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Troodontidae was a clade of bird-like theropod dinosaur which have been allied, at various times, with nearly every major coelurosaurian lineage.
Physical characteristics
They were a group of small-to medium sized theropods (~1-100 kg) with exceptionally long legs, forward-pointing eyes, and a small retractile claw on the second toe. Troodontids had unusually large brains among dinosaurs, comparable to those of living flightless birds. Although most paleontologists believe that they were predatory, the many small, coarsely serrated teeth and U-shaped jaws of some species (particularly Troodon) suggest that some species may have been omnivorous or herbivorous. In contrast, Byronosaurus had large numbers of needlelike teeth which seem best suited for picking up small prey such as birds, lizards and mammals.
A recently discovered troodont fossil, Mei long, demonstrates that the animals slept like birds, with their heads tucked under their arms. *
Classification
Troodontid fossils were among the first dinosaur remains ever described. Initially, Leidy (1856) assumed they were lacertilian, but by 1924, they were referred to Dinosauria by Gilmore, who stressed ostensible ornithischian affinities. It was only in 1945 that C.M. Sternberg recognized Troodontidae as a theropod family. Since 1969, Troodontidae has typically been allied with Dromaeosauridae in a clade known as Deinonychosauria, but this was by no means a consensus. Holtz (1994) erected Bullatosauria, uniting Ornithomimosauria and Troodontidae into a holophyletic clade on the basis of characters including, among others, an inflated parabasisphenoid and a long, low maxillary fenestra. The propubic pelvis also suggested they were less derived than dromaeosaurids. New discoveries of primitive troodontids from China, however (Sinovenator, Mei) display strong similarities between Troodontidae, Dromaeosauridae, and the primitive bird Archaeopteryx.
Taxonomy
"Troodontidae"