[4] The slightly older Koparion of Utah is only represented by a single tooth, and small maniraptoran teeth from the Middle Jurassic of England were identified as those of indeterminate troodontids in 2023.

[5] Over the Cretaceous, troodontids radiated throughout western North America, Asia, and Europe, suggesting a mostly Laurasian distribution for the group.

[6] The potential Gondwanan occurrence of troodontids is supported by the existence of Middle Jurassic remains, which suggest that they originated prior to the breakup of Pangaea.

Troodontids have sickle-claws and raptorial hands, and some of the highest non-avian encephalization quotients, suggesting that they were behaviourally advanced and had keen senses.

[8] They had unusually long legs compared to other theropods, with a large, curved claw on their retractable second toes, similar to the "sickle-claw" of the dromaeosaurids.

The specialization of the ears may indicate that troodontids hunted in a manner similar to owls, using their hearing to locate small prey.

Other morphological characteristics of the teeth, such as the detailed form of the denticles and the presence of blood grooves, also seem to indicate carnivory.

The proportions of the metatarsals, tarsals and unguals of troodontids appear indicative of their having nimbler, but weaker feet, perhaps better adapted for capturing and subduing smaller prey.

Initially, Leidy (1856) assumed they were lacertilian (lizards), but, by 1924, they were referred to Dinosauria by Gilmore, who suggested that they were ornithischians and allied them with the pachycephalosaurian Stegoceras in a Troodontidae.

Holtz (in 1994) erected the clade Bullatosauria, uniting Ornithomimosauria (the "ostrich-dinosaurs") and Troodontidae, on the basis of characteristics including, among others, an inflated braincase (parabasisphenoid) and a long, low opening in the upper jaw (the maxillary fenestra).

Sinornithoides Gobivenator Linhevenator Philovenator Troodon Saurornithoides Zanabazar In 2014, Brusatte, Lloyd, Wang and Norell published an analysis on Coelurosauria, based on data from Turner et al. (2012) who named a third subfamily of troodontids, Jinfengopteryginae.

Jinfengopteryx Mei Sinovenator Daliansaurus Sinusonasus Sinornithoides Troodon Zanabazar Saurornithoides Troodontinae is a subfamily of troodontid dinosaurs.

A few troodont fossils, including specimens of Mei and Sinornithoides, demonstrate that these animals roosted like birds, with their heads tucked under their forelimbs.

However, micro wear on the teeth indicated that dromaeosaurids likely preferred larger prey items than the troodontids with which they often shared their environment.

Crucially, the substantially complete Hesperornithoides ("Lori") is a troodontid from the Late Jurassic Morrison Formation, close to the time of Archaeopteryx.

The discovery of Jurassic troodonts is positive physical evidence that derived deinonychosaurs were present before the time that avians arose.

This fact strongly invalidates the "temporal paradox" cited by the few remaining opponents of the idea that birds are closely related to dinosaurs.