# Troodon formosus

> A small, big-brained, bird-like predator from the Late Cretaceous of North America, named in 1856 from a single tooth. Nests from Montana suggest that males brooded the eggs. Whether the name is still usable is debated.

- Classification: Dinosauria (clade) > Saurischia (clade) > Theropoda (clade) > Coelurosauria (clade) > Maniraptora (clade) > Troodontidae (family) > Troodon (genus) > Troodon formosus (species)
- Hip height: 0.8–1 m
- Age: 79.5–74.4 million years ago (Late Cretaceous)
- Fossils found in: Judith River Formation (Montana); Dinosaur Park Formation (Alberta); United States; Canada
- Diet: carnivore, omnivore
- Described: Joseph Leidy, 1856

Canonical page: https://dinodex.club/dinosaurs/troodon-formosus
Last updated: 2026-09-27

## Overview

Troodon formosus was named by Joseph Leidy in 1856 from one serrated tooth collected by Ferdinand Hayden in 1855 in the Judith River badlands of Montana. For more than a century it was one of the earliest-named North American dinosaurs, and many small troodontid skeletons from the Campanian of Montana and Alberta were referred to it.

Troodontids were slender, long-legged predators with large eyes, big brains for dinosaurs and an enlarged, retractable second toe claw. Clutches of paired, elongated eggs from Egg Mountain in Montana, some with an adult on top, have been attributed to Troodon (Varricchio et al. 1997).

The clutches are large relative to the adults, and the brooding adults lack the bone features of egg-laying females, which suggests that males cared for the eggs, as in many living birds (Varricchio et al. 2008). A single tooth cannot separate closely related species, so several authors treat Troodon formosus as a doubtful name and use Stenonychosaurus for the Canadian skeletons.

## Name meaning

"Wounding tooth", from Greek trōō ("to wound") and odous ("tooth"), for its serrated teeth; formosus, Latin for "beautiful" (Leidy 1856).

## Sources

1. (2026). Paleobiology Database: Troodon occurrences. The Paleobiology Database (paleobiodb.org), data service 1.2, accessed 2026-09-23. https://paleobiodb.org/data1.2/occs/list.txt?base_name=Troodon&show=coords,paleoloc,strat,loc,ref&pgm=gplates
2. Eberth, D. A., Evans, D. C., Ramezani, J., Kamo, S. L., Brown, C. M., Currie, P. J. & Braman, D. R. (2023). Calibrating geologic strata, dinosaurs, and other fossils at Dinosaur Provincial Park (Alberta, Canada) using a new CA-ID-TIMS U-Pb geochronology. Canadian Journal of Earth Sciences. https://doi.org/10.1139/cjes-2023-0037
3. 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.
4. Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs (2nd edition). Princeton University Press.
5. Rogers, R. R. et al. (2016). Age, correlation, and lithostratigraphic revision of the Upper Cretaceous (Campanian) Judith River Formation in its type area (north-central Montana), with a comparison of low- and high-accommodation alluvial records. The Journal of Geology 124(1). https://doi.org/10.1086/684289
6. 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: 247-250. https://doi.org/10.1038/385247a0
7. Varricchio, D. J., Moore, J. R., Erickson, G. M., Norell, M. A., Jackson, F. D. & Borkowski, J. J. (2008). Avian paternal care had dinosaur origin. Science 322: 1826-1828. https://doi.org/10.1126/science.1163245

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Cite as: "Troodon formosus". DinoDex. https://dinodex.club/dinosaurs/troodon-formosus (updated 2026-09-27).