Troodontidae
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 Troodon by Frederik Spindler
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Troodontidae is a family of bird-like theropod dinosaurs which have been allied, at various times, with nearly every major coelurosaurian lineage.
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[edit] Physical characteristics
Troodontids were a group of small- to medium-sized theropods (~1-100 kg) with unusually long legs compared to other theropods, with a disproportionately 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.
Troodontids had unusually large brains among dinosaurs, comparable to those of living flightless birds. Their eyes were also unusually large, and pointed forward, indicating that they had very strong, binocular vision. The ears of troodontids were also unusual among theropods, having extremely enlarged middle ear cavities, indicating acute hearing ability. The placement of this cavity near the eardrum may have aided in the detection of low-frequency sounds.[1] Troodontid ears were also asymmetrical, with one ear placed higher on the skull than the other, a feature shared only with some owls. The extreme specialization of the ears may indicate that troodontids hunted in a manner similar to owls, using their hearing to locate small prey.[2] Although most paleontologists believe that they were predatory carnivores, the many small, coarsely serrated teeth and U-shaped jaws of some species (particularly Troodon) suggest that some species may have been omnivorous or herbivorous.[3] In contrast, a few species, such as Byronosaurus, had large numbers of needle-like teeth, which seem best-suited for picking up small prey, such as birds, lizards and small mammals.
A few troodont fossils, including specimens of Mei and Sinornithoides, demonstrate that these animals slept like birds, with their heads tucked under their arms.[4] These fossils, as well as numerous skeletal similarities to birds and related feathered dinosaurs, support the idea that troodontids probably bore a bird-like feathered coat. The discovery of a fully-feathered, primitive troodontid (Jinfengopteryx) lends support to this.
[edit] Classification
Troodontid fossils were among the first dinosaur remains ever described. Initially, Leidy (1856) assumed they were lacertilian (lizards), but, by 1924, they were referred to Dinosauria by Gilmore, who suggested that they were ornithischians. It wan't until 1945 that C.M. Sternberg recognized Troodontidae as a theropod family. Since 1969, Troodontidae has typically been allied with Dromaeosauridae, in a clade (natural group) known as Deinonychosauria, but this was by no means a consensus. Holtz (in 1994) erected the clade Bullatosauria, uniting Ornithomimosauria (the "ostrich-dinosaurs") and Troodontidae, on the basis of characters including, among others, an inflated braincase (parabasisphenoid) and a long, low opening in the upper jaw (the maxillary fenestra). Features of the pelvis also suggested they were less advanced than dromaeosaurids. New discoveries of primitive troodontids from China (such as Sinovenator and Mei), however, display strong similarities between Troodontidae, Dromaeosauridae and the primitive bird Archaeopteryx, and most paleontologists, including Holtz, now consider troodontids to be much more closely related to birds than they are to ornithomimosaurs, causing the clade Bullatosauria to be abandoned.
[edit] Taxonomy
- Infraorder Deinonychosauria
- Family Troodontidae
[edit] References
- ^ Currie, P. J. (1985). "Cranial anatomy of Stenonychosaurus inequalis (Saurischia, Theropoda) and its bearing on the origin of birds." Canadian Journal of Earth Sciences, 22: 1643-1658.
- ^ Castanhinha, R., and Mateus, O. (2006). "On the left-right asymmetry in dinosaurs." Journal of Vertebrate Paleontology, 26 (Supp. 3): 48A.
- ^ Holtz, T.R., Jr., Brinkman, D.L. and Chandler, C.L. (1998). "Denticle morphometrics and a possibly omnivorous feeding habit for the theropod dinosaur Troodon." Gaia, 15: 159-166.[1]
- ^ Xu and Norell, (2004). "A new troodontid dinosaur from China with avian-like sleeping posture." Nature, 431: 838-841.
