Crocodile Evolution: From Four to Two Legs

KOMPAS.com – New fossil discoveries reveal interesting facts about one of the ancient relatives of crocodiles that lived more than 200 million years ago. This ancient reptile apparently had an unusual way of walking: when it was young it walked on four legs, but when it matured it changed to walking on two legs.

The species is given a name Sonselasuchus cedarand belongs to the shuvosaurid group, a group of ancient reptiles that lived in the Late Triassic period around 225–201 million years ago.

Scientists discovered that this animal had a very similar appearance to ornithomimid dinosaurs, runner dinosaurs that are often nicknamed “ostrich-like dinosaurs”. Interestingly, this similarity is not because the two are closely related, but because evolution has produced similar body shapes in the same environment.

These findings were published in the scientific journal Journal of Vertebrate Paleontology by a team of researchers from the University of Washington Department of Biology and the Burke Museum.

Also read: Fossils of long-legged crocodiles that lived 200 million years ago were found

Unusual Way of Walking

One of the most interesting findings from this research is the change in the way Sonselasuchus walked during its lifetime.

Analysis of the proportions of the leg bones shows that this reptile may have changed body shape as it grew.

The study’s lead author, Elliott Armor Smith, explained that these changes occurred due to different growth patterns between the front legs and the back legs.

“By analyzing the proportions of the leg skeletons of various animals, we found that standing on two legs is likely the result of different growth patterns,” explains Smith.

He added that when it was young, the size of the animal’s front and hind legs was relatively equal.

“We suspect Sonselasuchus had more balanced forelimbs and hindlimbs when young, but that its hindlimbs grew longer and stronger as it matured.”

As a result, the way this animal walks also changes.

“Essentially, we think these creatures start their lives walking on four legs… then switch to walking on two legs as they grow older.”

These changes are very rare in the fossil record of ancient reptiles.

Also read: Crocodile ancestors lived 240 million years ago, not dinosaurs

Incredible Fossil Site

Sonselasuchus fossils were found in Petrified Forest National Park, Arizona, United States.

In 2014, a research team discovered around 950 fossils of this species at the location. This discovery is part of a large excavation project that has been ongoing for more than a decade.