Measuring about 12 meters in length, four meters high to the hip and weighing between six and nine tons, the Tyrannosaurus rex it is the best known prehistoric predator on Earth. Carnivore, fearsome and magnificent, it nevertheless walked at a speed of about 4.6 kilometers per hour: similar to that of humans. A scientific team from the Netherlands has come to this conclusion that has not looked at the legs or hips of this tyrannosaurid theropod dinosaur, from the end of the Cretaceous period, but rather at the swaying of its tail. Although other studies had already analyzed the locomotion of the Tyrannosaurus rex when not hunting or running, the role of the tail did not stand out in the final calculations. Although it was more than half its length and could weigh 1,000 kilos. The new work has been published in the magazine Royal Society Open Science.

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Animals and people save energy by walking by moving at a regular cadence. Humans reach the proper pace for our body with our legs and a certain rhythmic accompaniment of the arms. The Tyrannosaurus rex It had very short front legs, very high back legs, and an imposing tail with vertebrae and ligaments, which allowed it to store and release energy when swinging. “When the beat of the tail reaches resonance, that is, it gets the most movement with the least effort, we can speak of a natural frequency: in this case, it would be that of the animal when it walks slowly ”, says Pasha van Bijlert, 27, the main author of the work, who is studying a Master in Movement Sciences at the Free University of Amsterdam (Vrije Universiteit) and analyzes the locomotion of dinosaurs.

For the research, a digital three-dimensional model of the skeleton of Trix, an adult female of the species, was created. About 66 million years old and about 12 meters long, its almost complete remains were excavated in 2013 in the United States. To this representation they then added the muscles and ligaments of the tail vertebrae, flexible and resistant to traction and compression. In order to observe the effect on locomotion, they subjected this digital copy of the bone and muscle system to a series of biomechanical analyzes. “The swaying of the tail thanks to the ligaments served as a counterweight. It is comparable to the suspension of a bridge, and produced part of the force necessary to rhythmically push the body forward on two legs. The speed of the footfall had to correspond to the natural frequency with which the tail rises and falls, and we indicated that the step of the Tyrannosaurus rex it was 1.28 meters per second, ”explains Van Bijlert.

We have helped each other to figure out the stride of fossil footprints, which we digitally scaled to conclude that it was 1.94 meters

You also had to know your stride, that is, the distance traveled with each of the steps you took. “We have helped ourselves with fossil footprints, which we digitally scale up to the size of Tyrannosaurus rex, to conclude that it was 1.94 meters. Combining both figures in all of our calculations, we estimate that he was walking at 4.6 kilometers per hour: more or less like people, ”explains Van Bijlert. In the same conversation, he stresses that other experts’ calculations based on hip height or leg length are crucial to knowing how he was walking. “According to our interpretation, also look at the queue it can help in the results ”. The pace of leisurely walking now proposed is relatively slow, “although similar to that of other living animals, with two and four legs, such as elephants or ostriches.”

Trix It is exhibited at the Naturalis Biodiversity Center in the Dutch city of Leiden, which has participated in the work, together with the universities of Utrecht and the Free University of Amsterdam itself. The echo of the publication has been immediate and one of the comments has made the researcher especially excited. It is signed by John Hutchinson, Professor of Evolutionary Biomechanics at the Royal Veterinary College, London, and has said that the role of the tail had been neglected in analyzes of this type. Some “very encouraging” words for Van Bijlert, who dropped out of medical studies when he realized that what interested him were dinosaurs. As there is no Paleontology career in Leiden, he studies them through Movement Sciences. Anyway, he recognizes that his passion comes from afar and has seen the film “dozens of times” Jurassic Park (1993) by the American director, Steven Spielgerg.

After measuring the pace of your favorite animal, do you know how fast they were running? He calculates that at about 30 kilometers per hour, although he says that there are studies that speak of between 20 and 30 kilometers. “There are those who aim even faster, but that way they could have broken their legs. Although I hope to study this in the future ”. Formidable and terrifying, but it seems less fast than Spielberg’s tyrannosaurus chasing humans who escape its bite in the film. As this young researcher says: “They exaggerate with the speed of their Tyrannosaurus rex, but it is a joyous recreation ”.

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