Open Paleo

Nanosaurus

Marsh, 1877 NAY-no-SAWR-us Disputed

From Greek 'nanos' (small or dwarf) and 'sauros' (lizard); meaning 'small or dwarf lizard'.

154.8–143.1 million years ago Kimmeridgian–Tithonian, Late Jurassic
Late Jurassic stages

Nanosaurus is an extinct genus of neornithischian dinosaur that lived about 155 to 148 million years ago, during the Late Jurassic in North America. Its fossils are known from the Morrison Formation of the south-western United States. The type and only species, Nanosaurus agilis, was described and named by Othniel Charles Marsh in 1877. The taxon has a complicated taxonomic history, largely the work of Marsh and Peter M. Galton, involving the genera Laosaurus, Hallopus, Drinker, Othnielia, and Othnielosaurus, the latter three now being considered to be synonyms of Nanosaurus. It had historically been classified as a hypsilophodont or fabrosaur, types of generalized small bipedal herbivore, but more recent research has abandoned these groupings as paraphyletic and Nanosaurus is today considered a basal member of Neornithischia.

Under dispute

Nanosaurus is treated as a valid basal neornithischian by Carpenter and Galton (2018), who synonymized Drinker, Othnielia, and Othnielosaurus with it, and by Fonseca and colleagues (2024). Boyd (2015) and Barrett and Maidment (2025) instead regard Nanosaurus agilis as a nomen dubium lacking diagnostic features.

Diagnosis

  1. Diminutive size, not larger than a cat
  2. Third trochanter present on femur
  3. Femur shorter than tibia
  4. Tooth crowns apparently compressed with distinct sockets
  5. Most bones hollow with thin walls

Species

Nanosaurus agilis Marsh, 1877 Disputed

Type species

From Latin 'agilis' meaning 'agile' or 'swift', in reference to the animal's presumed nimble locomotion.

Holotype
YPM 1913 , Yale Peabody Museum of Natural History
Material
Partial dentary with impressions, ilium, femur, tibia, fibula
Horizon
Morrison Formation, Kimmeridgian–Tithonian, Late Jurassic , 154.8–143.1 million years ago
Locality
Cope's Nipple, Garden Park Paleontological Recreation Area, Colorado, United States

Synonyms of the genus

Name Type Reason
Drinker junior subjective Material previously referred to Drinker is now considered referable to Nanosaurus, alongside Othnielia and Othnielosaurus.
Othnielia junior subjective Material went through Othnielia → Laosaurus consors → Othnielosaurus reclassifications and is now considered referable to Nanosaurus.
Othnielosaurus junior subjective Material went through Othnielia → Laosaurus consors → Othnielosaurus reclassifications and is now considered referable to Nanosaurus; a 2025 paper rejected the synonymy but lacks broader concurrence.

References

  1. Marsh, O. C. (1877). Notice of some new vertebrate fossils. American Journal of Science s3-14(81): 249-256. American Journal of Science (AJS). doi:10.2475/ajs.s3-14.81.249 marsh1877d
  2. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the hip height estimate.

    jones2026a
  3. Fonseca, A. O.; Reid, I. J.; Venner, A.; Duncan, R. J.; Garcia, M. S.; Müller, R. T. (2024). A comprehensive phylogenetic analysis on early ornithischian evolution. Journal of Systematic Palaeontology 22: 2346577. doi:10.1080/14772019.2024.2346577

    Comprehensive ornithischian phylogeny recovered Nanosaurus as valid basal neornithischian.

    fonseca2024a
  4. Boyd, C. A. (2015). The systematic relationships and biogeographic history of ornithischian dinosaurs. PeerJ 3: e1523. PeerJ. doi:10.7717/peerj.1523

    Regarded Nanosaurus agilis as a nomen dubium lacking diagnostic features.

    boyd2015a
  5. 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. Bulletin of the Peabody Museum of Natural History. doi:10.3374/014.066.0102

    Regarded Nanosaurus agilis as a nomen dubium lacking diagnostic features.

    barrett2025a
  6. Carpenter, K.; Galton, P. M. (2018). A photo documentation of bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation, USA. Geology of the Intermountain West 5: 167-207. doi:10.31711/giw.v5.pp167-207

    Treated Nanosaurus as a valid basal neornithischian, synonymizing Drinker, Othnielia, and Othnielosaurus with it.

    carpenter2018b