Troodon: one tooth and a disputed dinosaur name

Troodon formosus is one of palaeontology's most familiar disputed dinosaur names. Joseph Leidy named it in 1856 from a single tooth collected in Montana. The tooth is a real fossil and identifies a small theropod close to troodontids, but it cannot by itself reveal a complete animal, its size, feathers, brain or behaviour. Most of the body shown in books comes from other fossils whose connection to that tooth is contested.

Some researchers treat Troodon formosus as a nomen dubium and use Stenonychosaurus inequalis for better-preserved skeletons. A 2025 revision defended the older name, linked it to material from Montana's Two Medicine Formation and proposed a more informative specimen as a neotype. That proposal did not automatically replace the existing type, so both naming schemes remain in scientific use.

Quick facts

A name founded on one tooth

Geologist Ferdinand Vandeveer Hayden collected the type tooth in 1855 from Cretaceous deposits near the Judith River in central Montana. Joseph Leidy described it the following year as Troodon formosus. The generic name refers to a tooth that seemed unusually sharp or injurious; formosus means graceful or beautiful. The specimen is now catalogued as ANSP 9259.

Leidy initially thought the tooth belonged to a large lizard. Later authors compared its serrated edges with those of plant-eating dinosaurs and moved the name among several groups. That history reflects the limited information available in an isolated crown, not a sequence of complete skeletons changing identity. In the twentieth century, jaws and more complete bones established that comparable animals were small theropods.

Charles Sternberg named Stenonychosaurus inequalis in 1932 from partial remains collected in Alberta. The material included parts of the limbs, foot and vertebral column, giving a much fuller view of a troodontid. In the 1980s, Philip Currie compared its teeth with the Montana type tooth and treated the names as synonyms. Because Troodon had priority, familiar skeletons and nests were then widely called Troodon.

Why the name became controversial

Troodontid teeth vary along a jaw, between upper and lower jaws, through growth and with wear. Serrations once thought unique to Troodon occur in other members of the family. A tooth can therefore show that a small theropod was present without reliably identifying the genus or species. If no diagnostic feature distinguishes ANSP 9259 from other troodontids, skeletons cannot be attached to its name with confidence.

Studies of Alberta material in 2017 argued that the type tooth did not diagnose Troodon formosus and revived Stenonychosaurus inequalis for the skeletons. Some fossils previously assigned to the same broad animal were separated into other genera; a 2021 revision later returned some of that material to Stenonychosaurus. The important point is that different names can refer to the same general kind of animal while the researchers disagree about which fossils define each name.

The term nomen dubium does not mean that the tooth is fake, that no troodontids lived in Montana, or that every fossil once labelled Troodon is useless. It means the name cannot be applied securely to a distinct species on the evidence of its name-bearing specimen. This distinction prevents a familiar label from making uncertain associations appear settled.

What changed in the 2025 revision

Varricchio, Hogan and Gardner described additional troodontid material from the Two Medicine Formation of Montana and compared it with Canadian fossils. They argued that specimens from the two regions represent one species and that the longstanding concept of Troodon formosus can be retained. Their preferred classification gives the older name priority over Stenonychosaurus inequalis.

The authors proposed MOR 553, a multi-age collection that includes a frontal bone and other informative remains, as a neotype. It offers more diagnostic anatomy than the isolated tooth. But a published proposal is not itself the formal replacement of a surviving name-bearing type. The International Code of Zoological Nomenclature sets a separate process for exceptional neotype designation when the original type still exists. Until that step is resolved, it is accurate to say that the 2025 revision supports the name, not that it automatically changed the type.

Both treatments remain relevant. A page may use Troodon formosus under the 2025 interpretation while explaining that other authors prefer Stenonychosaurus. It should not silently present one scheme as an uncontested fact. This is a question of taxonomy and nomenclature, not evidence that two different animals fought or lived as one population.

What the fossils preserve

The holotype itself is one tooth crown. It preserves shape, enamel and serrations that can be compared with other theropod teeth. It does not preserve a skull, jaw joint, braincase, limb, feather, egg, nest or stomach contents. Every statement about the familiar animal's body depends on the decision to include additional material.

The broader North American troodontid sample includes jaws and teeth, partial skeletons, isolated bones, eggs with embryos and nesting traces. The Two Medicine material includes juvenile and adult remains, and MOR 553 represents several individuals rather than one articulated skeleton. These finds provide substantial information about troodontids, but the name attached to them changes with the taxonomic scheme. A fossil assemblage from a formation is not automatically a single species sample.

Some proposed diagnostic features in the 2025 revision come from the frontal, jaws and other bones rather than the original tooth. Linking those elements to the tooth is the central inference under debate. If that link is accepted, the name describes a much better-known dinosaur. If it is rejected, the tooth remains too limited to identify that complete form.

Size, anatomy and senses

The type tooth cannot be scaled into a body length or mass. When researchers include skeletons historically called Troodon or Stenonychosaurus, reconstructions are roughly 2–2.5 metres long and commonly weigh several tens of kilograms. Those figures describe the referred troodontid material, not a measurement of ANSP 9259.

The expanded reconstruction is a lightly built, two-legged theropod with a long tail, grasping forelimbs, a narrow skull and large eye sockets. The second toe carried an enlarged claw, though it was smaller than the specialised claw of many dromaeosaurids. These proportions come from partial skeletons. They should be labelled as a troodontid reconstruction or as Troodon/Stenonychosaurus under a stated classification.

Large eye sockets and features of the inner ear support well-developed vision and balance in troodontids. A relatively large braincase is not a direct measure of human-like intelligence. Endocasts reveal the size and shape of the cavity, while specific abilities such as planning, problem-solving and social learning are not preserved as simple measurements. Calling the animal the “smartest dinosaur” turns a complex comparison into a ranking the fossils cannot support.

Diet and possible omnivory

The tooth's serrations were once treated as evidence of a strict meat-eater, but tooth shape alone does not settle the diet. Comparisons of teeth and jaws, as well as chemical evidence discussed for troodontids, are consistent with flexible feeding. Small vertebrates and invertebrates could have been eaten alongside seeds, fruit or other plant matter. Omnivory is plausible, not directly demonstrated for the individual that carried ANSP 9259.

Some Two Medicine food remains have been associated with troodontids, but attributing them to Troodon formosus depends on the same disputed classification. A useful account names the formation and the fossil evidence, then states the taxonomic condition. No preserved meal reveals a complete menu or proves that every individual had the same diet.

Eggs, nests and growth

Montana sites preserve elongated eggs, embryos and adult troodontid remains. In some nests, eggs stood at an angle and were partly embedded in sediment. The layout is compatible with an adult sitting over the clutch so that exposed portions could receive warmth. Such evidence helps trace the evolution of nesting and incubation among theropods, including the lineage close to birds.

Whether these are properly called Troodon nests or troodontid nests depends on the taxonomy. Their arrangement does not by itself show how long an adult incubated, whether two parents took turns, or how hatchlings were fed. Embryos document development before hatching; they do not establish prolonged care after it.

Environment and limits of reconstruction

The Montana and Alberta fossils come from Campanian deposits of the Late Cretaceous. River systems, floodplains, wooded areas and seasonally dry ground formed a varied landscape. The Judith River, Two Medicine and Dinosaur Park formations represent different places and intervals; fossils from them should not be combined into one exact habitat scene without checking their age and locality.

The evidence directly establishes a diagnostic tooth from Montana and a wider record of small troodontid dinosaurs. The full feathered body, colours, voice, hunting scenes, precise mass and social behaviour are reconstructions. Feathers are plausible for this bird-related group, but the type tooth preserves none. The dinosaur catalogue presents Troodon as a historically important name and keeps its uncertain status visible.

Common claims that need qualification

Troodon is not securely known from one complete skeleton. The original type is one tooth; the complete-looking animal is assembled from other, debated specimens. Nor was the name simply “cancelled” in 2017: those studies questioned it, while the 2025 revision argued for its continued validity.

The 2025 authors did not instantly turn MOR 553 into the formal neotype. A proposed designation and a completed nomenclatural replacement are different things. The animal's size, nesting and likely feathers describe referred material, not the isolated name-bearing tooth. The responsible reconstruction shows what a troodontid may have looked like and states which taxonomic choice connects it to Troodon formosus.