Why Tyrannosaurus Could Not Run

🦴 Everybody asks how fast Tyrannosaurus could run, and a skeleton does not record speed. So in two thousand and two, John Hutchinson and Mariano Garcia asked a different question: how much leg muscle would running at speed actually require? That number can be calculated from joint moments, and it can be checked. Run the model on a chicken and it returns a few per cent of body mass per leg, which is what a chicken has. Run it on an adult Tyrannosaurus and it returns more than forty per cent per leg — over eighty per cent of the whole animal, before any of the rest of the body exists. Fifteen years later, William Sellers and colleagues came at it from a completely different direction: a dynamic simulation that plays the movement rather than holding a pose, with a constraint on bone stress written into it. The simulation is happy to produce faster gaits, and the leg bones fail when it does. Their ceiling is around five metres per second — roughly eighteen kilometres an hour, carried on very long legs. Two methods, two research groups, broadly one answer. But this is not a story about a slow animal. Juvenile tyrannosaurs had long lower legs and light bodies, and the same calculation puts them comfortably in the range of real runners. One species occupied two different positions in its ecosystem across a single lifetime. The episode also takes on the strongest objection — why build long legs at all — using work led by Thomas Dececchi showing that above a certain body mass, long legs buy cheap distance rather than top speed. 📜 What is known and what is reconstruction Known: the bone dimensions, limb proportions, growth rings and muscle attachment scars; the measured gaits of living animals; and the few tyrannosaurid trackways, which record walking. Reconstructed: the muscles themselves, since muscle does not fossilise, and every speed figure that follows from them. Scott Persons and Philip Currie have argued that the tail-to-femur muscle is reconstructed too small, which raises the available power; the counter-argument is that bone strength, not power, sets the ceiling. Body mass estimates also vary by a couple of tonnes between studies. All imagery is an artistic reconstruction, not a record of a living animal. 📚 Sources • Hutchinson J.R., Garcia M. (2002). Tyrannosaurus was not a fast runner. Nature 415 • Sellers W.I., Pond S.B., Brassey C.A., Manning P.L., Bates K.T. (2017). Investigating the running abilities of Tyrannosaurus rex using stress-constrained multibody dynamic analysis. PeerJ 5 • Hutchinson J.R. et al. (2011). A computational analysis of limb and body dimensions in Tyrannosaurus rex with implications for locomotion, ontogeny and growth. PLoS ONE 6 • Hutchinson J.R., Famini D., Lair R., Kram R. (2003). Are fast-moving elephants really running? Nature 422 • Persons W.S., Currie P.J. (2011). The tail of Tyrannosaurus: reassessing the size and locomotive importance of the M. caudofemoralis. The Anatomical Record 294 • Dececchi T.A. et al. (2020). The fast and the frugal: divergent locomotory strategies drive limb lengthening in theropod dinosaurs. PLoS ONE 15 • Farlow J.O., Smith M.B., Robinson J.M. (1995). Body mass, bone strength indicator, and cursorial potential of Tyrannosaurus rex. Journal of Vertebrate Paleontology 15 🎬 Mesozoic Mechanics takes one feature of one animal at a time and treats it as an engineering problem: what load it carried, what it was made of, where its limits were. 🦴 Which surprised you more, the muscle figure or the juveniles? And what should we take apart next? #tyrannosaurus #trex #biomechanics #palaeontology #mesozoicmechanics