An extinct species of reptile, said to have lived among dinosaurs, was discovered by researchers at the Smithsonian National Museum of Natural History.

The new species is the Opisthiamimus gregori and was in close relation to the living Sphenodon punctatus, also known as tuatara reptiles. Opisthiamimus inhabited earth 150 million years ago in Jurassic North America.

The remains of the reptile were found in the Morrison Formation, a sequence of sedimentary rock home to a variety of fossil remains. The Morrison Formation guided researchers to understand what prehistoric ecosystems were like.

Researchers discovered the fossil with the intention of reaching an Allosaurus nesting ground, but came across the Opisthiamimus fossil instead. It is considered one of the most complete fossil formations of small reptiles ever found in the formation.

While the reptile was estimated to be around a mere six inches from nose to tail, a well-preserved and complete fossil has more substantial implications on the history of life and its coexistence with other organisms compared to larger but more incomplete fossils.

Scientists excavating the site included the National Museum of Natural History’s curator of Dinosauria Matthew Carrano and research associate David DeMar Jr.

“What’s important about the Tuatara is that it represents this enormous evolutionary story that we are lucky enough to catch in what is likely its closing act,” Carrano said. “Even though it looks like a relatively simple lizard, it embodies an entire evolutionary epic going back more than 200 million years.”

Scientists had originally thought most of the lizard-like reptile remains were part of a genus called Opisthias, meaning that Opisthias were considered a dominant wide-ranging species. However, the discovery of the Opisthiamimus means other Tuatara species existed at the same time among one another.

Opisthiamimus’ similarity to lizards led researchers to believe they went extinct due to competition with lizards. Climate and habitat changes may also be factors.

Fossils like the Opisthiamimus, though small in relevance, are essential in understanding major environmental changes and evolution as one dominant species is replaced by the next.

“These animals have often taken a backseat to the dinosaurs which means there’s quite a lot left to discover, study, and understand about them,” Carrano said.

While the Morrison Formation has been researched and excavated for years, there is more to discover to truly understand Earth’s prehistoric organisms.