‘Enigmatic’ Dog-Sized Dinosaur Reveals a New Species That Scampered Around Jurassic North America
The speedy, plant-eating creature lived in what is now Colorado roughly 150 million years ago, and its skeleton went on display in London this week
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/accounts/headshot/SarahKuta.png)
Roughly 150 million years ago, a speedy, dog-sized dinosaur with big feet sprinted around what is now Colorado, dodging giants like Stegosaurus and Diplodocus.
Now, scientists have identified the nimble creature as a new species called Enigmacursor mollyborthwickae, they reported Wednesday in the journal Royal Society Open Science.
The small dinosaur’s skeleton is now on permanent display in the Earth Hall Mezzanine at London’s Natural History Museum, the institution announced in a statement this week. It’s the museum’s first new dinosaur to go on view since 2014, reports BBC News’ Georgina Rannard.
“[E. mollyborthwickae] represents one of the rarities from further down the food chain of the dinosaur era,” says David Norman, a paleobiologist at the University of Cambridge in England who was not involved with the new research, to New Scientist’s Chris Simms. “This newly described animal was clearly a small, wallaby-sized herbivore that scampered around the Late Jurassic countryside.”
Fossil hunters unearthed E. mollyborthwickae’s bones on private land between 2021 and 2022 in Moffat County, Colorado. The fossils were then sold through a commercial dealer and eventually made their way to a London art gallery, which informed Natural History Museum paleobiologists Susannah Maidment and Paul Barrett about them.
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/54/6f/546f93d4-9eed-4c67-b51a-0fd9e7a45e94/enigmacursor-reconstruction-painting-full-width-bob-nichollsjpgthumb19201920.png)
The two researchers were intrigued. Working with the art gallery, they found a donor—Molly Borthwick—who agreed to help them bring the unique specimen to the Natural History Museum. The species’ name “mollyborthwickae” is a nod of appreciation for Borthwick’s contribution, while the genus name “Enigmacursor” means “mysterious runner” in Latin.
The partial skeleton had originally been classified as a Nanosaurus, a type of plant-eating dinosaur first described in 1877. However, as Maidment and Barrett began their research, they realized the Nanosaurus genus had been created based on limited and somewhat flimsy evidence—that is, impressions made by bones in the sand and poorly preserved fossils. In the 1870s, scientists hadn’t unearthed and named many dinosaur fossils yet—so every novel discovery revealed exciting and new-to-them characteristics.
But a lot has changed in paleontology over the last 150 years. Today, scientists have discovered many more fossils around the globe, and the bar is much higher for the evidence needed to name a new genus and species. “As a result, it made sense to put [previously discovered Nanosaurus fossils] to one side and name Enigmacursor as a new species instead,” says Maidment in the museum statement.
Fun facts: Morrison Formation
- The Morrison Formation across the western United States preserves Late Jurassic fossils, from about 150 million to 157 million years ago.
- At the time, flowering plants and grasses likely hadn’t evolved—so the landscape was dominated by ferns, cycads and conifers alongside the large herbivores Stegosaurus and Apatosaurus and the carnivorous Allosaurus.
Against that backdrop, they suspect E. mollyborthwickae is probably just one of many small, unidentified (or misidentified) dinosaurs out there, either collecting dust in museum collections or still waiting to be unearthed from the ground. Even at well-studied fossil sites like the Morrison Formation, where E. mollyborthwickae was found, paleontologists have a tendency to focus on large, impressive creatures—while overlooking their petite peers.
“Smaller dinosaurs are often left behind, meaning there are probably many still in the ground,” says Maidment in the statement. “Enigmacursor shows that there’s still plenty to discover… and highlights just how important it is to not take historic assumptions about dinosaurs at face value.”
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/b5/e0/b5e0d3b2-71a0-4648-a230-11724a35d3e9/enigmacursor-mounted-skeleton-two-column-david-aaron-galleryjpgthumb19201920.png)
By analyzing E. mollyborthwickae’s bones, Maidment and Barrett—the new study’s two authors—have been able to learn a lot about this little creature. E. mollyborthwickae was a plant-eater that lived during the Late Jurassic, a time when the western United States was covered in floodplains and rivers. It walked around on its two long hind legs, using its tiny forearms to bring food to its mouth. It had “incredibly large” feet, Maidment tells New Scientist, which would have helped it dart around quickly to avoid being stepped on—or gobbled up—by a larger dinosaur.
“It also had a relatively small head, so it was probably not the brightest,” she tells the BBC.
E. mollyborthwickae is most closely related to a ten-foot-long dinosaur found in China called Yandusaurus hongheenis, which suggests it may have inhabited a wide range, reports CNN’s Issy Ronald.
The specimen now on view at the Natural History Museum is about two feet tall and nearly six feet long, with its tail making up roughly half of its length. It probably weighed about the same as a collie dog and was light on its feet, per CNN.
But the researchers suspect the dinosaur wasn’t done growing when it died. The top parts of its vertebrae—known as neural arches—were still separated from the lower sections rather than fused, which suggests the dinosaur was probably a teenager. However, because of the way the fossils were prepared before they reached the museum, the paleontologists can’t say for certain.
Another mystery? How the animal died. Maidment and Barrett found no evidence of sickness or injury that might hint at the cause of death.
Moving forward, the researchers hope E. mollyborthwickae will serve as a reminder to paleontologists that small dinosaurs are worth their time, too.
“When you’re looking for those very big dinosaurs, sometimes it’s easy to overlook the smaller ones living alongside them. But now I hope people will keep their eyes close to the ground looking for these little ones,” Barrett tells BBC News.
