Open Paleo

Stenonychosaurus

Sternberg, 1932 Sten-oh-NYE-ko-sore-us Disputed

From Greek 'stenos' meaning 'narrow', 'onyx/onychos' meaning 'claw', and 'sauros' meaning 'lizard'; meaning 'narrow claw lizard', referring to the narrow enlarged claw on the second toe.

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

Stenonychosaurus is a disputed genus of troodontid dinosaur from the Late Cretaceous Dinosaur Park Formation of Alberta, Canada, as well as possibly the Two Medicine Formation. The type and only species, S. inequalis, was named by Charles Mortram Sternberg in 1932, based on a foot, fragments of a hand, and some caudal vertebrae from the Late Cretaceous of Alberta. S. inequalis was reassigned in 1987 by Phil Currie to the genus Troodon, which was reverted by the recognition of Stenonychosaurus as a separate genus from the possibly dubious Troodon in 2017 by Evans and colleagues and also later in the same year by Van der Reest and Currie.

Under dispute

Stenonychosaurus inequalis was resurrected as a valid genus by van der Reest and Currie (2017), who treated Troodon formosus as a nomen dubium and referred the diagnostic Dinosaur Park Formation troodontid material to it. Cullen and colleagues (2021) synonymized Latenivenatrix into it, on extensive overlap in frontal morphospace and stratigraphic range, though they could not assess that taxon's pubic autapomorphies for want of overlapping material. That synonymy is not settled: Varricchio and colleagues (2025) adopt it for working purposes while stating the status of Latenivenatrix remains in question and reporting material that lends some support to its validity, and Rivera-Sylva and colleagues (2026) recover the two as a sister pair and find Dinosaur Park frontal variation consistent with more than one species. Whether Stenonychosaurus is itself valid or a junior synonym of Troodon likewise remains contested (Varricchio and colleagues, 2025). It is recovered as a derived troodontine.

Diagnosis

  1. Inequality in length of the three functional digits of the hind foot
  2. Ungual phalanges of the pes strongly laterally compressed
  3. Specialized digit II with moderately sized, slightly curved, laterally compressed ungual
  4. Metatarsal II much shorter than metatarsals III and IV
  5. Caudal vertebrae amphiplatyan with well-developed longitudinal groove on inferior surface
  6. Thick bone walls in phalanges of the manus, thicker than in Chirostenotes

Species

Stenonychosaurus inequalis Sternberg, 1932 Disputed

Type species

From Latin 'inequalis' (or 'inaequalis') meaning 'unequal' or 'uneven', referring to the unequal or distinctive enlarged claw on the second toe that was noted as a remarkable feature of the holotype.

Holotype
CMN 8539 , Canadian Museum of Nature
Material
Distal end of tibia with astragalus; complete left hind foot; left metacarpal I; distal end of Mc. II; distal ends of three phalanges of the manus and six caudal vertebrae
Horizon
lower Dinosaur Park Formation, Campanian, Late Cretaceous , 83.6–72.2 million years ago
Locality
about 2½ miles southwest of the mouth of Berry Creek, Steveville, Alberta, Canada

Notable specimens

  • MOR 748

    Museum of the Rockies reproductive

    Partial adult skeleton found in contact with a clutch of at least five eggs, probably in a brooding position. Bone histology lacked the resorption patterns of an egg-laying female, leading to the inference that the male brooded the eggs, a trait possibly shared by maniraptorans and basal birds.

    Varricchio et al. (1997)

References

  1. Varricchio, D. J.; Jackson, F.; Borkowski, J. J.; Horner, J. R. (1997). Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits. Nature 385(6613): 247-250. doi:10.1038/385247a0 varricchio1997a
  2. Sternberg, C. M. (1932). Two New Theropod Dinosaurs from the Belly River Formation of Alberta. The Canadian field-naturalist 46(5): 99-105. Biodiversity Heritage Library. doi:10.5962/p.339375 sternberg1932a
  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

    Resurrected Stenonychosaurus inequalis as valid, treating Troodon formosus as nomen dubium

    vanderreest2017a
  4. 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

    Assessed whether Stenonychosaurus is valid or a junior synonym of Troodon, and reported material lending some support to the validity of Latenivenatrix.

    varricchio2025a
  5. Cullen, T. M.; Zanno, L.; Larson, D. W.; Todd, E.; Currie, P. J.; Evans, D. C. (2021). Anatomical, morphometric, and stratigraphic analyses of theropod biodiversity in the Upper Cretaceous (Campanian) Dinosaur Park Formation. Canadian Journal of Earth Sciences 58(9): 870-884. Canadian Science Publishing. doi:10.1139/cjes-2020-0145

    Synonymized Latenivenatrix into Stenonychosaurus, without being able to assess that taxon's pubic autapomorphies.

    cullen2021a
  6. Rivera-Sylva, H. E.; Aguillón-Martínez, M. C.; Flores-Ventura, J.; Sánchez-Uribe, I. E.; Guzmán-Gutiérrez, J. R.; Longrich, N. R. (2026). A Thick-Skulled Troodontid Theropod from the Late Cretaceous of Mexico. Diversity 18(1). MDPI AG. doi:10.3390/d18010038

    Recovers Stenonychosaurus and Latenivenatrix as a sister pair and finds Dinosaur Park frontal variation consistent with more than one species.

    rivera-sylva2026a