Dog-sized, herbivorous and runner: Old creature newly discovered

The skeleton of this small dinosaur, about 1 meter long and 0.5 meters high, was unearthed in the Morrison Formation in Colorado by a private fossil company called Dinosaurs of America LLC between 2021 and 2022. The skeleton was later put up for sale as Nanosaurus, but when the Natural History Museum in London purchased the fossil and began examining it, they realized that it was not actually Nanosaurus.

Although the Morrison Formation is a region famous for its iconic Jurassic dinosaurs, such as Stegosaurus and Allosaurus, studies on small dinosaurs have been limited. Susannah Maidment, a researcher at the Natural History Museum, said that until now, the focus has been on large and impressive dinosaurs, while small dinosaurs have often been overlooked, and that there may still be many small dinosaurs waiting to be discovered in the region.

Co-author Paul Barrett said that Nanosaurus was originally named based on very few fossil bones, which were weak impressions in sandstone. When Barrett and Maidment re-examined fossils that had been previously included in the Nanosaurus group, they determined that by modern standards the species had no distinguishing features and were not considered valid.

In the new study, researchers examined the fossil known as the so-called Nanosaurus in detail, using 3D scans to determine that unique features on its legs set it apart from others. The analysis determined that the dinosaur was closely related to Yandusaurus hongheensis, a Jurassic herbivore from China.

The researchers named the new species Enigmacursor mollyborthwickae. The name “Enigmacursor” is derived from the Latin words “enigma” meaning “mystery” and “cursor” meaning “runner.” The species name “mollyborthwickae” is dedicated to Molly Borthwick, a donor who helped purchase the fossil for the museum.

Researchers emphasized that this discovery clarifies the diversity and evolutionary relationships of small dinosaur groups, and pointed out that the Jurassic Period still contains many unsolved mysteries.