Troodon, meaning “wounding tooth,” is a genus of bird-like dinosaur from the Late Cretaceous Period (76 to 65 million years ago). Troodon was discovered in 1855 in North America and was one of the first dinosaurs found there. Fossils of this specimen have been found from Alaska to Wyoming, with possible Troodon fossils also being found in Texas and New Mexico.

Troodon is known from the Judith River Formation and the upper Two Medicine Formation of Montana, the Judith River Group of Alberta, the Horseshoe Canyon Formation of Alberta, the Prince Creek Formation of Alaska, and the famous Hell Creek Formation of the USA.

Scientists have found some evidence that Troodon favored cooler climates, as it seems to have been abundant in the northern and arctic regions, and also from cooler time periods, such as the early Maastrichtian. Possible Troodon teeth have been found in the lower Javelina Formation of Texas and the Naashoibito Member of the Kirtland Formation in New Mexico, though this has not been confirmed.

It seems unlikely that the fossils discovered, coming from areas separated by thousands of miles, and by millions of years, are likely to be all from one Troodon species. Experts say more fossils, and further studies, are needed to determine how many species of Troodon existed.

Troodon was a small bird-like dinosaur, about 7.9 feet long and weighing around 110 pounds. It is the largest known troodontid. It had very long, slender hind limbs, suggesting that the animal was able to run quickly. It also had large, retractable sickle-shaped claws on its second toes, which were raised off the ground when running.

It had large eyes — which may have made it nocturnal in nature — that slightly faced forward, giving it some degree of depth perception. It had a light skull that contained a capsule similar to those found in ostrich-like dinosaurs. Troodon had one of the largest known brains of any dinosaur, relative to its body mass.

It’s teeth are different from those found in most other theropods. The teeth bear prominent “wounding” serrations that are similar to those of herbivorous reptiles, suggesting Troodon had a more omnivorous diet.

The Troodon tooth was originally classified as a “lacertilian” (lizard-like) by Joseph Leidy, which would have considered Troodon a lizard as well. But it was re-assigned as a megalosaurid dinosaur in 1901 (Megalosauridae having historically been a general taxon for most carnivorous dinosaurs). The tooth was suggested by Gilmore in 1924 as belonging to the herbivorous pachycephalosaur Stegoceras, and Stegoceras became the first junior synonym of Troodon. Troodon was classified as a pachycephalosaur for a number of years, until 1945, when Charles Mortram Sternberg rejected that theory based on the tooth’s stronger similarity to the teeth of carnivorous dinosaurs. Today, Troodon is classified as a theropod.

The first fossil currently assigned to Troodon that was not a tooth was named Stenonychosaurus in 1932 by Sternberg. The fossil included a foot, fragments of a hand, and some tail vertebrae found in Alberta.

Other fossils have been discovered that are now either considered to be Troodon fossils or junior synonyms of Troodon formosus. These include a complete skeleton of Stenonychosaurus inequalis described by Dale Russell in 1969, and two species: Polyodontosaurus grandis and Pectinodon bakkeri. Now all specimens once called Stenonychosaurus are now referred to as Troodon in scientific literature published by Currie in 1987.

However, the concept that all troodontids from the Judith River Group belong to one species was soon questioned soon after Currie’s paper was published, including by Currie himself. Currie and colleagues in 1990 noted that while they believed the Judith River troodontids were all T. formosus, troodontid fossils from other formations may belong to different species. In 1991, the Lance Formation fossils were assigned to T. bakkeri, and several other researchers (including Currie) have reverted to keeping the Dinosaur Park Formation fossils separate as T. inequalis.

Like other theropods, it is believed that Troodon was a predator. The view is supported by the sickle-shaped claw on its foot and its apparently good binocular vision. The teeth, however, could prove otherwise. One study suggests that Troodon could have been an omnivore. The jaws met in a broad, U-shaped junction similar to that of an iguana, a species of lizard that has adapted to a plant-eating lifestyle. Also, the teeth of Troodon bore large serrations each of which is called a denticle. The teeth show wear facets on their sides. It has also been noted that Troodon has several other features that are characteristic of herbivorous or omnivorous animals — particularly the grasping hands, large brain and stereoscopic vision.

But another study based on many Troodon teeth found in Alaska, suggest they were in fact carnivorous. The teeth of Alaskan troodontids were much larger than those collected from more southern sites, providing the evidence that northern populations grew to larger body sizes. The study suggests that Alaskan Troodons may have had access to large animals as prey because there were no tyrannosaurids in their habitat to provide competition for those resources. And based on wear patterns on Alaskan Troodon teeth, the study proposes that Troodons ate a diet primarily of meat.

Studies performed on fossilized remains of Troodon using growth ring counts suggest that Troodon reached its adult size in about 3 to 5 years.

John R. Horner discovered dinosaur eggs and nests in the Two Medicine Formation of Montana in 1983. Eight of these nests and their eggs have been described in 2002 by other scientists as belonging to Troodon. A partial skeleton of an adult Troodon along with a clutch of at least five eggs were described in 1997, which helped with describing the later eggs and nests.

The Troodon nests were built from sediments, they were dish shaped, about 40 inches in internal diameter, and with a pronounced raised rim encircling the eggs. More complete nests had contained between 16 and 24 eggs. The eggs are shaped like elongated teardrops, with the more tapered ends pointed downwards and imbedded about halfway in the sediment. The eggs are pitched at an angle so that, on average, the upper half is closer to the center of the nest. There is no evidence that plant matter was present in the nest.

Through extracting evidence from the fossilized nests and eggs, scientists were able to infer several characteristics of Troodon reproductive biology. Troodon appears to have a type of reproduction that is intermediate between crocodiles and birds, as phylogeny would predict. The eggs are mostly grouped in pairs, which suggests that the animal had two functional oviducts, like crocodiles, rather than one, as with birds. Crocodiles lay many eggs that are generally small proportional to adult body size. Birds lay fewer, larger eggs. Troodon was intermediate in this aspect, laying an egg about one-tenth its body size, which is ten times larger than reptiles of equivalent mass.

Further examinations of the fossilized nests led scientists to believe Troodons used repetitious laying, where the adult may lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. Some of the fossilized nests showed this evidence.

The fossilized embryos had an advanced degree of skeletal development, implying that they were precocial or perhaps superprecocial. It has been estimated that 45 to 65 total days of adult nest attendance for laying, brooding, and hatching occurred. No evidence was found that the young remained in the nest after hatching and suggested that they dispersed like hatching crocodiles instead.

Further analysis indicates that only the male broods the eggs. This characteristic may have been shared between maniraptoran dinosaurs and primitive birds.