Open Paleo

Troodon

Leidy, 1856 TROH-uh-don /ˈtroʊ.ədɒn/ Disputed

From Greek 'troo' (wounding) and 'odon' meaning 'tooth', hence 'wounding tooth', referring to the distinctive serrated teeth.

83.6–72.2 million years ago Campanian, Late Cretaceous
Late Cretaceous stages

Troodon is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at least one species, Troodon formosus, known from Montana. Discovered in October 1855, T. formosus was among the first dinosaurs found in North America, although it was thought to be a lizard until 1877.

Under dispute

The holotype of Troodon formosus, the type genus of Troodontidae, is an isolated tooth. Zanno and colleagues (2011) and Evans and colleagues (2017) suggest the name may be a nomen dubium, and van der Reest and Currie (2017) treat it as one and transfer its referred material to Stenonychosaurus. Varricchio and colleagues (2025) argue that the name is valid, propose a neotype, and report an ICZN petition in preparation. Where its referred material is scored it is recovered as a derived troodontine (with Saurornithoides and Zanabazar), where it is placed.

Diagnosis

  1. Compressed, curved, conical crown with trenchant edges
  2. Outer side more convex than inner side
  3. Trenchant edges coarsely denticulated
  4. Denticulations compressed conical with trenchant edges, bent toward crown apex
  5. Inner surface worn from friction of opposing tooth

Species

Troodon formosus Leidy, 1856 Disputed

Type species

From Latin 'formosus' meaning 'beautiful' or 'finely formed'.

Holotype
ANSP 9259 , Academy of Natural Sciences of Philadelphia
Material
Single tooth specimen with compressed, curved, conical crown with trenchant edges; base hollow (2 lines wide, 1.5 lines transversely); 3 lines long
Horizon
Judith River Formation, Campanian, Late Cretaceous , 83.6–72.2 million years ago
Locality
Judith River badlands, Montana, United States

References

  1. Leidy, J. (1856). Notices of remains of extinct reptiles and fishes, discovered by Dr. F. V. Hayden in the bad lands of the Judith River, Nebraska Territory. Proceedings of the Academy of Natural Sciences of Philadelphia 8: 72-73. leidy1856b
  2. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Theropods. Princeton University Press.

    Source of the body length estimate.

    jones2026c
  3. van der Reest, A. J.; Currie, P. J. (2017). Troodontids (Theropoda) from the Dinosaur Park Formation, Alberta, with a description of a unique new taxon: implications for deinonychosaur diversity in North America. Canadian Journal of Earth Sciences 54(9): 919-935. doi:10.1139/cjes-2017-0031

    Transferred diagnostic Troodon material to Stenonychosaurus, treating Troodon as nomen dubium.

    vanderreest2017a
  4. Evans, D. C.; Cullen, T. M.; Larson, D. W.; Rego, A. (2017). A new species of troodontid theropod (Dinosauria: Maniraptora) from the Horseshoe Canyon Formation (Maastrichtian) of Alberta, Canada. Canadian Journal of Earth Sciences 54(8): 813-826. Canadian Science Publishing. doi:10.1139/cjes-2017-0034

    Reassessed Troodon referred material, supporting transfer to Stenonychosaurus.

    evans2017a
  5. Zanno, L. E.; Varricchio, D. J.; O'Connor, P. M.; Titus, A. L.; Knell, M. J. (2011). A New Troodontid Theropod, Talos sampsoni gen. et sp. nov., from the Upper Cretaceous Western Interior Basin of North America. PLoS ONE 6(9): e24487. Public Library of Science (PLoS). doi:10.1371/journal.pone.0024487

    Regarded the Troodon holotype tooth as undiagnostic.

    zanno2011a
  6. Varricchio, D. J.; Hogan, J. D.; Gardner, J. D. (2025). Troodontid specimens from the Cretaceous Two Medicine Formation of Montana (USA) and the validity of Troodon formosus. Journal of Paleontology 99(1): 219-240. Cambridge University Press (CUP). doi:10.1017/jpa.2024.67

    Proposed a Troodon neotype and an ICZN petition to retain the name.

    varricchio2025a