Tyrannosaurus Rex is everyone’s favorite dinosaur, and we’ve all seen dozens of depictions of T. rex, and various more or less scientific reconstructions of its appearance and behavior.
One question has always been, “how fast did T. rex run?”
Experience from living land animals suggests that really large individuals are often slow-moving. On the other hand, T rex certainly looks like a fast runner, though it might have relied on surprise ambushes or even on harvesting carrion.
This month Sellers, William I published a study that uses mathematical models of the structure of the T rex skeleton, taking into account the strength of the bones . The idea is that running stresses the body, and ultimately an animal cannot run so fast that it breaks its bones and joints.
There is a long history of biomechanica studies of living animals which has been applied to fossils including T. rex. These methods use the measurements of the skeleton along with plausible hypotheses about the muscles and other tissues to estimate the “locomotor performance” of the ancient animals. The authors report that these studies have given a range of estimates for how fast a T rex could move, from 5 to 15 m/s, including walking and running gaits.
The current study refines these estimates using two simulations, a mechanical model of the skeleton and a model of the stress on the bones.
“Machine learning algorithms are used to generate the muscle activation patterns that simultaneously produce the maximum locomotor speed of a MBDA model of T. rex whilst maintaining defined skeletal safety factors.” (, p. 3)
These simulations were run driven by models of walking and running gaits. The detailed model involves all the muscle firings in the animal, so finding a stable gait is a huge computation. The system was run many times to search for maximum speed using the gaits. (See the paper for details.)
These computations indicate that the fast walking gait is consistent with bone stresses typically seen in living animals, which the running gaits often exceed typical stress levels. The authors argue that this indicates that adult T. rex did not run.
Considering the size of the animal, this isn’t a completely surprising conclusion. This fast walk may have been perfectly sufficient, given the size of their herbivore prey, which probably couldn’t run fast either.
The researchers are careful to point out that their simulations are simplified in order to make them computationally feasible. This method is effectively searching through all possible designs for a T. rex, which is a ludicrously large number of variables. In the future, more complete models may be possible, and the results may be refined.
They note that the behavior of a T. rex must have changed as it developed. The smaller young ones might have been fast runners, but reduced to walking as they grow enormous. But little is known about the developmental process.
The also note that their result overturned estimates based on analogy.
“It is somewhat paradoxical that the relatively long and gracile limbs of T. rex—long argued to indicate competent running ability […]—would actually have mechanically limited it to walking gaits, and indeed maximised its walking speed. This observation illustrates the limitation of approaches that rely solely on analogy and the importance of a full biomechanical analysis when investigating animals with extreme morphologies such as T. rex.” (, p..13)
Both dinosaurs and a neat example of multiphysics models, and an example of why HPC is relevant to lots of fields.
- William I. Sellers, Stuart B. Pond, Charlotte A. Brassey, Philip L. Manning, and Karl T. Bates, Investigating the running abilities of Tyrannosaurus rex using stress-constrained multibody dynamic analysis. PeerJ, 5:e3420, 2017/07/18 2017. https://doi.org/10.7717/peerj.3420