Troodon is a genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Cretaceous period (about 77 mya), though possible additional species are known from later in the Campanian and also from the early (and probably late) Maastrichtian age. It includes at least one species, Troodon formosus, though many fossils, possibly representing several species have been classified in this genus. These species ranged widely, with fossil remains recovered from as far north as Alaska and as far south as Wyoming and even possibly Texas and New Mexico. Discovered in 1855, T. formosus was among the first dinosaurs found in North America, although it was thought to be a lizard until 1877.
The genus name is Greek for “wounding tooth”, referring to the teeth, which were different from those of most other theropods known at the time of their discovery. The teeth bear prominent, apically oriented serrations. These “wounding” serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet. A partial Troodon skeleton has been discovered with preserved puncture marks.
Troodon were small dinosaurs, up to 0.9 metres (3.0 ft) in height, 2.4 metres (7.9 ft) in length, and up to 50 kilograms (110 lb) in mass. The largest specimens are comparable in size to Deinonychus and Unenlagia. They had very long, slender hind limbs, suggesting that these animals were able to run quickly. They had large, retractable sickle-shaped claws on the second toes, which were raised off the ground when running.
Their eyes were very large (perhaps suggesting a partially nocturnal lifestyle), and slightly forward facing, giving Troodon some degree of depth perception.
Troodon is famous for the great size of its brain cavity. Troodon and other troodontids may have had the largest brains, relative to body size, of any nonavian theropods. This has led many scientists to conclude that Troodon and its close relatives were more intelligent than other dinosaurs. Such a belief is based on the observation that in living animals there is rough correlation between relative brain size and degrees of intelligence. However, as intelligence leaves no trace in the fossil record, this must remain a matter of speculation. Troodon’s very large eye sockets have also led some researchers to believe that it may have been a nocturnal animal.
Troodon is considered to be one of the most derived members of its family. Along with Zanabazar, Saurornithoides and Talos it forms a clade of specialized troodontids.