DinoDex
Slender, fully feathered dark troodontid running in side view
Life restoration of a troodontid long known as Troodon formosus, based on a skeletal by Scott Hartman; Troodon itself is known only from teeth.Image credit: Jfstudiospaleoart, CC BY-SA 4.0, via Wikimedia Commons (unmodified except resizing)CC BY-SA 4.0 (unmodified)Source (opens in a new tab)

Troodon formosus

Pronounced TROH-oh-don for-MOH-sus

Maniraptora · Troodontidae

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.

  • Bipedal
  • Feathered
  • Sickle claw
  • Eggs known
Compare
Period
Cretaceous· Campanian
Diet
Carnivore
Length
Unknown
Mass
Unknown
Location
Montana, Alberta, United States, Canada

Quick answers

How big was Troodon formosus?
It stood about 0.8–1 m tall at the hips. (Paul 2016)
When did Troodon formosus live?
Troodon formosus lived during the Late Cretaceous, about 79.5–74.4 million years ago. (Paleobiology Database: Troodon occurrences 2026; Rogers et al. 2016)
Where have Troodon formosus fossils been found?
Fossils of Troodon formosus have been found in Judith River Formation (Montana), Dinosaur Park Formation (Alberta), United States, and Canada. (Leidy 1856; Varricchio et al. 1997)
What did Troodon formosus eat?
Troodon formosus was a carnivore or omnivore. (Leidy 1856; Varricchio et al. 1997)
Who named Troodon formosus, and when?
Troodon formosus was described by Joseph Leidy in 1856. (Leidy 1856; Paleobiology Database: Troodon occurrences 2026)

On this page

TROH-oh-don for-MOH-sus

Period
Cretaceous79.5–74.4 Ma
Diet
Carnivore
Length
Unknown
Mass
Unknown
Location
United States, Canada
Classification
Maniraptora · Troodontidae
Described
1856, Joseph Leidy

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).

Biology & evidence

What scientists think about how this animal lived, and how strong the evidence for each idea is.

Eggs

  • Troodon laid its eggs in pairs in a partly buried clutch.

    Evidence level: Moderate evidence

    Clutches from Montana show paired eggs, which suggests two working oviducts laying one egg each at intervals. The attribution of the eggs to Troodon itself rests on associated skeletons.

    Evidence for / against · 1
    Evidence for

    • Specimen · Egg Mountain troodontid clutches

      Clutches of paired, elongated eggs from the Two Medicine Formation of Montana, attributed to Troodon, some associated with adults.

Nesting

  • Troodon fathers brooded the eggs.

    Evidence level: Hypothesis

    The clutches are very large for the adults, as in birds where males incubate, and the adults on the nests show no signs of recent egg-laying. Care by males may have evolved before birds.

    Evidence for / against · 2
    Evidence for

    • Histology · Clutch volume and brooding-adult histology

      Large clutch volumes scale with a bird paternal-care model, and the adults found on clutches lack the bone features of egg-laying females.

    • Specimen · Egg Mountain troodontid clutches

      Clutches of paired, elongated eggs from the Two Medicine Formation of Montana, attributed to Troodon, some associated with adults.

Other

  • Troodon formosus, based on a single tooth, is a valid name.

    Evidence level: Disputed

    A 2017 study found the type tooth undiagnostic and revived Stenonychosaurus for Canadian skeletons; later work has proposed a neotype to keep Troodon. Which name applies to which fossils is unsettled.

    Evidence for / against · 1
    Evidence for

    • Specimen · Egg Mountain troodontid clutches

      Clutches of paired, elongated eggs from the Two Medicine Formation of Montana, attributed to Troodon, some associated with adults.

Replica of a troodontid nest with a clutch of eggs, exhibited at the Nagoya City Science Museum.Image credit: Kyu3a, CC BY-SA 4.0, via Wikimedia Commons (unmodified except resizing)CC BY-SA 4.0 (unmodified)Source (opens in a new tab)

Size

Length
Unknown
Height
0.9 mpublished range 0.8–1 m

(3 ft)

Mass
Unknown

Length unknown, so no size comparison yet.

Timeline

Lived from about 79.5 to 74.4 million years ago (Ma), within the Mesozoic era (252–66 Ma).

Periods: Campanian (83.6–72.1 Ma), Late Cretaceous (100.5–66 Ma)

Span of its formations: the Judith River Formation of the type area, about 79.5-75.2 Ma (Ar/Ar, Rogers et al. 2016, read in a secondary summary only), and the Dinosaur Park Formation, 76.47-74.44 Ma (Eberth et al. 2023). The type tooth is not tied to a dated level and referred material is debated.

Location

Formations

Map data: Natural Earth (public domain). Modern positions, not where the continents were then.

These are the places where fossils have been found, not the full area where the animal lived.

Ecosystem

Lived in: Dinosaur Park

76.5–74.4 million years ago · floodplain, coastal plain, river system

Late Campanian alluvial and coastal plains on the western shore of the Western Interior Seaway, in what is now southern Alberta.

Same ecosystem

Parasaurolophus walkeri

Cretaceous· CampanianHerbivore

Same ecosystem

Styracosaurus albertensis

Cretaceous· CampanianHerbivore

Same ecosystem

Gorgosaurus libratus

Cretaceous· CampanianCarnivore

Classification

  1. cladeDinosauria
  2. cladeSaurischia
  3. cladeTheropoda
  4. cladeCoelurosauria
  5. cladeManiraptora
  6. familyTroodontidae
  7. genusTroodon
  8. speciesTroodon formosus

Taxonomic status: Valid

Fossil record

The Troodon formosus holotype tooth, ANSP 9259, from the Judith River Formation of Montana, in three views.Image credit: Loris S. Russell, Public domain, via Wikimedia Commons, Academy of Natural Sciences of PhiladelphiaPublic domainSource (opens in a new tab)
Specimens
Catalog numberStatusInstitutionFound
ANSP 9259HolotypeAcademy of Natural Sciences of Drexel University (ANSP)1855

What has been found

Holotype: ANSP 9259

  • Preserved
  • Partial
  • Reconstructed
  • Unknown / not recorded
Region by region (table)
Skeleton regions of Holotype: ANSP 9259
RegionState
SkullUnknown / not recorded
MandibleUnknown / not recorded
TeethPartial · One tooth.
Cervical vertebraeUnknown / not recorded
Dorsal vertebraeUnknown / not recorded
SacrumUnknown / not recorded
Caudal vertebraeUnknown / not recorded
RibsUnknown / not recorded
GastraliaUnknown / not recorded
Scapula / shoulder girdleUnknown / not recorded
HumerusUnknown / not recorded
Forearm (radius / ulna)Unknown / not recorded
Hand (manus)Unknown / not recorded
PelvisUnknown / not recorded
FemurUnknown / not recorded
Tibia / fibulaUnknown / not recorded
Foot (pes)Unknown / not recorded

All specimens

Discovery

Year described
1856
Described by
Joseph Leidy
Type specimen
ANSP 9259

Discovery timeline

  1. 1856 · Formally described

    Leidy names Troodon from a tooth

    Leidy names Troödon formosus from a tooth collected by Hayden in 1855.

    People: Joseph Leidy, Ferdinand V. Hayden

  2. 1997 · Major study

    Troodon nests described from Egg Mountain

    Varricchio and colleagues describe clutches and nests from Montana.

    People: David J. Varricchio

  3. 2008 · Major study

    Paternal care proposed for Troodon

    Clutch size and bone histology suggest males brooded the eggs.

    People: David J. Varricchio

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 (opens in a new tab)
  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. doi:10.1139/cjes-2023-0037 (opens in a new tab)
  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). doi:10.1086/684289 (opens in a new tab)
  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. doi:10.1038/385247a0 (opens in a new tab)
  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. doi:10.1126/science.1163245 (opens in a new tab)