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A small dinosaur that once dashed along North American riverbanks has found a new home in London. The new species, named Enigmacursor mollyborthwickae, is the most complete named specimen of its kind and is now on permanent display at the Natural History Museum.

The new dinosaur has emerged from a "taxonomic tangle" more than a century in the making.

The U.S.'s Morrison Formation has produced some of the most famous dinosaurs in the world, such as and . But not all of its species are as well known, with many smaller herbivorous dinosaurs having been historically overlooked.

Now, researchers have named a new species of these little herbivores, calling it Enigmacursor mollyborthwickae. They hope that this work will shine some light on long-ignored animals of the formation and clear the way for more discoveries in the future.

Professor Susannah Maidment is one of our dinosaur experts and co-lead author of the research into Enigmacursor. She says that the new species could be the first of many small dinosaurs to be found from the western U.S.

"While the Morrison Formation has been well-known for a long time, most of the focus has been on searching for the biggest and most impressive dinosaurs," Susannah says. "Smaller dinosaurs are often left behind, meaning there are probably many still in the ground."

"Enigmacursor shows that there's still plenty to discover in even this well-studied region, and highlights just how important it is to not take historical assumptions about dinosaurs at face value."

The paper was in the journal Royal Society Open Science, and visitors can now see the new dinosaur on show in our .

What was Enigmacursor like?

The story of Enigmacursor begins during the closing years of the Late Jurassic between 150 and 145 million years ago. At the time, the Morrison Formation would have been a vast network of rivers and floodplains stretching across large parts of the western United States.

Huge, long-necked herbivores like would have roamed the landscape, while carnivorous theropods like would have stalked the riverside. Trying to keep out of their way would have been a variety of smaller dinosaurs, including Enigmacursor.

Its long legs would have allowed this small herbivore to dart away from danger, keeping it one step ahead of its predators, and this speedy lifestyle inspired the dinosaur's name. Enigmacursor means "mysterious runner," while the species name honors Molly Borthwick, whose generous donation allowed for the purchase and display of the dinosaur.

The dinosaur was only around one meter long, but there are signs that the dinosaur was not fully grown, says co-lead author Professor Paul Barrett.

"One feature we look at in dinosaurs are the neural arches," Paul explains. "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."

"We can speculate that Enigmacursor probably wasn't that old, as it doesn't seem to have many of its neural arches fused in place. However, the way the fossil was prepared before it was acquired by the Natural History Museum has obscured some of these details, so we can't be certain."

It's also unclear exactly how the animal died, as there are no obvious signs of injury or illness in the bones. In any case, the remains of Enigmacursor ended up buried within the Morrison Formation, waiting to be found millions of years later.

From Nanosaurus to Enigmacursor

The new fossils were unearthed on private land between 2021 and 2022, and put up for sale through a commercial fossil dealer. They were initially advertised as being from an animal called Nanosaurus, a poorly known species of dinosaur first named in the 1870s.

It was then brought it to the attention of Susannah and Paul, who were interested in finding out more about this enigmatic animal. After the fossils were purchased by the Natural History Museum, the paleontologists began digging into Nanosaurus's past, and were shocked by what they found.

"Nanosaurus wasn't named based on many fossilized bones, but largely that are very difficult to study," Paul says. "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."

"It just goes to show ," Susannah adds. "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."

"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. As a result, it made sense to put them to one side and name Enigmacursor as a new species instead."

"," some of its fossils still have scientific value. The few features that can be identified in the various fossils show that they're different to Enigmacursor. This suggests that further small dinosaur species could be discovered from the Morrison.

Some of these fossils may have already been uncovered. Susannah and Paul are aware of several well-preserved skeletons held in museums around the world that haven't yet been formally named, and might represent these missing dinosaurs.

The researchers hope to study some of these fossils in the future, and help to further clear up the muddled history of these dinosaurs.

"Taxonomic work is generally overlooked and not treated as a particularly important or employable skill," Susannah adds. "However, it's the foundation that all paleontology is built on. If it's wrong, then everything else collapses."

"We need more funding and support to ensure that this vital work doesn't stop, so that we can better understand how life on Earth evolved."