Ancient Croc Cousin Learned to Walk on Two Legs as It Grew

Imagine a creature that had to learn an entirely new way of walking as it grew up—not because of injury, but because its body naturally changed shape.

That's exactly what happened to Sonselasuchus cedrus, a newly discovered ancient relative of modern crocodiles. Scientists from the University of Washington and Burke Museum found that this poodle-sized reptile began life scampering on four legs, then gradually switched to walking upright on two as it reached adulthood.

"This is particularly peculiar," says lead author Elliott Armour Smith, a graduate student who analyzed the creature's unusual proportions. The team discovered that while young Sonselasuchus had evenly proportioned limbs, their back legs grew longer and stronger over time, eventually forcing them to adopt an upright stance.

The discovery came from an incredible treasure trove of 950 fossils unearthed in Arizona's Petrified Forest National Park in 2014. Over ten years of careful excavation, the site has revealed more than 3,000 fossil bones from various ancient animals.

Sonselasuchus lived about 215 million years ago during the Late Triassic period, standing roughly 25 inches tall. It had a toothless beak, large eye sockets, and hollow bones—features that made it look remarkably similar to certain dinosaurs, even though it was actually on the crocodile family tree.

The Bright Side

This discovery shows us that evolution tried out similar solutions in different animal lineages. While we often think of two-legged walking as a dinosaur trait, Sonselasuchus proves that the crocodile line of evolution also experimented with bipedalism millions of years ago.

Professor Christian Sidor, who led the dig team, says the site keeps delivering surprises. "We have collected over 3,000 fossils from the Sonselasuchus bonebed, and it doesn't seem to show any signs of petering out," he explains. The site has also yielded fossils of fish, amphibians, dinosaurs, and other reptiles.

More than 30 University of Washington students and volunteers have participated in the excavation over the years, gaining hands-on experience in paleontology while contributing to scientific understanding.

The creature's name honors both its discovery location and habitat—Sonselasuchus comes from the Sonsela geological formation, while cedrus refers to the cedar-like trees that filled Late Triassic forests where it lived.

This peculiar walking adaptation reminds us that nature has always been creative in solving the challenge of getting around.