(ORDO NEWS) — Most reptiles, both extinct and modern, move on four legs, but dinosaurs separated from their relatives in the early stages of their evolution, becoming bipedal. And now scientists have been able to find out how they did it.
Most reptiles – from primitive procolophons to modern lizards – are quadrupedal animals, with only rare exceptions rearing up.
However, there was a group of reptiles whose members either walked all their lives on two legs, or assumed a bipedal position with relative ease, say, to survey the surroundings or to reach for food. They were dinosaurs.
Their ancestors were quite ordinary four-legged lizard-like reptiles, but in a fairly short period of time – only about 50 million years – the heads of their femurs turned inward, which made it possible for dinosaurs to position their hips strictly under the body (and not on both sides of it, as in lizards) and start running smartly on two legs.
For a long time, scientists argued how the reorientation of the heads of the femurs occurred in the ancestors of dinosaurs: either due to the appearance of a new ligament that literally twisted their legs, or due to the gradual twisting of the bone itself.
Evidence for the first theory was found in birds and late dinosaurs, evidence for the second in early dinosaurs and crocodiles.
To answer this question, an international team of researchers studied several dozen fossils and modern embryos of reptiles and birds using computed tomography.
Surprisingly, their data demonstrates that both theories are correct: embryological data show one way of eversion of the femoral head, and paleontological evidence another.
As a result, scientists came to the conclusion that in early dinosaurs, the bones first turned inward due to new ligaments, and then twisting of the bone itself was added to this process.
Thus, the authors of the new work emphasized that seemingly contradictory theories can actually be part of a single process.
The formation of the heads of the femurs in the embryos of modern crocodiles and birds occurs in different ways, but this does not mean at all that their ancestors’ legs were formed in two alternative ways.
Rather, the descendants eventually settled on only one method.
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