Enigmacursor mollyborthwickae is a small-bodied ornithischian dinosaur named in 2025 from the Morrison Formation of Colorado. The fossil was collected in 2021–2022 and initially compared with Nanosaurus, a familiar but taxonomically difficult name in the Morrison record. Detailed study identified a distinctive combination of features and established a new genus.
Quick facts
| Species | Enigmacursor mollyborthwickae Maidment & Barrett, 2025 |
|---|---|
| Age | Late Jurassic, Morrison Formation |
| Locality | Moffat County, Colorado, USA |
| Type specimen | NHMUK PV R 39000; partial skeleton |
| Group | Basal neornithischian |
| Evidence limit | One immature individual; incomplete skeleton |
A specimen brought to the museum
The type specimen, NHMUK PV R 39000, is a partial skeleton preserved in three dimensions and includes teeth. It was collected in Moffat County, Colorado, by a commercial field team in 2021–2022 and acquired by the Natural History Museum in London in 2024. Susannah Maidment and Paul Barrett described it in a 2025 paper in Royal Society Open Science. The specific name honours Molly Borthwick, whose donation enabled the museum to acquire the specimen.
The specimen's route from field discovery to scientific description matters. Fossils first receive a field context and collection record; preparation then reveals which bones can be identified. A specimen may be provisionally assigned during early study and later receive a different interpretation when more anatomy is visible. That is what happened here: the initial comparison did not survive detailed anatomical analysis.
The generic name joins “enigma” with the Latin cursor, or runner. It points both to the taxonomic uncertainty around small-bodied Morrison ornithischians and to the animal's long-legged proportions. The species name honours Molly Borthwick, whose donation enabled the museum to acquire the fossil. These are naming explanations, not anatomical diagnoses.
What the bones reveal
NHMUK PV R 39000 preserves portions of the postcranial skeleton and teeth, providing a basis for comparison with early ornithischians. Three-dimensional preservation retains details that can be flattened or obscured in compressed fossils. The material is nevertheless incomplete, so not every part of the skeleton can be described and many characters remain unavailable for evolutionary analyses.
The associated teeth help identify the specimen as an herbivorous ornithischian, while the postcranial bones provide much of the evidence used to distinguish it from other Morrison forms. A taxonomic diagnosis depends on a combination of characters, not on the animal's small size or a general resemblance to another mount. The description sets out which features can be observed on the holotype and which comparisons remain provisional.
The authors interpreted the specimen as a basal neornithischian and discussed similarities to Yandusaurus. This is a phylogenetic hypothesis based on the characters that are preserved. The exact position of early ornithischians is sensitive to which taxa and character codings are included; the relationship should not be treated as a direct ancestor-descendant link.
Growth and the Nanosaurus problem
The individual was not fully grown. Bone fusion in the skeleton is incomplete, supporting an immature growth stage, although preparation obscures some details and does not provide a precise age. A juvenile can carry proportions and anatomical features that change as it matures. Its small size therefore cannot be assumed to represent the maximum size of the species.
The published estimate of about one metre describes the known individual as reconstructed from its partial skeleton. It is not an adult species average. Missing parts, body proportions and the animal's unfinished growth all affect total length; no second specimen currently supplies an adult comparison or a growth series.
The Morrison Formation has a long history of small dinosaur names based on fragmentary specimens. A 2025 first-hand review of the type material concluded that Nanosaurus agilis and several other small-bodied Morrison names lack unique diagnostic character combinations and should be treated as nomina dubia. That conclusion concerns the historic name-bearing specimens, not every later fossil once referred to them.
The confusion has more than one source. The Morrison record contains isolated small bones from different places and stratigraphic levels, while older descriptions used names that later workers could not diagnose securely. The 2025 review and the description of Enigmacursor separated the new skeleton from that unstable background. Future revisions may still alter how individual historic specimens are classified, but such changes require specimen-level evidence.
That distinction prevents a common shortcut: treating “Nanosaurus” as a single well-known small dinosaur and transferring every similar bone to a new genus. A taxonomic revision instead asks whether the diagnostic type material can be compared with each referred specimen. When the old type does not preserve enough distinguishing anatomy, its name cannot serve as a reliable catch-all for every small ornithischian from the formation.
A Morrison animal, with open questions
The Morrison Formation records a Late Jurassic system of rivers, floodplains and other continental environments across western North America. Its fossils accumulated through time and across a broad region. Finding Enigmacursor in the formation places it within that landscape, but does not by itself specify the exact habitat or ecological role of the individual.
Its hindlimbs support a bipedal body plan and likely terrestrial locomotion, but the name “runner” does not supply a measured speed. Estimates of running ability require models of muscle, joints, body mass and stride. The fossil also does not preserve a trackway that can be confidently linked to this genus. Its movement can be discussed from limb anatomy, but a dramatic chase scene would be reconstruction.
The Morrison Formation includes famous large sauropods and predators, yet the presence of those animals in the same broad formation does not show that this individual encountered any particular species. Deposits span substantial time and geography. A local food web requires tighter stratigraphic and ecological evidence than a formation-level list.
The description's phylogenetic result places Enigmacursor near Yandusaurus in the sampled tree. This comparison helps identify an early branch of neornithischians, but it does not mean that the Colorado species migrated directly from China or descended from the named genus. Related forms can share an older ancestor, and the proposed connection remains dependent on the character sample available from incomplete skeletons.
The paper estimated a body length of roughly one metre for the known immature animal; this is a reconstruction of an incomplete skeleton, not a direct measurement of a complete adult. Colour, skin, social behaviour, precise diet and mature proportions remain unknown. The 2025 description adds a testable taxon to the Morrison record and gives future work a clearly defined specimen. Compare it with other named dinosaurs in the A–Z dinosaur catalogue.
Frequently asked questions
When was Enigmacursor named?
The genus was described by Susannah Maidment and Paul Barrett in 2025 from a specimen collected in Colorado in 2021–2022.
What is its type specimen?
NHMUK PV R 39000 is a three-dimensionally preserved partial postcranial skeleton with teeth, now held by the Natural History Museum in London.
Was Enigmacursor an adult?
The specimen was still growing. Incomplete fusion in parts of the skeleton supports immaturity, but preparation and preservation limit a precise age estimate.
Why was it once identified differently?
The bones were initially compared with Nanosaurus, a historically complicated Morrison taxon. Re-examination found a distinct combination of features supporting a new genus.