Tyrannosaurus Had Extra Junk in the Trunk

Almost everyone has a pretty good idea of what Tyrannosaurus rex looked like. The massive head, scrawny arms, and the bird-like posture are all iconic parts of prehistory's most famous dinosaur, but its tail would probably be tacked on as an afterthought.

You can't have a good Tyrannosaurus without a tail, but our focus has traditionally been on the business end of the animal. In a new Anatomical Record paper, however, scientists W. Scott Persons IV and Philip Currie have taken another look at the caudal portion of this animal and found it to be a bit more beefy than has been previously thought.

Except in cases of truly exceptional, three-dimensional preservation, we usually can't directly study the muscles of dinosaurs. More often, scientists must rely on muscle scars visible on bones and the musculature of extant animals to reconstruct details of soft anatomy. This is not quite as straightforward as it sounds.

Birds and crocodiles are the closest living relatives of non-avian dinosaurs, but many dinosaurs were significantly different from both in their anatomy. In the case of tails, especially, birds do not have the long, muscular tails of dinosaurs, and while crocodylians do possess long tails, their posture and mode of life are very different from those of dinosaurs. This uncertainty has led to the reconstruction of dinosaur tails as relatively thin structures which, Persons and Currie state, "appear altogether emaciated when compared to the tails of modern reptiles."

Yet, despite being evolutionary cousins with a very different natural history, crocodylians may be good proxies for understanding dinosaur tail and leg anatomy after all. As Persons and Currie point, one of the primary reasons for this association is a muscle called the M. caudofemoralis. This is a tail muscle that inserts onto the top of the femur and helps to retract that upper leg bone while walking. Its presence in dinosaurs has been noted for over 150 years, but this same muscle was reduced or lost in many birds during their evolution. This large retractor muscle is present and remains important in living reptiles such as crocodiles, however, meaning that these animals are more useful in reconstructing the tail anatomy of dinosaurs.

To better understand the role of this muscle in reptile anatomy, Persons and Currie dissected the pelvic and post-pelvic muscles of a brown basilisk, spectacled caiman, veiled chameleon, green iguana and Argentinean black and white tegu to see how the muscles in this area corresponded to the tail anatomy of the theropod dinosaurs Gorgosaurus, Ornithomimus and Tyrannosaurus. What they found was that the dinosaurs had scars related to the important M. caudofemoralis muscle stretching back to around the 12th to 14th tail vertebra in each dinosaur, but the question was how thick this muscle was at the base of the tail.

In crocodylians, the M. caudofemoralis muscle creates a thick bulge just behind the hips, and it is likely that it did the same in dinosaurs. By combining anatomical measurements from the modern reptiles with the known anatomy of the dinosaurs, Persons and Currie used computer modeling to recreate dinosaurs with thick, crocodile-like tails, and the scientists argue that this arrangement is supported by a subtle anatomical feature.

In many theropod dinosaurs, the three to four tail vertebrae behind the hips have wings of bone called transverse processes, and these flattened structures are angled upwards. As reconstructed by Persons and Currie, this arrangement would have provided expanded space for the M. caudofemoralis muscle, although they note that the transverse processes of both Gorgosaurus and Tyrannosaurus were not oriented in the same upward diagonal fashion. Nevertheless, given how many theropod dinosaurs had this expanded space near the base of the tail, it is possible that a large M. caudofemoralis muscle was a common feature of these dinosaurs stretching all the way back to early forms such as the circa 228-million-year-old Herrerarasaurus.

This new reconstruction of dinosaur tails has some important implications for how these animals moved. As a prominent retractor of the upper leg, especially, the M. caudofemoralis would have been one of the primary muscles involved in locomotion. Yet larger muscle size did not necessarily translate to greater speed. Persons and Currie found that this muscle would have been relatively larger in Tyrannosaurus than in the juvenile Gorgosaurus they examined, but the overall anatomy of Tyrannosaurus indicates that it would have been a slower runner than its more slender relative. The larger size of the M. caudofemoralis muscle in Tyrannosaurus may have been the result of being a much bigger animal and requiring more muscle power to get around. Still, Persons and Currie argue that the size of this muscle may have allowed Tyrannosaurus to achieve speeds towards the higher end (more than 10 meters per second) of what has been estimated for it, and future tests will have to incorporate the new anatomical data in order to better understand how this dinosaur moved.

Persons and Currie ask that paleoartists take note, too. Even though theropod dinosaurs are often restored with thin, "athletic" tails, the new study suggests a different sort of shape in which the tail is thick and almost square near the base, is tall and thin in the middle, and then tapers into a circular shape at the tip. Even though this arrangement enlarges the posteriors of these dinosaurs, it actually makes them more powerful runners than the thin restorations. We should expect to see more big-bootied tyrannosaurs in the near future.

References:

Persons, W., & Currie, P. (2010). 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 DOI: 10.1002/ar.21290

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.