Troodon formosus is one of the first dinosaurs from North America ever named. Leidy (1856) described it for a single tooth with denticles on both the anterior and posterior carina. As of now, Troodon material includes the holotype tooth, the foot and caudal vertebra of the Stenonychosaurus holotype, the dentary of Polyodontosaurus, and multiple specimens referred to Stenonychosaurus, Troodon and Pectinodon. What is really interesting is how none of the holotypes of the above genera have overlapping material, and yet, a paleontological majority considers then synonyms. Not only that, but those genera also span some 10 million years and a large chunk of North America.
The one specimen that seems key to this assignment is CMN (or NMC) 12340, which includes the postorbital skull, multiple vertebral bones, and most of the hindlimb. As far as I know, this specimen is the basis on which nearly all specimens can be assigned to Troodon, being the only one including cranial and post cranial material. But there is a problem, CMN 12340 should be kept separate from Stenonychosaurus.
|This is NMC 8539, the holotype of Stenonychosaurus from Sternberg (1932)|
Now, as you can see below, in CMN 12340, the third metatarsal is longer than the fourth, but in the above picture, looking closely, it is reversed in NMC 8539.
|CMN 12340, the linking specimen between Troodon and Stenonychosaurus. From Russell (1969)|
|From right to left: Polyodontosaurus, ROM 1445, PMA A P67.14.39 from Currie (1969), Russell (1948) and Sues (1977) respectively|
- Leidy, J. (1856). “Notice of remains of extinct reptiles and fishes, discovered by Dr. F. V. Hayden in the Bad Lands of the Judith River, Nebraska Territories”. Proceedings of the Academy of Natural Science Philadelphia 8: 72–73.
- Sternberg, C.M. (1932). Two new theropod dinosaurs from the Belly River Formation of Alberta. Canadian Field-Naturalist 46(5):99-105.
- Russell, D.A. (1969). A new specimen of Stenonychosaurus from the Oldman Formation (Cretaceous) of Alberta. Canadian Journal of Earth Sciences, 1969, 6(4): 595-612, 10.1139/e69-059
- Russell, L.R. (1948). The Dentary of Troödon, a Genus of Theropod Dinosaurs. Journal of Paleontology. 22(5): pp. 625-629.
- Sues, H.D. (1977). Dentaries of small theropods from the Judith River Formation (Campanian) of Alberta, Canada. Canadian Journal of Earth Sciences, 1977, 14(4): 587-592, 10.1139/e77-061.
- C. W. Gilmore. 1932. A new fossil lizard from the Belly River Formation of Alberta. Transactions of the Royal Society of Canada, section 4, series 3. 16:117-119.