Chinese scientists have unlocked the secret of when dinosaurs and crocodiles evolved to walk upright on all fours – instead of sprawling like lizards – with the unearthing of a new species dating from more than 250 million years ago.
The team from the Chinese Academy of Sciences said the discovery of a hind limb belonging to a small, lizardlike animal from the upper Permian showed the erect walking posture evolved before Earth’s biggest mass extinction at the end of that period.
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Some scientists have previously proposed that archosaurs – represented today by birds and crocodiles – became capable of walking with their limbs perpendicular to the ground, like mammals, to avoid the heat of the Earth’s surface during the following Triassic period.
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But the latest study, published in December by peer-reviewed journal The Science of Nature, has pushed forward the timeline for archosaurs – “ruling reptiles” – which once included dinosaurs and pterosaurs.
Co-author Chen Jianye, a researcher at the Institute of Vertebrate Palaeontology and Palaeoanthropology, said the development of the necessary structure to walk upright had given archosaurs the ability to adapt to global warming.
“The evolution of walking ability was not prompted by climate change. But having such a structure made it easier for archosaurs to adapt to the warming conditions in the early Triassic,” he said.
Like today’s lizards, early reptiles crawled with their limbs almost flat and their bellies close to the ground, bending from side to side. But to run fast, like a velociraptor – a dinosaur which could reach 40km (25 miles) per hour – an animal needed to be upright. The erect posture developed by later archosaurs was “the key to running”, Chen said.
“The new posture opened up a lot of opportunities for occupying new ecological niches, providing new possibilities in animal evolution.”
The fossil hind limb, around 10cm long, was unearthed in Xinjiang, near the city of Turpan, an area rich in vertebrate and plant fossils from the Permian and Triassic periods.
The specimen has been named Vigilosaurus gaochangensis. Gaochang is the name of an ancient kingdom believed to have existed in the region where the fossil was discovered, according to the paper.
Chen said the animal was estimated to be less than 50cm (20 inches) long, unlike its contemporaries who were often top predators measuring up to three metres (10 feet). “The limb is slender, possibly pointing to its agility,” he said.
The team also found that the animal had a longest third toe and reduced fifth toe – a classic archosaur feature shared by dinosaurs, crocodiles and birds. In contrast, the fourth toe is the longest in early reptiles.
“With the longest toe being its third, its feet point and walk forward, unlike reptiles’ feet which point to the side,” Chen said.
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