The mystical “runner” dinosaurs, only about dog size, ran around the ancient flood plains of Colorado 150 million years ago, new research finds.
The two-legged herbivore, named Enigmacursor Mollyborthwickae, was about 3 feet (1 meter) long and 1.5 feet (0.5 m) tall. However, despite its small stature, the discovery of this dinosaur has had a major impact on scientists’ understanding of some Jurassic species.
The private fossil company unearthed E. Mollyborthwickae in Colorado between 2021 and 2022. The skeleton was later sold and promoted as a nanosaurus, another type of dinosaur from the same rock formation. After the Natural History Museum in London purchased the skeleton, researchers discovered that the new specimen was not a nanosaurus, and the scientific classification of nanosaurus and several other dinosaurs from the Morrison Formation was unreliable.
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The Morrison Formation is home to many iconic dinosaur fossils, including the plated stegosaurus and the ferocious predator Allosaurus, who would have lived with E. Mollybos Wiccae during the Jurassic period (2013 million to 145 million years ago). A new study published on Wednesday (June 25) in the Journal Royal Society Open Science highlights that there is still much to learn about the little dinosaurs of the formation.
“Morrison’s formation has long been well-known, but most focused on finding the biggest and most impressive dinosaurs,” says Susanna Maidment, a researcher and senior researcher at the Museum of Natural History, in an article published by the museum. “The little dinosaurs are often left behind, which means that there are probably many people still on the ground.”
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According to the research, the skeleton was unearthed by a company called Dinosaurs of America, LLC, and was acquired by London’s David Aaron Art Gallery in 2024 before purchasing the fossil. When researchers got the suspected nanosaurus, they began to look closely at the group, along with other small-bodied Morrison layer dinosaurs.
“The nanosaurus were not named based on many fossilized bones, but the preserved impressions of bones tucked into hardened sand, which are mostly difficult to study, mentions Paul Barrett, a paleontologist at the Museum of Natural History, in a research museum article. “So we looked at other bones that have been featured in groups over the past century, but these were not particularly preserved either.”
The fossil records of some small-bodied Morrison Formation dinosaurs are poor and incomplete, and research has experienced some scientific revisions. Barrett and Maidment reviewed Morrison strata species similar to nanosaurus in a study published in the Journal of the Peabody Museum of Natural History on April 25th, and concluded that by modern standards none of them have a unique combination of valid traits.
For the new study, researchers collaborated on so-called nanosaurus specimens and scanned them to create virtual 3D images of fossilized bones. They determined that the animal stands out well from other dinosaurs, especially due to its unique combination of foot features. Their analysis revealed that their closest known relative was Yandusaurus Honghensis, a Chinese Jurassic herbivore.
Barrett and Maidments created a new genus (group) for the specimen. The genus name, Enigmaursor, combines the word “Enigma,” a reference to the mystery surrounding scientific classification, with the Latin word “cursor,” meaning “runner.” The dinosaur species name “Mollyborthwickae” honors museum donor Molly Borthwick. The donation allowed the Museum of Natural History to purchase specimens. The museum did not disclose the cost of the specimen.
“A detailed study of its anatomy has allowed us to clarify the evolutionary relationships of this type, its taxonomy, and the diversity of small dinosaur groups that were previously less understood,” Madement said in a statement.
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