An early relative of crocodiles spent its juvenile years walking on all fours, then stood up on two legs as an adult.

Its arm and leg bones grew at different rates to enable this transition. “The forelimb starts out like 75 per cent the length of the hind-limb, and then it ends up being more like 50 [per cent],” says Elliott Armour Smith at the University of Washington in Seattle.

The finding adds to evidence that crocodile-like animals in the early dinosaur era were extremely diverse, with some even adopting bodies and lifestyles similar to those of modern ostriches.

With Christian Sidor, also at the University of Washington, Armour Smith excavated Kaye Quarry in Petrified Forest National Park in Arizona. It consists of mudstones and sandstones deposited by a river that flowed about 215 million years ago, in the Triassic period.

Armour Smith and Sidor found more than 3000 bones belonging to early relatives of crocodiles called shuvosaurids. “It’s a jumbled mess of individual limb bones that don’t necessarily have an association between the individual animals,” says Armour Smith.

Nevertheless, the pair were able to identify a new shuvosaurid, which they called Sonselasuchus cedrus. Over 950 of the bones belonged to this species. It didn’t resemble a modern crocodile, but instead looked more like a flightless bird or theropod dinosaur. Its arms were short and instead of a mouth filled with teeth, it had a toothless beak.

Other shuvosaurids have similar bodies. “Shuvosaurids are these absolute weirdos that live in the late Triassic,” says Michelle Stocker at Virginia Tech. “They really look like dinosaurs.” They most resemble ornithomimids, which were ostrich-like dinosaurs that lived in the late Cretaceous period, more than 100 million years after the shuvosaurids.

Sonselasuchus cedrus seems to have started life walking on all fours. Bones from younger individuals show the front and rear limbs were relatively similar in size. But in older individuals, the hind-limbs grew much more and also showed signs of bearing more weight. “The larger femurs in the population are rather robust,” says Armour Smith, whereas “even the largest humerus is relatively delicate”.

This is unusual, but not unique. A 2019 study found evidence of two dinosaur species transitioning from quadrupedal to bipedal walking as they grew. One was a sauropodomorph, an ancestor of the huge sauropods like Brachiosaurus, and the other was an early ceratopsian distantly related to Triceratops.

It may be that juvenile and adult S. cedrus lived fairly separate lives and even ate different diets, as some crocodilians do today, says Stocker.

Popular descriptions of the dinosaur era often give the impression that dinosaurs, especially birds, were evolving in very creative ways, while crocodiles pretty much stayed the same. That misrepresents the diversity of pseudosuchians – the branch of the reptile tree that includes crocodiles, says Stocker. “They’re actually doing a lot of the really unique, crazy stuff first, and then dinosaurs are picking it up later.”

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