The troodontids are best characterized as small, rather delicate theropods with relatively the largest brains of any classic dinosaur, long and slender legs, and unusual teeth that have been mistaken for those of the ornithischian group Pachycephalosauria. These teeth have led some researchers to suggest that they were omnivorous. Like ornithomimosauroids, they had large eyes. Troodontidae has proven hard to classify, and can be linked by various characteristics with Spinosauridae, Dromaeosauridae, Oviraptorosauria, and Ornithomimosauria. In the recent past (1990s), they were combined with the ornithomimosaurians into a taxon called Bullatosauria, meaning "inflated reptiles" (referring to a "swollen" bone in the braincase). Part of the difficulty is that they are still rather poorly known; only one really complete skeleton, that attributed to Sinornithoides, has been uncovered. In addition, until the 1990s there had been no good basal troodontid remains known, hindering comparisons. Generally, troodontids tend to plot out as either the sister group of Dromaeosauridae (forming Deinonychosauria), or somewhere in the neighborhood.
    For a long time, this family was known as Saurornithoididae, because Troodon was thought to be a dubious pachycephalosaurid. It may return to this name if recent investigations into Troodon show that the type teeth are not "troodontid".

                          |     `--Philovenator

Troodontidae: So far, all valid, well-established members of Troodontidae are from the Laurasian continents Asia and North America (which isn't to say that Europe hasn't been trying). There is a good skeleton with a nice skull of a troodontid from the Morrison, but let's see the thing published before we go too nuts. 

Taxon or Taxa: Time/Place: Comments:
Sinovenator changiae Xu, Norell, Wang, Makovicky, and Wu, 2002 early Aptian (EK) of China A very new taxon, the latest in a growing assortment of small Yixian theropods (although slightly older and found at a level not conducive to preservation of soft remains like "feathers"), Sinovenator lacks many derived characteristics of later troodontids, like an inflated bony "capsule" in the braincase and arctomet metatarsals. It is based on a partial skull and skeleton, with another referred skeleton; restorations show it to be a gracile, long-legged animal, much like Sinornithoides, the only other troodontid known from a comparable amount of material. Interestingly, the hips are opisthopubic, like a dromaeosaurid's. 
Mei long Xu and Norell, 2004 early Aptian (EK) of China Based on a spectacular, beautifully preserved, essentially complete, and fully articulated skeleton of a half-meter long subadult troodontid, Mei (new holder of shortest dinosaur name, beating Minmi and fellow maniraptoran Khaan) appears to have died in a bird-like sleeping posture; its name means "soundly-sleeping dragon". The tail is curled around the crouched body, the neck flexed back along the left side, the skull tucked back posterior to the long bones of the left arm in a pose very much like that assumed by sleeping birds. Various skeletal characteristics suggest a basal position relative to other troodontids. It comes from low in the Yixian, in the Lujiatun beds; these layers do not preserve integumentary features as consistently as the upper beds, but they do preserve 3D specimens, unlike the flattened fuzzy wonders above. This specimen does not preserve integument.
Like Dilong, this specimen is worth seeing in color; if you're interested, the full reference is: Xu X., and M. A. Norell. 2004. A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431:838-841.
Talos sampsoni Zanno, Varricchio, O'Connor, Titus, and Knell, 2011 early late Campanian (LK) of Utah Talos, from the Kaiparowits Formation, is known primarily from parts of the legs and pelvis, with a few verts, chevrons, and a ulna also present. Size-wise, it appears to fit into a typical troodontid slot of a couple of meters long. The foot was on the gracile side compared to Troodon and other derived troodontids. One of the "killer claws" of the type individual features a pathogenic phalanx that was significantly modified during life, providing additional evidence that these toes weren't used for walking. The cause of the injury is of course unknown, and could be anything from prey capture to the old classic of accidentally kicking a rock in the dark.
Byronosaurus jaffei Norell, Mackinovicky, and Clark, 2000 late Campanian (LK) of Mongolia This is a new troodontid based on the remains of two individuals. Included among the remains are two good skulls. Its teeth seem to lack serrations.
Sinornithoides youngi Russell and Dong, 1993 Aptian-Albian (EK) of China Based on a mostly complete skeleton and skull, Sinornithoides revealed for the first time the arm, back, and neck structure of the troodontids.
Linhevenator tani Xu X., Tan Q., Sullivan, Han F., and Xiao D., 2011 Campanian (LK) of China Linhevenator is known from a single partial semi-articulated specimen that is sort of a knot of troodontid verts and limb, girdle, and skull bones. It is notable for its short stout humerus, and like other derived troodontids had a large second foot claw.
Philovenator curriei Xu X., Zhao J., Sullivan, Tan Q.-W., Sander, and Ma Q.-Y., 2012 Campanian (LK) of China Philovenator is known only from a left leg, originally interpreted as that of a juvenile Saurornithoides. The specimen has since been reinterpreted as that of an older individual of a distinct, small-bodied species. Philovenator is a derived troodontid. Its name can be taken two ways, as a reference to Phil Currie (also honored in the species name), and as a play on the Greek "philein", which become "philo" in this context for a lover of hunting.
Gobivenator mongoliensis Tsuihiji, Barsbold, Watabe, Tsogtbaatar, Chinzorig, Fujiyama, and Suzuki, 2014 late Campanian (LK) of Mongolia Gobivenator is based on an excellent articulated skeleton missing parts of the neck and feet, most of the gastralia, and the hands. The skull is quite well-preserved, with the palate a particular point of interest.
Saurornithoides mongoliensis Osborn, 1924 late Campanian (LK) of Mongolia Closely related to Troodon, Saurornithoides is a rare component of LK Mongolian dinosaur assemblages. It was a contemporary of Velociraptor mongoliensis, a smaller but better armed theropod. Second species S. junior has been reassigned as Zanabazar.
Troodon: Leidy, 1856 (including Stenonychosaurus Sternberg, 1932) T. formosus (type) Leidy, 1856 middle-late Campanian (LK) of Montana The ultimate taxonomic survivor, Troodon started scientific life as a lizard species based on distinctive teeth, then was reclassified as a pachycephalosaurian, first as a synonym of Stegoceras, then as a dubious tooth taxon. For a short period of time ending in the early 1980s, it was thought to be a carnivorous hypsilophodont (ironically, eggs once attributed to the hypsilophodont Orodromeus were recently thought to belong to Troodon, but new nests show they could not belong to it, but instead pertain to a currently-unknown theropod. On the other hand, embryos have been assigned to this genus, which suggest that the juveniles had unusually long legs, even for troodontids). Later, a partial jaw was found containing Troodon teeth, and it was found to be the same (or maybe not!-stay tuned for future developments) animal as a "saurornithoidid" named Stenonychosaurus inequalis Sternberg, 1932. It is now considered one of the most brainy of all known classic dinosaurs, but is still incompletely known.
In the 1980s, under its alter-ego Stenonychosaurus, this dinosaur was the subject of an imaginative study on what dinosaurs might have evolved into had they not gone extinct, resulting in the human-like "dinosauroid".
The long temporal span of remains attributed to this genus (covering maybe 15 million years) strongly suggests that more than one closely-related species is represented by the available remains. One suggestion has been to use T. inequalis for the Dinosaur Park material.
T. inequalis (Sternberg, 1932 [originally Stenonychosaurus] middle-late Campanian (LK) of Alberta
Zanabazar junior Norell, Makovicky, Bever, Balanoff, Clark, Barsbold, and Rowe, 2009 (originally Saurornithoides junior Barsbold, 1974) early Maastrichtian (LK) of Mongolia Zanabazar started out as a species of Saurornithoides (above; if they keep ending up as sister taxa, though...). Like S. mongoliensis, it is based on an incomplete skull and some postcranial pieces, mostly from the hind legs. It is the largest known troodontid.

Troodontidae i.s.:

Taxon or Taxa: Time/Place: Comments:
?Archaeornithoides deinosauriscus Elzanowski and Wellnhofer, 1992 late Campanian (LK) of Mongolia Based on an enigmatic partial juvenile skull with unserrated teeth (very juvenile, maybe even a hatchling) described as having both troodontid and spinosaurid characteristics, and thought of early on as a possible aublysodont, Archaeornithoides is in need of more material.
Borogovia gracilicrus Osmolska, 1987 (?Saurornithoides) early Maastrichtian (LK) of Mongolia Named after Lewis Carroll's fictional "borogoves," Borogovia is at the end of a trend of second pedal claw size reduction in troodontids. It could be the same as Saurornithoides junior.
?Elopteryx nopcsai (N.D.) Andrews, 1913 early-mid Maastrichtian (LK) of Romania What may be more accurately called the Elopteryx complex of remains has long been considered an early bird. Recent restudy has suggested it could be a troodontid, but not a dromaeosaurid, as had also been suggested.
Geminiraptor suarezarum Senter, Kirkland, Bird, and Bartlett, 2010 Barremian (EK) of Utah Known from a partial maxilla found slightly higher (stratigraphically speaking) than the Crystal Geyser Falcarius quarry, Geminiraptor appears to be more derived than Anchiornis and Sinovenator. Also unlike the two more basal genera, Geminiraptor was similar in size to the larger LK troodont models.
?Koparion douglassi Chure, 1994 late Kimmeridgian (LJ) of Utah The earliest known named possible troodontid, Koparion is a rarity for theropods: a (potentially) viable tooth taxon. It is based on a tiny, unique tooth discovered through sieving.
"Laelaps" cristatus (N.D.) Cope, 1876 late middle Campanian (LK) of Montana Indeterminate troodontid teeth, with the only point of interest being the typo in The Dinosauria's provenance section that assigns it to the Lance Formation of Wyoming rather than the Judith River Formation of Montana.
Pectinodon bakkeri Carpenter, 1982 (?Troodon) late Maastrichtian (LK) of Wyoming and Montana Pectinodon is a tooth taxon here tentatively retained as a hedge against a potential dissolution of Troodon. It represents an end-K North American form.
"Saurornitholestes" robustus (N.D.) Sullivan, 2006 middle late Campanian (LK) of New Mexico "Saurornitholestes" robustus was originally described as a dromaeosaurid; coincidentally enough, Saurornitholestes itself was first described as a "saurornithoidid" (troodontid). It is based on a frontal, with teeth and a pedal ungual referred to it. 
Sinusonasus magnodens Xu and Wan, 2004 early Aptian (EK) of China This is a new troodontid from the Yixian, based on a partial skeleton apparently missing presacrals, the shoulder girdle, and forelimbs (the wording is a bit odd). Interesting diagnostic features include relatively large teeth, a sinusoidal nasal profile (doubtless the source of the generic name), and "plate-like chevrons forming a band along most of the length of the tail."
Tochisaurus nemegtensis Kurzanov and Osmolska, 1991 early Maastrichtian (LK) of Mongolia Tochisaurus is based on a metatarsus from the Nemegt Formation with a reduced second metatarsal, suggesting its "killer claw" was not particularly large.
"Troodon" asiamericanus (N.D.) Nesov, 1995 early Cenomanian (LK) of Uzbekistan This is an indeterminate troodontid based on teeth.
Urbacodon itemirensis Averianov and Sues, 2007 Cenomanian (LK) of Uzbekistan This troodontid is based on a partial dentary with teeth (unserrated). It had less teeth and a straighter lower jaw in dorsal/ventral view than more derived troodontids.
Xixiasaurus henanensis Lü J., Xu L., Liu Y., Zhang X., Jia S., and Ji Q., 2010 Coniacian-Campanian (LK) of China Xixiasaurus is known from a partial skull and lower jaw, a partial hand, and chunks of the forearm. The teeth are unserrated. Its lower jaw is similar in some respects to that of Urbacodon, which also has unserrated teeth, as does Byronosaurus, so perhaps they are related.
Yaverlandia bitholus Galton, 1971 Barremian (EK) of England An unusual Wealden dinosaur, Yaverlandia is based on a skull roof originally interpreted as belonging to an early, generalized pachycephalosaurid. However, upon further review, it is instead a troodontid, which brings things full-circle from the days of Troodon as a pachycephalosaurid..


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