Ancient Reptile Learned to Walk Upright as It Grew Up

Imagine a baby animal that starts life crawling on all fours, then stands up and walks on two legs as it grows up. That's exactly what a newly discovered ancient reptile did 215 million years ago in what is now Arizona.

Scientists have named the creature Sonselasuchus cedrus, and it looked nothing like you'd expect from a crocodile relative. About the size of a poodle, it had large eyes, a toothless beak, and short arms that would eventually become less useful as it matured.

Since 2014, researchers have unearthed more than 950 bones from at least 36 individuals at Petrified Forest National Park in eastern Arizona. The variety of ages among the fossils gave scientists a unique window into how these animals changed throughout their lives.

The bones tell a fascinating story of transformation. In younger animals, the front and hind leg bones were roughly the same size, perfect for walking on all fours. But older individuals had much larger, stronger hind leg bones while their front legs stayed relatively small.

Lead researcher Elliott Armour Smith found that front limbs started at 75 percent the length of hind limbs in juveniles but shrank to just 50 percent in adults. This dramatic shift shows the species transitioned from four-legged walking to two-legged striding as it grew.

This kind of change is surprisingly rare in nature. Scientists know of only two dinosaur species that probably made this switch, plus humans, who crawl as babies before learning to walk upright.

The Bright Side

This discovery helps scientists understand why walking upright became such an important evolutionary advantage. For our human ancestors, standing on two legs freed up hands for using tools and allowed longer distance travel.

Sonselasuchus belonged to an extinct family called shuvosaurids, which one paleontologist not involved in the study called "absolute weirdos." Despite being early crocodile relatives, they looked strikingly similar to ostrich-like dinosaurs that wouldn't appear for another 100 million years.

The team believes both groups evolved similar features separately because they lived in similar ecosystems and filled similar ecological roles. It's a powerful example of nature finding the same solutions to similar challenges across millions of years.

The fossil site shows no signs of slowing down. Researchers have collected over 3,000 specimens from the bonebed, with plenty more discoveries waiting beneath the Arizona soil.

Every new fossil brings us closer to understanding the incredible diversity of life that walked, crawled, and eventually stood upright on Earth long before humans arrived.