Troodon (or Troödon in older sources) is a genus of relatively small, bird-like dinosaur from the Late Cretaceous Period (75-65 mya). Discovered in 1855, it was among the first dinosaurs found in North America.
Its name ("TROE-odon") is Greek for "wounding tooth", referring to the dinosaur's teeth, which bore prominent serrations. These "wounding" serrations, however, have similarities to those of herbivorous reptiles, and suggest an omnivorous diet.
It seems unlikely that all of these fossils, which come from localities hundreds or thousands of miles apart, separated by millions of years of time, represent a single species of Troodon. However, further study and more fossils are needed to determine how many species of Troodon existed.
Varricchio et al. (1997) made this determination with even more certainty when they described a partial skeleton of an adult Troodon (MOR 748) in contact with a clutch of at least five eggs (MOR 750), probably in a brooding position.
Varricchio et al (1997) described the exact structure of Troodon nests. They were built from sediments, they were dish shaped, about 100 cm in internal diameter, and with a pronounced raised rim encircling the eggs. The more complete nests had between 16 (minimum number in MOR 246) and 24 (MOR 963) 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.
Varricchio et al.(1997) were able to extract enough evidence from the nests to infer several characteristics of Troodon reproductive biology. The results are that Troodon appears to have a type of reproduction that is intermediate between crocodiles and birds, as phylogeny would predict. The eggs are statistically grouped in pairs, which suggests that the animal had two functional oviducts, like crocodiles, rather than one, as in birds. Crocodiles lay many eggs that are small proportional to adult body size. Birds lay fewer, larger, eggs. Troodon was intermediate, laying an egg of about 0.5 kg for a 50kg adult. This is 10 times larger than reptiles of the same mass, but two Troodon eggs are roughly equivalent to the 1.1 kg egg predicted for a 50 kg bird. Varricchio et al. also found evidence for iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. MOR 363 was found with 22 empty (hatched) eggs, and the embryos found in the eggs of MOR 246 were in very similar states of development, implying that all of the young hatched simultaneously. The embryos had an advanced degree of skeletal development, implying that they were precocial or even superprecocial. The authors estimated 45 to 65 total days of adult nest attendance for laying, brooding, and hatching. The authors found no evidence that the young remained in the nest after hatching and suggested that, instead, they dispersed like hatchling crocodiles or megapode birds.
The Troodon tooth was originally classified as a "lacertian" (lizard) by Leidy, but re-assigned as a megalosaurid dinosaur by Nopsca in 1901 (Megalosauridae having historically been a wastebin taxon for most carnivorous dinosaurs). In 1924, Gilmore suggested that the tooth belonged to the herbivorous pachycephalosaur Stegoceras, and that Stegoceras was in fact a junior synonym of Troodon (the similarity of troodontid teeth to those of herbivorous dinosaurs continues to lead many paleontologists to believe that these animals were omnivores). In 1945, Charles Mortram Sternberg rejected the possibility that Troodon was a pachycephalosaur due to its stronger similarity to the teeth of other carnivorous dinosaurs.
The first specimen of Troodon that was not a tooth, then referred to its own genus (Stenonychosaurus), was named by Sternberg in 1932, based on a foot, fragments of a hand, and some caudal vertebrae from Alberta. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of Deinonychosauria. Sternberg initially classified Stenonychosaurus as a member of the family Coeluridae. Later, Sternberg (1951) speculated that since Stenonychosaurus had a "very peculiar pes" and Troodon "equally unusual teeth", they may be closely related. Unfortunately, no comparable specimens were available at that time to test the idea.
A more complete skeleton of Stenonychosaurus was described by Dale Russell in 1969, which eventually formed the scientific foundation for a famous life-sized sculpture of Stenonychosaurus accompanied by its fictional, human-like descendant, the "dinosauroid". Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Phil Currie, reviewing the known Troodontidae in 1987, reclassified Stenonychosaurus inequalis as a junior synonym of Troodon formosus. This synonymy has been widely adopted by other paleontologists, and therefore all of the specimens once called Stenonychosaurus are now referred to as Troodon in the recent scientific literature.
In 1982, paleontologist Dale Russell, curator of vertebrate fossils at the National Museum of Canada in Ottawa, speculated on how evolution would have proceeded if the troodonts had survived the extincton of the dinosaurs. Russell speculated that a species like Troodon would have grown smarter and taken on a human-like appearance. Russell partnered with taxidermist and artist Ron Sequin and together they made a model of what a derived, intelligent Troodon would look like, naming their fantasy creation a "Dinosauroid" (Russell & Séguin, 1982). While a few paleontologists, such as David Norman (1985) and Cristiano dal Sasso (2004) have regarded this as a plausible line of reasoning, others, such as Gregory S. Paul (1988) and Thomas R. Holtz Jr., consider it "suspiciously human" (Paul, 1988) and argue that a large-brained, highly intelligent troodontid would retain a more standard theropod body plan. Darren Naish has suggested the ground hornbill as a better model for a hominid-mimicking terrestrial theropod.
In fiction, Troodon feature prominently in the 1994 science-fiction novel End of an Era by Robert J. Sawyer, in which they are the preferred host vehicle for intelligent Martian viral invaders. Also, an intelligent saurian species very like the Troodon, called the saurs, appears in Ken MacLeod's "Engines of Light" series of books.
In the Star Trek novel First Frontier by Diane Carey and James Kirkland, several members of the Enterprise crew travel back in time and find that Troodon has rudimentary language and were well coordinated hunters. They also encounter evolved Troodons, called Clan Ru, from the future that were transplanted before the extinction by the Preservers and were sentient beings capable of warp travel.
The Troodon to "Dinosauroid" 'Thought Experiment', as proposed by Dr. Dale Russell, has also fueled conversation in the UFO community. John Rhodes, the original proponent for the potential for intelligence in "Dinosauroid" like cryptids, called Reptilian-Humanoids or Reptoids, has an entire web site dedicated to this subject.