Troodon formosus

Troodon formosus inhabited the seasonal lacustrine and fluvial environments of the Two Medicine Formation (Montana) during the Campanian (~77-74 Ma). This region was part of Laramidia, the western continent of North America separated from the east by the Western Interior Seaway. The climate was seasonal and semi-arid, with marked wet and dry seasons, and vegetation included open forests of angiosperms and gymnosperms. It coexisted with other dinosaurs such as Maiasaura, Hypacrosaurus, and ceratopsids like Einiosaurus. The Alaskan population, from the Prince Creek Formation, inhabited Arctic environments with 120-day winters of darkness and temperatures between 2 and 12°C.

Troodon formosus's diet was likely omnivorous: teeth with apically oriented serrations are morphometrically more similar to those of herbivorous reptiles than to carnivorous theropods, but enamel hardness and wear patterns are inconsistent with strict herbivory on tough plants. Gastric pellets with mammal bones from the Egg Mountain site (Freimuth and Varricchio, 2021) confirm that small mammals such as Alphadon were part of the diet. Fiorillo (2008) demonstrated that dental wear is inconsistent with bone chewing, suggesting soft prey. Insects, lizards, and eggs of other dinosaurs are also likely prey items.

Troodon formosus exhibited highly developed nesting behaviors documented by multiple studies (Varricchio et al., 1997, 1999). It built nests with capacity for 16 to 24 eggs, incubated by partial body contact. Isotopic analysis by Tagliavento et al. (2023) suggests possible communal nesting, with eggs from multiple females in the same nest. The large forward-facing eyes indicate binocular vision and possible crepuscular or nocturnal activity. The exceptional encephalization quotient suggests more complex social behavior than other dinosaurs, possibly including vocal communication and individual recognition.

The bone microstructure of Troodon formosus, analyzed by Varricchio (1993), reveals rapid growth with highly vascularized fibro-lamellar bone in juvenile phases, reaching adult size in approximately 3 to 5 years. This is consistent with elevated metabolism and endothermy. Clumped isotope thermometry analysis by Tagliavento et al. (2023) indicates heterothermic endothermy: variable body temperature, lower than that of modern birds during egg calcification. Four of the five periotic pneumatic systems of birds are present in the Troodon skull (Currie and Zhao, 1993), and the brain has morphology similar to birds, with a characteristic triangular-shaped telencephalon.

This is the founding paper of Troodon formosus: Joseph Leidy describes a single serrated tooth collected by F.V. Hayden from the Judith River Formation of Montana, naming it Troödon formosus. Leidy initially interpreted it as a lizard, not a dinosaur, demonstrating the limited paleontological knowledge of the time. The holotype tooth (Academy of Natural Sciences of Philadelphia specimen no. 9259) bears characteristic apical serrations that distinguish troodontids from other theropods. This single tooth generated over 160 years of taxonomic debate about the validity and identity of the genus.

Holotype tooth of Troodon formosus (specimen ANSP 9259), photographed by Loris S. Russell in 1948, from the Judith River Formation of Montana. This is the sole specimen on which Leidy based the original 1856 description.

Illustrated skull comparison of Troodon (then called Troodon validus) by Charles Gilmore in 1924, showing the characteristic dental and cranial anatomy of the genus first described by Leidy.

Charles Mortram Sternberg describes Stenonychosaurus inequalis based on a foot, hand fragments, and caudal vertebrae from the Dinosaur Park Formation of Alberta, Canada. This material would become central to the taxonomic debate about Troodon decades later. Phil Currie synonymized Stenonychosaurus with Troodon formosus in 1987, but van der Reest and Currie reversed this synonymy in 2017, recognizing Stenonychosaurus as a distinct genus. Sternberg's work established the first documentation of troodontids in Canada.

Outcrop of the Two Medicine Formation near 'Egg Mountain' in northern Montana (1989 photograph). This Campanian geological formation (~83-74 Ma) is where the most complete Troodon formosus material was collected, including Sternberg's Stenonychosaurus material.

Skeletal diagram of Stenonychosaurus by Scott Hartman (CC BY 2.5), based on all known material, with missing skull and vertebral column based on Saurornithoides. Stenonychosaurus was the taxon described by Sternberg in 1932 from the Dinosaur Park Formation, subsequently synonymized with Troodon by Currie (1987).

Dale Russell describes new Stenonychosaurus material (later referred to Troodon) and performs the first encephalization quotient estimate for the genus using Jerison's EQ concept. Results indicate Troodon had the largest proportional brain volume among all non-avian dinosaurs, comparable to modern ratites. This publication was the starting point for decades of speculation about dinosaur intelligence, culminating in Russell and Séguin's famous 'dinosauroid' thought experiment published in 1982.

Drawing of the dinosauroid, the hypothetical humanoid figure imagined by Dale Russell and Ron Séguin in 1982 as a speculative projection of how Troodon might have evolved if dinosaurs had not gone extinct, directly inspired by Russell's 1969 findings about the animal's intelligence.

Reconstruction of the troodontid originally published to represent Troodon, updated to reflect its possible identity as Stenonychosaurus. The image has a white background and shows the general morphology of the animal whose brain Russell studied in 1969.

Phil Currie performs a systematic review of the dental and mandibular characteristics of North American troodontids, demonstrating that supposed differences between Stenonychosaurus, Pectinodon, and other genera reflect ontogenetic variation and tooth position in the jaw, not species differences. Based on this, Currie synonymized all North American material under Troodon formosus, a classification that dominated the literature for 30 years. The paper also highlights avian characteristics of troodontid jaws, reinforcing the group's role in understanding the dinosaur-bird transition.

Quantitative morphotypes of small theropod teeth with their stratigraphic ages, published by Larson and Currie in PLOS ONE (2013). The dental morphological variation illustrated here is directly relevant to Currie's (1987) thesis that dental differences among troodontids reflect position and ontogeny, not specific distinction.

Simplified cladogram of Paraves showing phylogenetic relationships between Dromaeosauridae, Troodontidae, and birds. The derived position of troodontids as close relatives of birds is at the center of Currie's observations about avian characteristics of troodontid jaws.

Currie and Zhao describe a well-preserved braincase of Troodon formosus from the Dinosaur Park Formation of Alberta, the first to reveal the internal anatomy of the lower part of the cranial box. CT scanning reveals the anatomy of the inner ear and the course of pneumatic ducts diverging from the middle ear. The authors demonstrate that four of the five periotic pneumatic systems present in bird skulls are present in Troodon, reinforcing close kinship with birds. The work provides detailed data on the brain volume and morphology of the most encephalized non-avian dinosaur.

Diversity of Deinonychosauria dinosaurs, including troodontids. The Troodon braincase studied by Currie and Zhao in 1993 revealed that the animal's brain had morphology similar to modern birds, with a characteristic triangular-shaped telencephalon.

Reconstruction of Troodon formosus based on Scott Hartman's skeletal diagram. The animal's relatively large skull, studied in detail by Currie and Zhao in 1993, is visible in the proportions of the head relative to the body.

David Varricchio analyzes the bone microstructure of Troodon formosus through histology, identifying three ontogenetic growth stages: rapid fibro-lamellar, moderate lamellar-zonal, and slow avascular lamellar growth. Highly vascularized fibro-lamellar bone accounted for most growth, with the animal potentially reaching adult size in 3 to 5 years. This rapid growth pattern is more similar to birds and mammals than to reptiles, supporting the hypothesis of elevated (endothermic) metabolism for Troodon.

Reconstruction of Troodon formosus with feathers by Conty, depicting an active, high-metabolism animal consistent with the rapid growth documented by Varricchio (1993) through bone microstructure.

Artistic reconstruction of Troodon formosus by El fosilmaníaco, showing the dinosaur in an active posture. The rapid growth studied by Varricchio (1993) implies the animal was endothermic, maintaining posture and activity similar to birds.

Varricchio and collaborators describe nests and egg clutches of Troodon formosus from the Two Medicine Formation of Montana, published in Nature. The study demonstrates that Troodon produced two eggs simultaneously at daily or longer intervals, and incubated eggs using a combination of soil and direct body contact. Each clutch contained up to 24 elongated eggs positioned with their smaller ends pointing toward the center of the nest. This complex reproductive behavior is intermediate between reptiles and modern birds, providing direct fossil evidence for the evolution of avian reproductive traits.

Clutch of Troodon formosus eggs in matrix, specimen from the Museum of the Rockies on loan to the Burke Museum. The eggs are partially encased in sediment, consistent with the mixed incubation mode (soil and body contact) described by Varricchio et al. (1997).

Replica of a Troodon formosus nest fossil displayed at the Nagoya City Science Museum in 2025. The nest structure, with eggs arranged vertically, reflects the pattern described by Varricchio et al. (1997) as intermediate between reptiles and birds.

Varricchio and Jackson describe a nesting trace with eggs of Troodon formosus from the Two Medicine Formation of Montana, with detailed analysis of nest geometry and inferred incubation behavior. The eggs were positioned with their smaller ends downward, partially buried in the substrate, with the larger ends exposed to the air. This arrangement suggests that only the lower half of the eggs was in contact with moist sediment, while the adult covered the eggs with its body. The work complements the 1997 findings and reinforces Troodon's role as a model of parental behavior intermediate between reptiles and birds.

Reconstruction of three Troodon formosus individuals by ABelov2014 (2019). The complex nesting behavior documented by Varricchio and Jackson (1999) suggests animals with sophisticated parental behavior, possibly including nocturnal nest vigil, consistent with the nocturnal vision inferred for the species.

Reconstruction of Troodon formosus by Danny Cicchetti, showing the animal in an active posture. The nesting behavior studied by Varricchio and Jackson (1999) implies that Troodon exercised active parental care, similar to modern birds.

Varricchio and Barta review paravian eggs from the Cretaceous of Mongolia and compare them to the reproductive pattern established for Troodon formosus. The work elucidates the evolution of reproductive traits in paravians, especially the transition from the partially buried incubation of troodontids to the completely aerial incubation of modern birds. Direct comparisons with Two Medicine material show how Troodon's nesting mode represents a critical intermediate stage in the evolution of avian reproductive behavior.

Reconstruction of Deinonychus and Troodon together, two North American Cretaceous paravians. The comparison of reproductive modes of different paravians, a central theme of Varricchio and Barta's (2015) work, is facilitated by studies of multiple species within the group.

Life reconstruction of Stenonychosaurus by Tomopteryx (CC BY-SA 4.0). The Canadian Stenonychosaurus material, separated from Troodon by van der Reest and Currie (2017), was the focus of comparative studies of reproductive modes in paravians by Varricchio and Barta (2015).

Fiorillo and collaborators describe two partial Troodon braincases from the Prince Creek Formation of Alaska, documenting for the first time the presence of the genus in the Arctic Circle. The Alaskan troodontid's teeth are approximately twice as large as specimens from the Two Medicine Formation, suggesting an animal nearly 4 meters long. This size variation is consistent with Bergmann's Rule: animals in cold climates tend to be larger. Troodon was the most common theropod in the Prince Creek Formation, representing nearly two-thirds of all theropod specimens.

Reconstruction of two Stenonychosaurus inequalis individuals in snow by Midiaou Diallo (CC BY 3.0). The Alaskan troodontid population described by Fiorillo et al. (2009) inhabited Arctic environments with temperatures of 2 to 12°C and 120 days of darkness per winter.

Geological map showing the surface exposure of the Two Medicine Formation by Montanoceratops (CC BY 4.0). The Alaskan troodontid described by Fiorillo et al. (2009) comes from the Prince Creek Formation, paleogeographically correlatable with the Two Medicine Formation of Montana.

Zanno and collaborators describe Talos sampsoni, a new gracile troodontid from the Kaiparowits Formation (upper Campanian) of Utah, published in PLOS ONE. Phylogenetic analysis recovers Talos in a position close to Troodon formosus, Saurornithoides, and Zanabazar within Troodontinae. The specimen includes a pathological pedal phalanx, the first evidence of pathology in a troodontid and possible evidence of interspecific predation. The work also documents the evolution of an enlarged second pedal claw in derived troodontids, similar to the sickle claw of dromaeosaurids.

Centrosaurine evolution in the Two Medicine Formation, scientific publication illustrating the faunal diversity of the Campanian of Montana. Talos sampsoni, described by Zanno et al. (2011), coexisted with similar ceratopsids in the same Laramidian paleogeographic region.

Map of North America with the Western Interior Seaway during the Campanian (Upper Cretaceous). Talos sampsoni, described by Zanno et al. (2011), lived in the Kaiparowits Formation of southern Laramidia, while Troodon formosus inhabited the northern portion in the Two Medicine Formation.

Van der Reest and Currie review troodontid material from the Dinosaur Park Formation of Alberta and propose that Stenonychosaurus inequalis be recognized as a valid genus separate from Troodon formosus, reversing the synonymy established by Currie in 1987. The authors also describe Latenivenatrix mcmasterae, the largest known troodontid, with an estimated height of 180 cm and length of 350 cm, from the upper portion of the formation. This work represents the main modern taxonomic revision of the group and triggered the reassessment of the validity of Troodon formosus itself, culminating in the 2025 neotype proposal.

Panoramic reconstruction of a Maiasaura herd in the Two Medicine Formation. Van der Reest and Currie (2017) described that the troodontid material from the Dinosaur Park Formation (correlatable with the Two Medicine) belonged to Stenonychosaurus and the new taxon Latenivenatrix, not to Troodon formosus.

Dinosaurs from the west coast of Laramidia, including Campanian fauna of North America. The troodontid material from the Dinosaur Park Formation described by van der Reest and Currie (2017) as Stenonychosaurus and Latenivenatrix was part of this diverse west coast Laramidian fauna.

Freimuth and Varricchio describe gastric pellets containing small mammal bones from the Egg Mountain site (Two Medicine Formation), possibly produced by Troodon formosus. This is the first direct record of pellet regurgitation (similar to the behavior of modern raptorial birds) in non-avian dinosaurs. The study provides concrete evidence that Troodon included mammals such as Alphadon in its diet, supporting the omnivory hypothesis. The spatial association of the pellets with Troodon nests at Egg Mountain reinforces the troodontid attribution.

Prismatoolithus levis (troodontid) eggs from the Egg Mountain site (Two Medicine Formation, Upper Cretaceous, Teton County, Montana), specimen MOR 299 from the Museum of the Rockies. The obliquely arranged eggs are from the same site where Freimuth and Varricchio (2021) found mammal-bearing gastric pellets attributed to Troodon formosus.

Map of North America at ~75 Ma (Campanian), published in PLOS ONE, showing the Western Interior Seaway and Laramidia. The Egg Mountain site (Two Medicine Formation, Montana) where Freimuth and Varricchio (2021) found the mammal-bearing gastric pellets is located on the northwestern margin of Laramidia.

Tagliavento and collaborators apply clumped isotope thermometry to Troodon formosus eggshells from the Oldman Formation, revealing that egg calcification occurred at lower temperatures than in modern birds. This indicates heterothermic endothermy (variable body temperature) and the presence of two simultaneous functional ovaries, unlike birds which have only one. Despite two ovaries, egg production was limited, suggesting that eggs in clutches were laid by multiple females in communal nests, as observed in some modern birds.

Late Cretaceous metatherian localities, published in ZooKeys. The eggs analyzed by Tagliavento et al. (2023) are from troodontids that coexisted with primitive marsupials like Alphadon in the Two Medicine Formation, whose mammal localities are plotted on similar maps.

Reconstruction of embryonic Maiasaura emerging from egg, cast from the Museum of the Rockies (2013). The isotopic analysis by Tagliavento et al. (2023) on Troodon formosus eggs complements nesting studies of other dinosaurs from the same Egg Mountain site, such as Maiasaura.

Varricchio, Barta, and Riggs redescribe the troodontid material from Egg Mountain (Two Medicine Formation, Montana), including specimen MOR 553, a collection of elements representing multiple individuals of different ontogenetic stages. The authors propose MOR 553 as the neotype of Troodon formosus, preserving the genus's validity and treating Stenonychosaurus as a possible junior synonym. This paper represents the most recent resolution of a century-long taxonomic controversy and consolidates Troodon formosus as the valid name for Two Medicine Formation material, with implications for the paleoecology of Campanian Laramidia.

Distribution of chasmosaurine ceratopsids in Late Cretaceous Laramidia. Troodon formosus coexisted with ceratopsids in the Two Medicine Formation of Montana. Varricchio et al.'s (2025) work consolidates the Troodon neotype from the same paleoenvironment where these ceratopsids lived.

Map of North America with the Western Interior Seaway during the Campanian (PNG version). The neotype proposal MOR 553 by Varricchio et al. (2025) comes from Egg Mountain (Montana, USA), in the northwestern portion of Laramidia where Troodon formosus was most abundant.

Full-size T-rex animatronic from the Jurassic Park franchise, with the iconic red Jeep — Amaury Laporte · CC BY 2.0