Troodontidae/troʊ.əˈdɒntɪdiː/ is aclade of bird-liketheropoddinosaurs from theLate Jurassic toLate Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with manydinosaurian lineages. More recent fossil discoveries of complete and articulated specimens (including specimens which preservefeathers,eggs,embryos, and complete juveniles), have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, likeSinovenator, demonstrate striking anatomical similarities withArchaeopteryx and primitivedromaeosaurids, and demonstrate that they are relatives comprising aclade calledParaves.
The oldest definitive troodontid known isHesperornithoides from theLate Jurassic ofWyoming.[4] The slightly olderKoparion ofUtah is only represented by a single tooth, and small maniraptoran teeth from theMiddle Jurassic ofEngland were identified as those of indeterminate troodontids in 2023.[5] Over theCretaceous, troodontids radiated throughout westernNorth America,Asia, andEurope, suggesting a mostlyLaurasian distribution for the group. However, in 2013, a single diagnostic tooth from the latest Cretaceous (Maastrichtian)Kallamedu Formation of southernIndia was identified as a troodontid, suggesting that troodontids either also inhabitedGondwana or managed to disperse to India from elsewhere prior to its separation as anisland continent.[6] The potential Gondwanan occurrence of troodontids is supported by the existence of Middle Jurassic remains, which suggest that they originated prior to the breakup ofPangaea. 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.[7]
Troodontids are a group of small, bird-like,gracilemaniraptorans. All troodontids have unique features of the skull, such as large numbers of closely spacedteeth in the lower jaw. Troodontids have sickle-claws andraptorialhands, and some of the highest non-avianencephalization quotients, suggesting that they were behaviourally advanced and had keen senses.[8] 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 thedromaeosaurids. However, the sickle-claws of troodontids were not as large or recurved as in dromaeosaurids, 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 large, and pointed forward, indicating that they had goodbinocular 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.[9] In some troodontids, ears were also asymmetrical, with one ear placed higher on the skull than the other, a feature shared only with someowls. The specialization of the ears may indicate that troodontids hunted in a manner similar to owls, using their hearing to locate small prey.[10]
Although mostpaleontologists believe that they werepredatorycarnivores, the many small, coarsely serrated teeth, large denticle size, and U-shaped jaws of some species (particularlyTroodon) suggest that some species may have beenomnivorous orherbivorous. Some suggest that the large denticle size is reminiscent of the teeth of extantiguanine lizards.[11][12] In contrast, a few species, such asByronosaurus, had large numbers of needle-like teeth, which seem best-suited for picking up small prey, such as birds,lizards and smallmammals. 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.[13] Analyses ofbarium/calcium andstrontium/calcium ratios, which are higher in carnivores due tobioaccumulation, found low ratios in teeth ofStenonychosaurus[verification needed], suggesting that it had a diet ranging from mixed to plant-dominant omnivory.[14] 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 morecursorial lifestyle, though the study indicates that troodontids were still likely to have used theunguals 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.[15]
Troodontid fossils were among the first dinosaur remains described. Initially,Leidy (1856) assumed they werelacertilian (lizards), but, by 1924, they were referred toDinosauria byGilmore, who suggested that they wereornithischians and allied them with thepachycephalosaurianStegoceras in a Troodontidae. It was not until 1945 thatC.M. Sternberg recognized Troodontidae as a theropod family. Since 1969, Troodontidae has typically been allied withDromaeosauridae, in aclade (natural group) known asDeinonychosauria, but this was by no means a consensus.Holtz (in 1994) erected the clade Bullatosauria, unitingOrnithomimosauria (the "ostrich-dinosaurs") and Troodontidae, on the basis of characteristics including, among others, an inflated braincase (parabasisphenoid) and a long, low opening in the upper jaw (themaxillary fenestra). Features of the pelvis also suggested they were less advanced than dromaeosaurids. New discoveries of primitive troodontids fromChina (such asSinovenator andMei), however, display strong similarities between Troodontidae, Dromaeosauridae and the primitive birdArchaeopteryx, and most paleontologists, including Holtz, now consider troodontids to be much more closely related to birds than they are to ornithomimosaurs, causing the cladeBullatosauria to be abandoned.
One study of theropod systematics by members of the Theropod Working Group has uncovered striking similarities among the most basal dromaeosaurids, troodontids, andArchaeopteryx. This clade is together calledParaves by Novas and Pol.[16] The extensive cladistic analysis conducted by Turneret al., (2012) supported the monophyly of Troodontidae.[17]
There are multiple possibilities of the genera included in Troodontidae as well as how they are related. Very primitive species, such asAnchiornis huxleyi, have alternately been found to be early troodontids, early members of the closely related group Avialae, or more primitive paravians by various studies. Thecladogram below follows the results of a study by Lefèvreet al. 2017.[18]
Shenet al. (2017a) explored troodontid phylogeny using a modified version of the Tsuihijiet al. (2014) analysis.[19] It was in turn based on data published by Gaoet al. (2012), a slightly modified version of the Xuet al. (2011) analysis,[20] focusing on advanced troodontids. A simplified version is shown below.[21]
In 2014, Brusatte, Lloyd, Wang and Norell published an analysis onCoelurosauria, based on data from Turneret al. (2012) who named a third subfamily of troodontids, Jinfengopteryginae.[17] 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).[22] An updated version of the Brusatteet al. analysis was provided by Shenet at. (2017b), who included more taxa and recovered greater resolution. Shenet at. named a fourth subfamily of troodontids, the Sinovenatorinae. A simplified version of their analysis is shown below.[23]
Troodontinae is asubfamily oftroodontid dinosaurs. The subfamily was first used in 2017 for the group of troodontids descended from the last common ancestor ofGobivenator mongoliensis andZanabazar junior, but has been redefined to be the least inclusive clade containingSaurornithoides mongoliensis andTroodon formosus, utilizing thetype species of the clade.[24][2][25] Below is acladogram of the Troodontinae as published by Aaron van der Reest andPhil Currie, in 2017.[24]
| Troodontidae | |
Many troodontid nests, including eggs that contain fossilized embryos, have been described. Hypotheses about troodontid reproduction have been developed from this evidence (seeTroodon). A few troodontfossils, including specimens ofMei andSinornithoides, demonstrate that these animals roosted like birds, with their heads tucked under their forelimbs.[26] 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 asJianianhualong, 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 ofoviraptorid eggs in the Djadokhta Formation of Mongolia. The nest is quite certainly that of anoviraptorosaur, since an oviraptorid embryo is still preserved inside one of the eggs. The two partial troodontid skulls were first described by Norell et al. (1994) as dromaeosaurids, but reassigned to the troodontidByronosaurus after further study.[11][27] The troodontids were either hatchlings or embryos, and fragments of eggshell are adhered to them although it seems to be oviraptorid eggshell. The presence of tiny troodontids in an oviraptorid nest is an enigma. Hypotheses explaining how they came to be there include that they were the prey of the adult oviraptorid, that they were there to prey on oviraptorid hatchlings, or that some troodontids may have beennest parasites.[28]
Troodontid feeding was discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method seen also in such theropods as thedromaeosauridae andtyrannosauridae. Studies of wear patterns on the teeth of dromaeosaurids by Angelica Toriceset 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 asDromaeosaurus andSaurornitholestes (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.[29][30] Ba/Ca and Sr/Ca ratios found in the enamel ofOldman Formation troodontids indicate that they were mixed-feeding to plant-dominant omnivores.[31]
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 likeMicroraptor andRahonavis.[32]
Troodontids are important in research into the origin of birds because they share many anatomical characters with early birds. Crucially, the substantially completeHesperornithoides ("Lori") is a troodontid from theLate JurassicMorrison Formation, close to the time ofArchaeopteryx. The discovery of Jurassic troodonts is positive physical evidence that derived deinonychosaurs were present before the time that avians arose. This fact strongly invalidates the "temporal paradox" cited by the few remaining opponents of the idea that birds are closely related to dinosaurs.[33]
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