Ancient ‘weird’ reptiles graduated from 4 legs to 2 in adolescence

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

The arm and leg bones grew at different rates to make this transition possible. “The forelimb starts out as 75 percent of the length of the hind limb, and then it ends up being more like 50 (percent),” says Elliott Armor Smith of the University of Washington in Seattle.

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

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

Armor Smith and Sidor found more than 3,000 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 Armor Smith.

Nevertheless, the pair managed to identify a new shuvosaurid, which they named Sonselasuchus cedrus. Over 950 of the bones belonged to this species. It did not 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 lived in the late Triassic,” says Michelle Stocker of Virginia Tech. “They really do look like dinosaurs.” They are most similar to ornithomimids, which were ostrich-like dinosaurs that lived in the late Cretaceous period, more than 100 million years after the shuvosaurids.

Sonselasuchus cedrus appears to have started life walking on all fours. Bones from younger individuals show that the fore and hind limbs were relatively equal in size. But in older individuals, the hind legs grew much more and also showed signs of carrying more weight. “The larger femurs in the population are quite robust,” says Armor Smith, while “even the largest humerus is relatively delicate”.

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

It can be young and adult S. cedrus lived quite separate lives and even ate different diets, as some crocodiles do today, says Stocker.

Popular descriptions of the age of dinosaurs often give the impression that dinosaurs, especially birds, evolved in very creative ways, while crocodiles remained largely the same. That misrepresents the diversity of pseudosuchians — the branch of the reptile tree that includes crocodiles, Stocker says. “They actually do a lot of the really unique, crazy stuff first, and then the dinosaurs pick it up later.”

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