Among the rarest of dinosaurian fossils are those of the smaller flesh eaters. One of these, Troodon formosus, is particularly interesting.

Its eyes were enormous—larger, even, than those of most modern land animals. Also, its brain was proportionally much larger than in living reptiles, and approached that of some living birds and mammals in relative size. This embodies a widespread trend for the brain to increase in size throughout the history of life.

This dinosaur was less than 3 m (10 ft.) long, and weighed only about 45 kg (99 lb.). It was a biped, and could rotate its lower arm to grasp objects with a three-fingered hand.

Troodon was first described in 1856, based upon a single tooth. It was originally thought to have been a lizard. Later discoveries of more complete specimens revealed it to be a dinosaur. Troodon formosus had distinctive curved and serrated teeth, and it is for these that the species is named. The genus name Troodon, which is derived from Greek words, means "wounding tooth". Formosus is Latin for "beautiful". Another name used in the past for Troodon was Stenonychosaurus.

Evidence from fossilized Troodon nests found in Montana, U.S.A., suggests that Troodon parents may have brooded the eggs or hatchlings. Other discoveries from bonebeds hint that Troodon may have lived in small social groups.

Troodon remains have been found in many Late Cretaceous-age localities of North America, including the 76 million-year-old sediments of Dinosaur Provincial Park in Alberta. Dinosaurs related to Troodon are also known from north-eastern Asia.