Enigmacursor mollyborthwickae

Enigmacursor mollyborthwickae is a small-bodied neornithischian dinosaur from the Upper Jurassic Morrison Formation of western North America. It was described in 2025 based on a three-dimensionally preserved partial skeleton from Moffat County, Colorado. The name Enigmacursor (meaning “mysterious runner”) reflects the uncertain history of small ornithischian taxonomy from the Morrison Formation, while the species name honors philanthropist Molly Borthwick.

The discovery of Enigmacursor sheds light on the underappreciated diversity of small bipedal herbivores in the Morrison ecosystem, which has long been dominated by more famous taxa like Allosaurus, Stegosaurus, and Diplodocus. Its anatomy places it close to Yandusaurus hongheensis from China, suggesting complex biogeographic patterns in Late Jurassic ornithischians.

Geographic Coordinates

Type locality: Skull Creek Estates, Moffat County, Colorado, USA

Physical Characteristics

Enigmacursor was a small, gracile dinosaur, measuring approximately 2–3.0 m (6.5–9.8 ft) in length and estimated to weigh around 18–25 kg (40–55 lb). These estimates are based on proportional scaling from the preserved limb bones, particularly the 169 mm long femur and 209 mm tibia, and comparison with related neornithischians such as Hypsilophodon.

Key features include:

• Elongated hind limbs and lightweight construction indicative of cursorial (running) behavior
• Absence of a supracetabular crest on the ilium
• A unique femoral anatomy including a trochanteric fossa, low anterior trochanter, and medially directed hook-like posterior condyle of the tibia
• Premaxillary teeth with subtle ridges and marginal denticles, likely used for cropping vegetation

Unlike larger ornithopods, Enigmacursor lacks robust chewing adaptations and had a lightly built axial skeleton, more akin to basal neornithischians.

Paleontological Material

Holotype (NHMUK PV R 39000) includes:

• 3 teeth
• 3 cervical, 11 dorsal, 2 dorsosacral, and 5 caudal vertebrae
• 10 dorsal ribs, 5 chevrons
• Right sternum
• Both scapulae, humeri, ulnae, radii, and 3 metacarpals
• Left ilium, right pubis, right ischium
• Both femora, tibiae, fibulae, right astragalus
• Both feet (pedes)

Diet and Feeding Habits

Enigmacursor was herbivorous, feeding on low vegetation such as ferns, cycads, and primitive flowering plants. Its simple, conical premaxillary teeth with fine denticles suggest it was a selective browser or nipper. Its small size and cursorial limbs imply it may have foraged in open areas, relying on speed to evade predators.

Habitat and Distribution

Enigmacursor lived during the Kimmeridgian–Tithonian stages of the Late Jurassic (approximately 157–145 million years ago), in what is now the western United States. The Morrison Formation represented a semi-arid, seasonal environment of floodplains and river channels, supporting a rich dinosaur fauna.

Coexisting taxa included:

• Large sauropods (Diplodocus, Camarasaurus)
• Stegosaurs (Stegosaurus)
• Large theropods (Allosaurus, Torvosaurus)
• Other small ornithischians of uncertain taxonomy

Behavior and Social Structure

Although no direct evidence exists, the long limbs and lightly built skeleton suggest that Enigmacursor was a swift, solitary or possibly gregarious forager. Its anatomical traits point to an active lifestyle, possibly avoiding predators through agility rather than defense.

Discovery and Research

The specimen was discovered in 2021–2022 by Dinosaurs of America LLC and later acquired by the Natural History Museum, London, with support from private donors. It was formally described in 2025 by Susannah C. R. Maidment and Paul M. Barrett.

Phylogenetic analysis placed Enigmacursor as a non-cerapodan neornithischian, closely related to the Asian taxon Yandusaurus. This relationship supports possible faunal exchange between Asia and North America during the Late Jurassic.

Significance and Notable Facts

• First new small neornithischian named from the Morrison Formation in over a century
• Provides the most complete data on Morrison small-bodied ornithischians
• Its unique femoral morphology distinguishes it from other known ornithischians
• Highlights the previously underestimated diversity of small herbivores in the Late Jurassic

Conclusion

Enigmacursor mollyborthwickae is a key addition to the dinosaurian diversity of the Morrison Formation. As a fast-running, lightly built herbivore, it likely played a significant ecological role among the vast sauropods and theropods of its time. Its anatomical distinctiveness and phylogenetic placement fill a gap in our understanding of early neornithischian evolution and highlight the importance of continued exploration of less charismatic dinosaur clades.