Troodon

Troodon is a genus with a complex and often problematic history. Joseph Leidy first named Troodon formosus in 1856 based on a single tooth. Initially believed to be from a lizard, the specimen was not identified as a theropod until 1877. For much of the 20th century, the name served as a "wastebasket" category for various small, intelligent-looking Late Cretaceous theropods found across North America. However, recent studies (van der Reest & Currie 2017; Varricchio et al. 2018) argue that the original tooth lacks enough unique traits to define a genus. Most fossils once called "Troodon" have now been reassigned to Stenonychosaurus or Latenivenatrix. Consequently, the name Troodon is now frequently considered a nomen dubium.

The anatomy of these reclassified fossils remains consistent. They represent a predator about 2.4 m long and weighing around 50 kg. The dinosaur had a long snout and serrated teeth; its name translates to "wounding tooth." It possessed large, forward-facing eyes and a brain that was exceptionally large for a non-avian dinosaur. Its build included long, slender legs, a stiff tail, and the characteristic sickle-shaped claw on each foot. Like its relatives in the clade Deinonychosauria, it was covered in pennaceous feathers.

Fossil evidence, such as the material from Egg Mountain in Montana, reveals that these dinosaurs engaged in bird-like brooding behaviour. One preserved nest shows an adult sitting on a clutch of eggs in a posture nearly identical to that of modern birds (Varricchio et al. 1997). The eggs were laid in pairs and partially buried, representing a transitional nesting style between primitive reptiles and modern avian species.

Its large eyes suggest an adaptation for low-light vision, leading researchers to believe "Troodon" may have been a nocturnal or crepuscular hunter. While direct evidence of its diet is missing, its teeth suggest it was a generalist omnivore. It likely fed on small vertebrates and insects, potentially supplementing this with plant matter.

• The original tooth — collected by Hayden 1855, described by Leidy 1856, retroactively the holotype.
• Egg Mountain brooding adult (MOR 748) — Museum of the Rockies; Varricchio et al. 1997.
• Most museum displays labelled "Troodon" are now technically Stenonychosaurus or Latenivenatrix — Royal Tyrrell, AMNH, ROM.

Genus validity — Strong recent argument (van der Reest & Currie 2017) that the type tooth is undiagnostic and "Troodon formosus" should be considered nomen dubium, with most material reassigned to Stenonychosaurus and Latenivenatrix. Not all workers agree, and "Troodon" remains in popular use. Brooding behaviour — well-supported by Egg Mountain. Nocturnality / sensory ecology — supported by sclerotic ring measurements (Schmitz & Motani 2011) but precise activity pattern debated.

"Troodon" is famous for being the canonical "smart dinosaur" — including the speculative "Dinosauroid" thought experiment by Dale Russell (1982) imagining a hypothetical descended-from-Troodon humanoid. Dinosaur Train and various children's media feature it as the brainy theropod.

• theropoda
• dromaeosauridae
• oviraptor

• van der Reest, A. J., & Currie, P. J. (2017). Troodontids (Theropoda) from the Dinosaur Park Formation, Alberta. Canadian Journal of Earth Sciences, 54, 919–935.
• Varricchio, D. J., Jackson, F., Borkowski, J. J., & Horner, J. R. (1997). Nest and egg clutches of the dinosaur Troodon formosus. Nature, 385, 247–250.
• Russell, D. A., & Séguin, R. (1982). Reconstruction of the small Cretaceous theropod Stenonychosaurus inequalis. Syllogeus, 37, 1–43.
• Schmitz, L., & Motani, R. (2011). Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. Science, 332, 705–708.