Troodontidae
Troodontids | |
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Collection of four troodontids, clockwise from top left: Alaskan Troodon, Jianianhualong, Sinovenator .
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Clade: | Paraves |
Family: | †Troodontidae Gilmore, 1924 |
Type species | |
† Troodon formosus
, 1856 | |
Clade | |
Synonyms | |
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Troodontidae .
Evolution
The oldest definitive troodontid known is Hesperornithoides from the Late Jurassic of Wyoming.[1] The slightly older Koparion of Utah is only represented by a single tooth, and small maniraptoran teeth from the Middle Jurassic of England were identified as those of indeterminate troodontids in 2023.[2] Over the Cretaceous, troodontids radiated throughout western North America, Asia, and Europe, suggesting a mostly Laurasian distribution for the group. However, in 2013, a single diagnostic tooth from the latest Cretaceous (Maastrichtian) Kallamedu Formation of southern India was identified as a troodontid, suggesting that troodontids either also inhabited Gondwana or managed to disperse to India from elsewhere prior to its separation as an island continent.[3] The potential Gondwanan occurrence of troodontids is supported by the existence of Middle Jurassic remains, which suggest that they originated prior to the breakup of Pangaea. However, due to the lack of other remains from the region, it has been suggested that the existence of Gondwanan troodontids should be regarded as provisional.[4]
Description
Troodontids are a group of small, bird-like,
Troodontids had unusually large brains among dinosaurs, comparable to those of living flightless birds. Their eyes were also large, and pointed forward, indicating that they had good binocular vision. The ears of troodontids were also unusual among theropods, having 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.[6] In some troodontids, ears were also asymmetrical, with one ear placed higher on the skull than the other, a feature shared only with some owls. The specialization of the ears may indicate that troodontids hunted in a manner similar to owls, using their hearing to locate small prey.[7]
Diet
Although most paleontologists believe that they were predatory carnivores, the many small, coarsely serrated teeth, large denticle size, and U-shaped jaws of some species (particularly Troodon) suggest that some species may have been omnivorous or herbivorous. Some suggest that the large denticle size is reminiscent of the teeth of extant iguanine lizards.[8][9] 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. Other morphological characteristics of the teeth, such as the detailed form of the denticles and the presence of blood grooves, also seem to indicate carnivory.[10] Analyses of barium/calcium and strontium/calcium ratios, which are higher in carnivores due to bioaccumulation, found low ratios in teeth of Stenonychosaurus, suggesting that it had a diet ranging from mixed to plant-dominant omnivory.[11] Though little is known directly about the predatory behavior of troodontids, Fowler and colleagues theorize that the longer legs and smaller sickle claws (as compared to dromaeosaurids) indicate a more cursorial lifestyle, though the study indicates that troodontids were still likely to have used the unguals for prey manipulation. The proportions of the metatarsals, tarsals and unguals of troodontids appear indicative of their having nimbler, but weaker feet, perhaps better adapted for capturing and subduing smaller prey. This suggests an ecological separation from the slower but more powerful Dromaeosauridae.[12]
Classification
Troodontid fossils were among the first dinosaur remains described. Initially,
One study of theropod systematics by members of the Theropod Working Group has uncovered striking similarities among the most basal dromaeosaurids, troodontids, and Archaeopteryx. This clade is together called Paraves by Novas and Pol.[13] The extensive cladistic analysis conducted by Turner et al., (2012) supported the monophyly of Troodontidae.[14]
Taxonomy
- Family Troodontidae
- †Albertavenator
- †Almas
- †Geminiraptor
- †Jianianhualong
- †Liaoningvenator
- †Sinornithoides
- †Talos
- †Tochisaurus
- †Xixiasaurus
- Subfamily Jinfengopteryginae
- †Jinfengopteryx
- IGM 100/1128
- Subfamily Sinovenatorinae
- †Daliansaurus
- †Mei
- †Sinovenator
- †Sinusonasus
- Subfamily Troodontinae
Phylogeny
There are multiple possibilities of the genera included in Troodontidae as well as how they are related. Very primitive species, such as Anchiornis huxleyi, have alternately been found to be early troodontids, early members of the closely related group Avialae, or more primitive paravians by various studies. The cladogram below follows the results of a study by Lefèvre et al. 2017.[15]
Eumaniraptora
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Shen et al. (2017a) explored troodontid phylogeny using a modified version of the Tsuihiji et al. (2014) analysis.[16] It was in turn based on data published by Gao et al. (2012), a slightly modified version of the Xu et al. (2011) analysis,[17] focusing on advanced troodontids. A simplified version is shown below.[18]
Deinonychosauria |
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In 2014, Brusatte, Lloyd, Wang and Norell published an analysis on Coelurosauria, based on data from Turner et al. (2012) who named a third subfamily of troodontids, Jinfengopteryginae.[14] Their analysis included more basal troodontid species but failed to resolve many of their interrelationships, resulting in large "polytomies" (sets of species where the branching order in the family tree is uncertain).[19] An updated version of the Brusatte et al. analysis was provided by Shen et at. (2017b), who included more taxa and recovered greater resolution. Shen et at. named a fourth subfamily of troodontids, the Sinovenatorinae. A simplified version of their analysis is shown below.[20]
Deinonychosauria |
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Paleobiology
Many troodontid nests, including eggs that contain fossilized embryos, have been described. Hypotheses about troodontid reproduction have been developed from this evidence (see Troodon). A few troodont fossils, including specimens of Mei and Sinornithoides, demonstrate that these animals roosted like birds, with their heads tucked under their forelimbs.[21] 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 fully feathered, primitive troodontids, such as Jianianhualong, lend support to this.
In 2004, Mark Norell and colleagues described two partial troodontid skulls (specimen numbers IGM 100/972 and IGM 100/974) found in a nest of
Feeding
Troodontid feeding was discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method seen also in such theropods as the dromaeosauridae and tyrannosauridae. Studies of wear patterns on the teeth of dromaeosaurids by Angelica Torices et al., indicate that dromaeosaurid teeth share similar wear patterns to those seen in the aforementioned groups. However, micro wear on the teeth indicated that dromaeosaurids likely preferred larger prey items than the troodontids with which they often shared their environment. Such differences in dietary preferences likely allowed them to inhabit the same ecosystems. The same study also indicated that dromaeosaurids such as Dromaeosaurus and Saurornitholestes (two dromaeosaurids analyzed in the study) likely included bone in their diet and were better adapted to handle struggling prey while troodontids, equipped with weaker jaws, preyed on softer-bodied animals and prey items such as invertebrates and carrion that either was immobile or could likely be swallowed whole.[24][25]
Flight
Compared to most other paravians, troodontids are unspecialised for aerial locomotion. However, Jinfengopteryx ranks closely with non-avian theropods known to engage in powered flight like Microraptor and Rahonavis.[26]
Bird evolution
Troodontids are important in research into the origin of birds because they share many anatomical characters with early birds. Crucially, the substantially complete
See also
References
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- ^ Holtz, T.R. Jr.; Brinkman, D.L.; Chandler, C.L. (1998). "Denticle morphometrics and a possibly omnivorous feeding habit for the theropod dinosaur Troodon" (PDF). Gaia. 15: 159–166.
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- ^ Guarino, Ben (27 April 2018). "Scratches on dinosaur teeth reveal their fierce, efficient eating habits - The Washington Post". The Washington Post.
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