamic with thin tails and appear superficially faster and more athletic. In reality, a lean-tailed theropod would be intrinsically less athletic than one with a beefy tail. A bigger M. caudofemoralis could support greater maximum running speeds and greater endurance. This is true not just in T. rex, but in the majority of theropod dinosaurs.
Just how fast was T. rex? Well, that’s a hard question, and it will take more than the tail to tell, but I think speed estimations that only put T. rex on par with similarly sized modern elephants are missing a big piece of the puzzle. I’ll say to you what, in a few short weeks, I’ll be saying over-and-over to my relatives: When you think about T. rex and its tail, don’t think of a seesaw. Instead, think of a souped-up Volkswagen, because it’s what was in the trunk that provided all the locomotive power.
W. Scott Persons, Philip J. Currie. The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non-Avian Theropods. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 2010; DOI: 10.1002/ar.21290