Enigmacursor

Enigmacursor Temporal range: Late Jurassic, 155–148  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Skeletal and life reconstructions
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	† Ornithischia
Clade:	† Neornithischia
Genus:	† Enigmacursor Maidment & Barrett, 2025
Species:	†E. mollyborthwickae
Binomial name
†Enigmacursor mollyborthwickae Maidment & Barrett, 2025

Enigmacursor (meaning "puzzle runner") is an extinct genus of neornithischian dinosaur from the Late Jurassic Morrison Formation of Colorado, United States. The type species is Enigmacursor mollyborthwickae. ^[1]

Discovery and naming

The holotype specimen, NHMUK PV R 39000, was discovered between 2021 and 2022 in Moffat County, Colorado by private company Dinosaurs of America, LLC. and acquired by the Natural History Museum, London (NHM) in 2024. It represents a three-dimensionally preserved partial postcranial skeleton with associated teeth. ^[1]

Speculative life restoration

It was described as a new genus and species by Susannah Maidment & Paul Barrett in 2025. The generic name, Enigmacursor, comes from the word enigma, referring to the taxonomic confusion of Morrison Formation neornithischians such as Nanosaurus , ^[2] and the Latin word cursor, meaning "runner", after its cursorial hindlimbs. The specific name, mollyborthwickae, honors Molly Borthwick, who donated the funds for the acquisition of the specimen by the NHM. ^[1]

Classification

Maidment & Barrett (2025) entered Enigmacursor into a phylogenetic analysis and placed as the sister taxon to the Chinese Yandusaurus . A cladogram adapted from their analysis is shown below: ^[1]

Neornithischia	Agilisaurus Hexinlusaurus Yandusaurus Enigmacursor Cerapoda

Paleoecology

Torvosaurus mounted as if chasing small neornithischians

Enigmacursor was one of the smaller members of the diverse Morrison Formation dinosaur fauna, diminutive in comparison to the giant sauropods. ^[3] The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains. ^[4] Vegetation varied from river-lining gallery forests of conifers, tree ferns, and ferns, to fern savannas with rare trees. ^[5] It has been a rich fossil hunting ground, holding fossils of green algae, fungi, mosses, horsetails, ferns, cycads, ginkgoes, and several families of conifers. Other fossils discovered include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphs, several species of pterosaur, numerous dinosaur species, and early mammals such as docodonts, multituberculates, symmetrodonts, and triconodonts. Such dinosaurs as the theropods Ceratosaurus , Allosaurus , Ornitholestes , and Torvosaurus , the sauropods Apatosaurus , Brachiosaurus , Camarasaurus , and Diplodocus , and the ornithischians Camptosaurus , Dryosaurus , and Stegosaurus are known from the Morrison. ^[6]

References

1. Maidment, S. C. R.; Barrett, P. M. (2025). "Enigmacursor mollyborthwickae, a neornithischian dinosaur from the Upper Jurassic Morrison Formation of the western USA". Royal Society Open Science. 12 (6). 242195. doi: 10.1098/rsos.242195 .
2. Barrett, P. M.; Maidment, S. C. R. (2025). "A Review of Nanosaurus agilis Marsh and Other Small-Bodied Morrison Formation "Ornithopods"". Bulletin of the Peabody Museum of Natural History. 66 (1): 25–50. doi:10.3374/014.066.0102.
3. Foster, John R. (2003). "Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A.". New Mexico Museum of Natural History and Science Bulletin. 23: 29.
4. Russell, Dale A. (1989). An Odyssey in Time: Dinosaurs of North America. Minocqua, Wisconsin: NorthWord Press. pp. 64–70. ISBN   1-55971-038-1.
5. Carpenter, Kenneth (2006). "Biggest of the big: a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 131–138.
6. Chure, Daniel J.; Litwin, Ron; Hasiotis, Stephen T.; Evanoff, Emmett; Carpenter, Kenneth (2006). "The fauna and flora of the Morrison Formation: 2006". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 233–248.