Enigmacursor, new neornithischian from Upper Jurassic Morrison Formation of Colorado (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

Enigmacursor mollyborthwickae gen. et sp. nov.

 
Susannah C. R. Maidment and Paul M. Barrett (2025)
Enigmacursor mollyborthwickae, a neornithischian dinosaur from the Upper Jurassic Morrison Formation of the western USA
Royal Society Open Science 12(6):242195
doi: https://doi.org/10.1098/rsos.242195
https://royalsocietypublishing.org/doi/10.1098/rsos.242195

Free pdf:
https://royalsocietypublishing.org/doi/epdf/10.1098/rsos.242195


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.

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News:

https://www.nhm.ac.uk/discover/news/2025/june/new-species-mystery-dinosaur-unveiled-natural-history-museum.html

https://www.newscientist.com/article/2485569-small-and-speedy-dinosaur-recognised-as-a-new-species/

https://www.bbc.com/news/articles/cglzy4zndp0o

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[Mickey Mortimer]

Well now we see why they declared Nanosaurus indeterminate.  I'm no ornithischian expert, and I'm away from my files, but that ilium looks awfully similar to what I remember of Nanosaurus. Anyone able to check the fourth trochanter position and tibiofemoral ratio that the authors inaccurately simplified to "proximally placed" and "greater than 1" in order to invalidate Galton's characters?

Mickey Mortimer

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[Jacqueline Silviria]

Somebody call a doctor! Those authors need to un-discretize their characters, and fast! (cf. Farris, 1990; Goloboff et al., 2006; Wheeler et al., 2024).

Jacqueline S. Silviria

she/her/hers

The Last King of the Jungle

Department of Earth & Space Science
University of Washington
Seattle, WA, USA
jsi...@uw.edu, sympan...@gmail.com

ResearchGate profile

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[Tim Williams]

Mickey Mortimer <therizino...@gmail.com> wrote:

> Well now we see why they declared Nanosaurus indeterminate.  

Because it's based on indeterminate material?

[Mickey Mortimer]

Well now that I'm back with my computer, Enigmacursor has a fourth trochanter index of ~0.46 (measured at the level used for Nanosaurus, as the former has an autapomorphic non-pendant trochanter, which cannot be determined for Nanosaurus' holotype whose trochanter is broken at its base) and a tibiofemoral ratio of 124%, compared to Galton's (1978) proposed Nanosaurus characters of "Fourth trochanter index of femur about 0.43, tibia about 115 % of femur length." However, reading Galton's text indicates "The fourth trochanter index (minimum distance between proximal end of femur and distal edge of fourth trochanter/length of femur) of the femur YPM 1913d (Fig. 5B) is about 0.43 as in the femora of Othnielia and Hypsilophodon whereas in Lesothosaurus it is 0.36", so this was merely meant to distinguish Nanosaurus from Lesothosaurus whose dentary and teeth Galton felt were similar. Note while the distal femur of Nanosaurus is unpreserved, the shaft dimensions show the trochanter was indeed similarly placed. The tibiofemoral ratio character can largely be ignored because it turns out Galton had no firm reason to place the proximal tibial edge where he did, as the bone is broken off proximally in both tibiae, and in fact Carpenter and Galton (2018) later reconstructed a longer proximal tibia for Nanosaurus that results in a ratio of 115%. Notably as tibiofemoral ratios might expect to decrease with age/size, Nanosaurus' femur is ~64 mm while Enigmacursor's is 169 mm. So this would not explain the difference, and Nanosaurus' tibia does look more stout than Enigmacursor's. 

Additionally, Nanosaurus has a supracetabular crest on its ilium unlike Enigmacursor, though the two do plesiomorphically share the absence of a ligament sulcus on their proximal femora (assuming Galton's shading is accurate). So I don't think the taxa are the same, contra the ambiguity Maidment and Barrett leave when only saying "neither [femur of Nanosaurus] are well enough preserved to determine any morphological features [3], so they cannot be compared to the femora of Enigmacursor. Barrett & Maidment [3] concluded that YPM VP 001913 was an indeterminate neornithischian."  I do agree with Maidment and Barrett that Othnielia and Othnielosaurus "are clearly not referrable to Enigmacursor, Dryosaurus or Fruitadens and this indicates that there was at least one other small-bodied neornithischian taxon present in the Morrison Formation fauna." At least Othnielosaurus has a ligament sulcus so would not be Nanosaurus either. While I don't have the time to examine the entire issue, I do think my conclusion would be to not throw out all the old holotypes like Barrett and Maidment but also to not lump them all together like Carpenter and Galton.

References- Galton, 1978. Fabrosauridae, the basal family of ornithischian dinosaurs (Reptilia: Ornithopoda). Paläontologische Zeitschrift. 52, 138-159.

Carpenter and Galton, 2018. A photo documentation of bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation, USA. Geology of the Intermountain West. 5, 167-207.

Barrett and Maidment, 2025. A Review of Nanosaurus agilis Marsh and other small-bodied Morrison Formation 'ornithopods'. Bulletin of the Peabody Museum of Natural History. 66, 25-50.

Maidment and Barrett, 2025. Enigmacursor mollyborthwickae, a neornithischian dinosaur from the Upper Jurassic Morrison Formation of the western USA. Royal Society Open Science. 12(6), 242195.