The femur was the secret evolutionary boot of giant bipedal dinosaurs – Technologist

How could the giant dinosaurs that appeared successively starting 168 million years ago, such as Spinosaurus, Giganotosaurus, Carnotaurus, and of course, the inescapable Tyrannosaurus Rex, support the weight of an elephant on just two legs? The answer lies in their femurs and was just provided in an article published on May 2 in the journal Paleobiology, which reprinted the thesis work of Romain Pintore, a young Ph.D. researcher at the Museum of Natural History in Paris.

A decade ago, Roger Benson of Oxford University showed that 12 lineages of theropod dinosaurs, bipedal carnivores, evolved independently into a form of gigantism. “Every large giant theropod descended from a small ancestor,” said Pintore. “The question was whether they relied on the same adaptations.” The femur, the central bone for studying bipedalism and locomotion, was therefore the focus of the young researcher’s efforts during the Covid-19 pandemic.

“My thesis initially focused on less popular dinosaurs, but with the lockdowns, it was impossible for me to travel to make 3D measurements of their bones for comparison,” he said. With 3D digital databases for theropod femurs more plentiful, he branched out to these animals and benefited from the help of many paleontologists around the world. “They’ve been super collaborative! In Argentina, a student took the trouble to take 300 photos of a 1.37-meter Giganotosaurus femur, so that I could reconstruct it using photogrammetry,” he said. This technique, together with 3D digital files obtained with more expensive surface scanners, enabled him to compare 68 femurs from 41 theropod species.

‘A mosaic of evolution’

The analysis highlights two points. The first is that the femurs of giants are of course more robust, with wider extremities and a larger central cylindrical part than in smaller species. “But the key adaptation concerns a muscular attachment, central to balance and locomotion, between the leg and tail, which migrates lower in the most massive specimens,” said Pintore.

The second point concerns birds, whose ancestors were theropods. T. Rex, which was not a theropod, walked using the entire leg from the hip upward, whereas walking for birds involves the knee joint, with a more static, horizontal femur. The researchers wondered whether these two potentially opposing factors might have hindered the emergence of massive species in avian theropods. “This is not the case; they are observed jointly in Utahraptor, for example, despite a mass of around 550 kilos. This is what we call mosaic evolution, which led to the emergence of large bipedal dinosaurs very similar to birds, combining large size and highly dynamic locomotion.”

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