The Enigmatic Runner

A labrador-sized animal fossil from 150 million years ago was believed to be a Nanosaurus. 

Now, a new study shows that the remains, found at the Morrison Formation in the western United States, belong to a new, previously uncategorized species that once lived in North America.  

“While the Morrison Formation has been well-known for a long time (for dinosaur remains), most of the focus has been on searching for the biggest and most impressive dinosaurs,” study co-author Susannah Maidment said in a statement. “Smaller dinosaurs are often left behind, meaning there are probably many still in the ground.” 

Scientists named the species Enigmacursor mollyborthwickae. Enigmacursor, which means mysterious runner, lived in the Late Jurassic, together with famous giants like the Stegosaurus.  

At about 25 inches tall and 71 inches long, it was tiny compared with the big dinosaurs of the time. Its long legs and speedy lifestyle, after which it’s named, likely allowed it to stay out of the way of dangerous predators. 

However, scientists say there are signs indicating that Enigmacursor was not fully grown, so the remains likely belonged to a teenage dinosaur, and it’s unclear why it died young. 

“One feature we look at in dinosaurs are the neural arches,” co-study author Paul Barrett said. “These are the top section of vertebrae, and form separately from the lower parts. They gradually merge as an animal gets older, so by examining them you can see whether it was still growing.” 

The Enigmacursor remains were unearthed on private land between 2021 and 2022 and sold through a commercial fossil dealer. They were advertised as belonging to a species called Nanosaurus and first described in the late 19th century.  

Most of the small dinosaurs named since the 1870s were named Nanosaurus but scientists have long suspected the categorization was incorrect. To prove their theory, researchers went to the US from London with scans of the skeleton and detailed photographs to examine the original Nanosaurus, which is considered the archetypal specimen, explained the BBC. 

“Nanosaurus wasn’t named based on many fossilized bones, but largely the preserved impressions of bones pressed into hardened sand that are very difficult to study,” said Barrett. “So, we turned to the other bones that have been referred to the group over the past century, but these weren’t particularly well-preserved either.” 

The scientists concluded that those impressions could have belonged to any dinosaur, while the remains they were examining maintained unique features that had to belong to a distinct species. 

“When Nanosaurus was named in 1877, there weren’t that many named dinosaurs so the few characteristics that its fossils preserved would have been unique,” said Maidment. “Now, however, we have found hundreds of small dinosaurs from all over the world and know that the fossils of Nanosaurus just aren’t that useful, let alone enough to name a species with.”