Period: Late Cretaceous
Order, Suborder, Family: Saurischia, Theropoda, Troodontidae
Location: North America (Canada, United States)
Length: 8 feet (2.4 meters)
Troodon was described in 1856 by Joseph Leidy on the basis of a single small tooth. It was one of the first North American dinosaurs described. However, it was first thought to be the tooth of a lizard. It was not until 1901 that it was known to be a dinosaur tooth.
In the early days of paleontology it was common to name a dinosaur only on the basis of teeth. But often these names were put aside because scientists could not tell what dinosaur the tooth belonged to even with a complete skull with teeth for comparison. Despite this, the name Troodon was kept because its tooth was so unusual. But for many years some scientists thought the tooth of Troodon was from a pachycephalosaur.
Many scientists believed the original Troodon tooth belonged to a theropod. Troodon was placed in the family Troodontidae in 1948. Then, in 1983, Jack Horner found the lower jaw of a small theropod with the same type of teeth. This jaw proved that Troodon was a theropod. It was also found that all the teeth of Troodon were not the same; the shape of the tooth depended on where the tooth was in the jaw. This jaw also showed that the animal called Stenonychosaurus was the same as Troodon. A close relative of Troodon was Saurornithoides.
Troodon is now known from several partial skulls and skeletons. These skulls show that it had a large brain for its size, and it probably had the most developed brain of any dinosaur. Its eyes were large, taking up a big part of the skull. Each side of the lower jaw had 35 teeth-more than any other theropod.
Its hands had slender fingers, and the inner finger ended in a large, thin, sharply pointed claw. The foot was similar to the foot of dromaeosaurids, with a large claw on the second toe. The claw was used to slash its prey. However, the claw was higher on the foot and smaller than the claw of a dromaeosaur. The claw on the first finger of the hand of Troodon was much larger than the largest claw on its foot. Also, like the dromaeosaurids, the back part of the tail was stiff. The dromaeosaurs may have been the ancestors of the troodontids.