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| Enigmacursor mollyborthwickae Maidment & Barrett, 2025 Artwork by Bob Nicholls. |
Abstract
Although their remains have been known since the 1870s, the small, bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation of the western USA remain poorly known. The historic type specimens are incomplete and poorly preserved and have recently been designated as nomina dubia. Here, we describe a recently collected, partial but three-dimensionally preserved skeleton of a new small-bodied ornithischian from the Morrison Formation of Colorado, USA, that we name Enigmacursor mollyborthwickae gen. et sp. nov. The skeleton includes substantial portions of the axial and appendicular skeleton and, when scored into a phylogenetic analysis, is shown to be a non-cerapodan neornithischian, whose closest relative is Yandusaurus hongheensis from the Late Jurassic of China. The discovery of Enigmacursor enhances the diversity of ornithischian dinosaurs from the Morrison Formation and provides new information on their anatomy. In addition, it demonstrates that there is additional cryptic diversity of small-bodied Morrison Formation ornithischians, suggesting they were a more diverse component of these Late Jurassic ecosystems than was previously realized.
Systematic palaeontology
Dinosauria (Owen 1842)
Ornithischia (Seeley 1888)
Neornithischia (Cooper 1985)
Enigmacursor mollyborthwickae gen. et sp. nov.
Holotype—NHMUK PV R 39000, a partial skeleton that includes three teeth, three cervical, 11 dorsal, two dorsosacral and five caudal vertebrae, 10 dorsal ribs, five chevrons, right sternum, both scapulae, both humeri, both radii, both ulnae, three metacarpals, left ilium, right ischium, right pubis, both femora, both tibiae, both fibulae, right astragalus and both pedes.
Diagnosis—Enigmacursor mollyborthwickae differs from all other ornithischian dinosaurs in possessing the following unique combination of features and one potential autapomorphy (the latter marked with an asterisk): (1) posterior articular facets offset ventrally relative to anterior articular facets on proximal dorsal vertebrae*; (2) absence of a supracetabular crest on the ilium; (3) femoral head separated from the greater trochanter by a trochanteric fossa; (4) apex of anterior trochanter situated level with the ventral margin of the femoral head; (5) absence of a ligament sulcus on the posterior surface of the femoral head; (6) ventral surface of fourth trochanter straight or slightly convex in medial or posterior view; (7) medially directed, hook-like posterior condyle of the proximal end of the tibia.
Etymology—Enigma, meaning a puzzle or mystery, in reference to the convoluted taxonomic history of small-bodied ornithischians from the Morrison Formation; cursor, from the Latin for ‘runner’, in reference to the cursorial morphology of the elongated hind limb and pes. The species name honours Molly Borthwick, whose generous donation allowed the NHMUK to acquire the specimen.
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| Enigmacursor mollyborthwickae a) Skeletal reconstruction of with preserved elements shaded. b) Life reconstruction. Artwork by Bob Nicholls. |
Susannah C. R. Maidment and Paul M. Barrett. 2025. Enigmacursor mollyborthwickae, a neornithischian dinosaur from the Upper Jurassic Morrison Formation of the western USA. R. Soc. Open Sci. 12: 242195. DOI: doi.org/10.1098/rsos.242195 [25 June 2025]
https://www.nhm.ac.uk/discover/news/2025/june/new-species-mystery-dinosaur-unveiled-natural-history-museum.html
