Enigmacursor mollyborthwickae: North America’s “Mysterious Runner” – Dog-Sized Jurᴀssic Enigma Finally Solved

In a June 2025 scientific triumph published in Royal Society Open Science, paleontologists have officially named Enigmacursor mollyborthwickae — a speedy, dog-sized herbivorous dinosaur whose discovery finally untangles one of the Morrison Formation’s longest-standing taxonomic puzzles.

Unearthed in 2021–2022 from Moffat County, Colorado, by Dinosaurs of America LLC, the three-dimensionally preserved partial skeleton (NHMUK PV R 39000) represents the most complete small ornithischian ever recovered from the famous Late Jurᴀssic Morrison Formation. At just 1.8 metres long and standing 64 cm at the hip, this bipedal “runner” weighed roughly 20–30 kg and browsed low vegetation in the floodplains of what is now the western United States 150–145 million years ago.

Named by Susannah Maidment and colleagues at the Natural History Museum in London, Enigmacursor (“puzzle runner”) honours both its convoluted history — small Morrison ornithischians have long been synonymised or misidentified as Nanosaurus — and generous donor Molly Borthwick. Its elongated hind limbs and cursorial anatomy confirm it was built for speed, darting through dense undergrowth to evade predators like Allosaurus.

The specimen is now on permanent public display at the Natural History Museum, London, where it dramatically increases known ornithischian diversity in the Morrison and provides crucial anatomical data for understanding early neornithischian evolution. Maidment called it “a long-awaited key to a 150-year-old mystery.”

With this elegant little dinosaur finally decoded, researchers believe many more small herbivores await discovery in the American West — proving that even the smallest Jurᴀssic players can deliver outsized scientific revelations.