Researchers have announced the discovery of a previously unknown dinosaur species that fills a long‑standing gap in the evolutionary record. Between 2021 and 2022, a series of fossils was uncovered on a stretch of private land in Colorado, all belonging to a small animal just under six feet long.
Initially identified as a member of the long‑neglected genus Nanosaurus, the specimen was overlooked until Professors Susannah Maidment and Paul Barrett of the Natural History Museum in London acquired the fossils for detailed study. Their investigation revealed that the remains belonged to a new species, and that the name Nanosaurus never truly represented a distinct dinosaur.
Comparing the Colorado specimens with other “Nanosaurus” fossils housed worldwide, Maidment and Barrett found none to be truly identical. The findings point to a diverse family of tiny dinosaurs that require a fresh, comprehensive review. In the June 2025 issue of Royal Society Open Science, they formally named the species Enigmacursor mollyborthwickae.
Enigmacursor mollyborthwickae combines the Greek word “enigma” (riddle) with the Latin “cursor” (runner), reflecting both the mystery surrounding small-bodied dinosaurs and the species’ powerful, bipedal locomotion. The specific epithet honors Molly Borthwick, whose generous donation enabled the museum’s purchase of the fossils.
The fossils originate from the Morrison Formation, a vast Late Jurassic sedimentary basin that stretches from Montana to New Mexico. While the Morrison is renowned for its giant sauropods and theropods, its smaller fossils have historically been overlooked, giving rise to the ill‑defined Nanosaurus grouping. In the April 2025 issue of the Bulletin of the Peabody Museum of Natural History, Maidment and Barrett argued for phasing out the Nanosaurus designation entirely.
After removing the outdated label, the researchers could profile the species as an independent dinosaur. The skeleton measured 180 cm in length and 64 cm in height—about six feet long and two feet tall—with roughly half of its length contributed by a long tail.
Its name’s “cursor” element reflects the evidence of rapid running: elongated hind legs, robust feet, and a femur with distinctive muscle‑attachment sites not seen in related taxa. A primary research goal is to reconstruct its locomotion and ecological niche. Though its forelimbs were short—resembling the arm morphology of a T. rex—Enigmacursor was a small, herbivorous dinosaur. Its stubby teeth were adapted for chewing ferns and cycads, as grasses and flowering plants had not yet evolved.
Dating places the fossils in the Late Jurassic, approximately 145 to 150 million years ago, during the formation of the Morrison Basin. The ecosystem that once hosted Enigmacursor also included giants such as Stegosaurus, five times longer, and Diplodocus, which reached about 85 feet (roughly 15 Enigmacursor individuals) in length.
By studying the morphological traits of Enigmacursor, paleontologists are exploring how early bipedal dinosaurs may have given rise to the massive quadrupeds that dominated the Jurassic. This discovery brings us closer to understanding the evolutionary transition from small, agile dinosaurs to the colossal giants of later periods.
