Ancient Croc Relative Walked on Two Legs Like Dinosaurs

A crocodile relative from 215 million years ago pulled off a trick rarely seen in nature: switching from four legs to two as it matured.

Scientists at the University of Washington and Burke Museum discovered this unusual growth pattern while studying fossils of Sonselasuchus cedrus, a small reptile that lived alongside early dinosaurs. Graduate student Elliott Armour Smith and Professor Christian Sidor analyzed over 950 fossils unearthed from Arizona's Petrified Forest National Park in 2014.

The fossils revealed something surprising. Young Sonselasuchus had proportional front and back legs, suggesting they walked on all fours. As they grew, their hind legs became longer and more robust while their front limbs stayed smaller, eventually forcing them upright onto two legs.

"This is particularly peculiar," Smith noted in the study published in the Journal of Vertebrate Paleontology. This developmental shift is extremely rare among reptiles, making the discovery all the more remarkable.

Despite coming from the crocodile family tree, Sonselasuchus looked nothing like modern crocs. Standing about 25 inches tall, it sported a toothless beak, large eye sockets, and hollow bones. It actually resembled fast-running, ostrich-like dinosaurs called ornithomimids far more than its own relatives.

The resemblance wasn't coincidence or shared ancestry. Both groups lived in the same Late Triassic forests and faced similar survival challenges. Nature independently arrived at the same solution twice, a phenomenon scientists call convergent evolution.

Why This Inspires

This discovery reminds us that Earth's history holds countless surprises still waiting to be found. The Petrified Forest site has yielded over 3,000 fossils in the past decade, including fish, amphibians, dinosaurs, and other reptiles, with no signs of slowing down.

Professor Sidor's decade-long collaboration with the National Park Service has involved over 30 University of Washington students and volunteers. Each fossil adds another piece to the puzzle of how life adapted and thrived in ancient ecosystems.

The Sonselasuchus findings show that features we associate exclusively with dinosaurs, like bipedalism and toothless beaks, actually evolved multiple times across different animal groups. The crocodile line experimented with body plans just as diverse and creative as their dinosaur cousins.

These ancient forests were more complex and diverse than scientists previously imagined, filled with creatures pushing evolutionary boundaries in unexpected ways. Every new fossil reveals that life has always been remarkably inventive at finding ways to thrive.