Troodontid
| Troodontids Temporal range: Late Jurassic–Late Cretaceous, 160–66 Ma |
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| Mounted skeletal cast of Troodon inequalis, Perot Museum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Clade: | Averaptora |
| Family: |
†Troodontidae Gilmore, 1924 |
| Type species | |
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Troodon formosus Leidy, 1856 |
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| Subgroups | |
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See text |
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| Synonyms | |
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Elopterygidae? Lambrecht, 1933 |
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See text
Elopterygidae? Lambrecht, 1933
Saurornithoididae Barsbold, 1974
Archaeornithoididae? Elzanowski & Wellnhofer, 1992
Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens (including specimens which preserve feathers, eggs and embryos, and complete juveniles), have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.
Troodontids are a group of small, bird-like, maniraptorans. All troodontids have unique features of the skull, such as large numbers of closely spaced teeth in the lower jaw. Troodontids have sickle-claws and raptorial hands, and some of the highest non-avian encephalization quotients, suggesting that they were behaviourally advanced and had keen senses. The largest troodontid was Troodon, and the smallest was Anchiornis, which is also the smallest known non-avian dinosaur. They had unusually long legs compared to other theropods, with a large, curved claw on their retractable second toes, similar to the "sickle-claw" of the dromaeosaurids. However, the sickle-claws of troodontids were not as large or recurved as in their relatives, and in some instances could not be held off the ground and "retracted" to the same degree. In at least one troodontid, Borogovia, the second toe could not be held far off the ground at all and the claw was straight, not curved or sickle-like.
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