Troodontidae /troʊ.əˈdɒntɪdiː/ is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete 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, 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.
|Mounted skeletal cast of an unnamed Alaskan troodontid, Perot Museum|
Saurornithoididae Barsbold, 1974
Troodontids are a group of small, bird-like, gracile 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. 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 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 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. 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.
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. 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. 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.
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. 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 oviraptorid eggs in the Djadokhta Formation of Mongolia. The nest is quite certainly that of an oviraptorosaur, 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 troodontid Byronosaurus after further study. 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 been nest parasites.
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.
Troodontids are important in research into the origin of birds because they share many anatomical characters with early birds. Crucially, the substantially complete fossil identified as WDC DML 001 ("Lori") is a possible troodontid from the Late Jurassic Morrison Formation, close to the time of Archaeopteryx. 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.
Troodontid fossils were among the first dinosaur remains 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 and allied them with the pachycephalosaurian Stegoceras in a Troodontidae. It was not 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 characteristics 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.
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. The cladogram published in Hwang et al. found that Archaeopteryx represents a more basal branch of Paraves, and places dromaeosaurids and troodontids as more derived. This raises the possibility that aerodynamic behaviors could be ancestral to all of Deinonychosauria. The extensive cladistic analysis conducted by Turner et al. (2012) supported the monophyly of Troodontidae.
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.
Shen et al. (2017a) explored troodontid phylogeny using a modified version of the Tsuihiji et al. (2014) analysis. It was in turn based on data published by Gao et al. (2012), a slightly modified version of the Xu et al. (2011) analysis, focusing on advanced troodontids. A simplified version is shown below.
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. 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). 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.
Borogovia is a troodontid theropod dinosaur genus which lived during the Late Cretaceous Period, in what is now Mongolia.
In 1971, a Polish-Mongolian expedition discovered the remains of a small theropod at the Altan Ula IV site, in the Nemegt Basin of Ömnögovĭ province. In 1982, the find was reported by Halszka Osmólska and considered by her to be a possible specimen of Saurornithoides. Later she concluded that it represented a species new to science.
In 1987, Osmólska named and described the type species Borogovia gracilicrus. The generic name is derived from the fantasy creatures known as 'borogoves' in the Lewis Carroll poem "Jabberwocky", in his Alice's Adventures in Wonderland. The specific name is a combination of Latin gracilis, "lightly built", and crus, "shin", in reference to the elegant build of the lower leg.The holotype specimen, ZPAL MgD-I/174, was found in the Nemegt Formation, dating from the early Maastrichtian. It consists of two lower legs of a single individual, including fragments of both tibiotarsi, the undersides of both metatarsi and the second, third and fourth toes of each foot.The tibiotarsi have an estimated length of twenty-eight centimetres. Borogovia is about two meters (6 feet) long, weighing some twenty kilograms (forty-five pounds). The tibiotarsus is very elongated. The third toe is narrow. The second phalanx of the second toe is short. The claw of the second toe is short and relatively flat. Osmólska claimed that the second toe could not be hyperextended, and suggested that it had regained a weight-bearing function, compensating for the weakness of the third toe.Borogovia was assigned by Osmólska to the Troodontidae in 1987.Byronosaurus
Byronosaurus is a genus of troodontid dinosaur from the Late Cretaceous Period of Mongolia.Gobivenator
Gobivenator is an extinct genus of troodontid theropod dinosaur known from the late Campanian Djadokhta Formation of central Gobi Desert, Mongolia. It contains a single species, Gobivenator mongoliensis. G. mongoliensis is known from a single individual, which represents the most complete specimen of a Late Cretaceous troodontid currently known.Koparion
Koparion is a genus of small coelurosaurian theropod dinosaurs (probably troodontids), from the late Jurassic Period (Kimmeridgian stage), of Utah. It contains the single named species Koparion douglassi which is known only from a single isolated tooth.Liaoningvenator
Liaoningvenator (meaning "Liaoning hunter") is a genus of troodontid theropod dinosaur from the Early Cretaceous of China. It contains a single species, L. curriei, named after paleontologist Phillip J. Currie in 2017 by Shen Cai-Zhi and colleagues from an articulated, nearly complete skeleton, one of the most complete troodontid specimens known. Shen and colleagues found indicative traits that placed Liaoningvenator within the Troodontidae. These traits included its numerous, small, and closely packed teeth, as well as the vertebrae towards the end of its tail having shallow grooves in place of neural spines on their top surfaces.
Within the Troodontidae, the closest relative of Liaoningvenator was Eosinopteryx, and it was also closely related to Anchiornis and Xiaotingia; while these have traditionally been placed outside the Troodontidae, the phylogenetic analysis conducted by Shen and colleagues offered evidence supporting the alternative identification of these paravians as troodontids. Compared to its close relatives, however, Liaoningvenator had relatively long legs, in particular the femora. As the fifth troodontid known from geographically and temporally comparable strata, Liaoningvenator increases the diversity of Chinese troodontids.Linhevenator
Linhevenator is a genus of short-armed troodontid theropod dinosaur from the Late Cretaceous (Campanian) Bayan Mandahu Formation of Bayan Mandahu, Inner Mongolia, China.Maniraptoriformes
Maniraptoriformes is a clade of dinosaurs with pennaceous feathers and wings that contains ornithomimosaurs and maniraptors. This group was named by Thomas Holtz, who defined it as "the most recent common ancestor of Ornithomimus and birds, and all descendants of that common ancestor."Philovenator
Philovenator (literally meaning "love hunter") is an extinct genus of troodontid paravian dinosaurs from the Wulansuhai Formation (dated to the Campanian age, sometime between 75 and 71 million years ago) of Inner Mongolia, China. Its specific name honors Phillip J. Currie.Saurornithoides
Saurornithoides ( saw-ROR-ni-THOY-deez) is a genus of troodontid maniraptoran dinosaur, which lived during the Late Cretaceous period. These creatures were predators, which could run fast on their hind legs and had excellent sight and hearing. The name is derived from the Greek stems saur~ (lizard), ornith~ (bird) and eides (form), referring to its bird-like skull.Sinovenator
Sinovenator (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period.Sinusonasus
Sinusonasus is a genus of dinosaurs from the Early Cretaceous Period, recovered from the Yixian Formation. It lived in what is now the Liaoning Province of China. Sinusonasus was a theropod, specifically a troodontid dinosaur.
The type species, Sinusonasus magnodens, was named and described by Xu Xing and Wang Xiaolin in 2004. The generic name, derived from Latin sinus, "wave", and nasus, "nose", refers to the sinusoid form, in lateral view, of the nasals. The specific name means "big-toothed" from Latin magnus, "large" and dens, "tooth". In a later publication the species is referred to as "Sinucerasaurus" but this is a junior objective synonym.
The holotype, IVPP V 11527, was found in the Lujiatun Member of the Yixian Formation, dating from the Hauterivian. It consists of a partial skeleton including skull and lower jaw fragments and partial tail, pelvis and hindlimbs. The fossil is compressed and partially articulated.Sinusonasus is a small troodontid. In 2010, Gregory S. Paul estimated its length at one metre, its weight at 2.5 kilogrammes. The femur is 141 millimetres long.In 2004, several distinguishing traits were established. An interantorbital channel, connecting the antorbital fenestrae at each skull side, is lacking. The nasal bone has an undulating profile. The middle maxillary teeth are rather large. The chevrons on the rear caudal vertebrae are so long, measured from the front to the back, that they connect, forming a continuous plate at the underside of the tail. The neck of the thighbone is elongated.More generally, the head is relatively short, equalling 77% of the length of the thighbone. There are at least nineteen maxillary teeth per side. The front teeth are not serrated: those more to the rear only have denticles at the trailing edge. Five sacral vertebrae are present; the tail probably consisted of about thirty vertebrae. The pubic bone probably pointed obliquely to the front. The ischium is elongated. Sinusonasus has a long lower leg, indicating a good running capacity. The foot is "arctometatarsal", with a 'pinched' upper third metatarsal. The second metatarsal is distinctly shorter than the fourth. The second toe bears a retractable 'sickle claw'.Sinusonasus was in 2004 placed in the Troodontidae. It was presumed to have had a rather derived position, despite living in the Early Cretaceous. This was by the describing authors not interpreted as an indication for a long ghost lineage, troodontids developing earlier during the Jurassic than had been thought, but explained by rapid evolutionary change after a Cretaceous origin of the group.Timeline of pachycephalosaur research
This timeline of pachycephalosaur research is a chronological listing of events in the history of paleontology focused on the pachycephalosaurs, a group of dome-skulled herbivorous marginocephalian dinosaurs. One of the first major events related to the history of pachycephalosaur research actually regards the discovery of an unrelated dinosaur called Troodon, reported from the western United States by Joseph Leidy in 1856. The type specimen of Troodon was simply an unusual tooth, but the close resemblance between Troodon teeth and pachycephalosaur teeth would cause taxonomic confusion for over a century. This was resolved by Phil Currie in 1987, who realized that Troodon belonged to a group of bird-like carnivores then known as saurornithoidids, but since renamed Troodontidae after Troodon itself. The first scientifically documented true pachycephalosaur remains were discovered in Early Cretaceous rocks from England and named Stenopelix not long after Troodon was named in America. Other notable early finds include the well-known pachycephalosaur Stegoceras validum.In 1924, Charles Whitney Gilmore named the family Troodontidae after Troodon, but most of its members would be recognizable today as pachycephalosaurs. Seven years later, Gilmore named the new species "Troodon" wyomingensis which would be formally reclassified as Pachycephalosaurus in 1943. Pachycephalosaurus was so unusual that Sternberg named a new family for it, the Pachycephalosauridae. From the time paleontologists identified the pachycephalosaurs as a distinct group of dinosaurs, the chief mystery surrounding their biology has been the function of their distinctive cranial domes. Edwin Colbert interpreted the structure as a biological battering ram in 1955, but never specified who or what may have been on its receiving end. The idea that it was used in head butting between members of the same pachycephalosaur species was first proposed by science fiction writer Sprague de Camp. From then it became a staple of both scientific and cultural reconstructions of these animals.Nevertheless, this perennial hypothesis would come to be criticized by researchers like Hans-Dieter Sues as less likely than "flank butting" where pachycephalosaurs' domed heads would be aimed at rivals' bodies rather than in head-to-head combat. Others, like Goodwin and others have thought the dome purely for display because its high density of internal blood vasculature may have rendered it too fragile for combat. Meanwhile, in 1998 Chapman and others found the biomechanics of pachycephalosaur domes consistent with the old head-butting hypothesis, suggesting that the idea retains scientific merit.Timeline of troodontid research
This timeline of troodontid research is a chronological listing of events in the history of paleontology focused on the troodontids, a group of bird-like theropod dinosaurs including animals like Troodon. Troodontid remains were among the first dinosaur fossils to be reported from North America after paleontologists began performing research on the continent, specifically the genus Troodon itself. Since the type specimen of this genus was only a tooth and Troodon teeth are unusually similar to those of the unrelated thick-headed pachycephalosaurs, Troodon and its relatives would be embroiled in taxonomic confusion for over a century. Troodon was finally recognized as distinct from the pachycephalosaurs by Phil Currie in 1987. By that time many other species now recognized as troodontid had been discovered but had been classified in the family Saurornithoididae. Since these families were the same but the Troodontidae named first, it carries scientific legitimacy.Many milestones of troodontid research occurred between the description of Troodon and the resolution of their confusion with pachycephalosaurs. The family itself was named by Charles Whitney Gilmore in 1924. That same year Henry Fairfield Osborn named the genus Saurornithoides. In the 1960s and 1970s researchers like Russell and Hopson observed that troodontids had very large brains for their body size. Both attributed this enlargement of the brain to a need for processing the animal's especially sharp senses. Also in the 1970s, Barsbold described the new species Saurornithoides (now Zanabazar) junior and named the family Saurornithoidae, but as noted this was just a junior synonym of the Troodontidae in the first place.In the 1980s Gauthier classed them with the dromaeosaurids in the Deinonychosauria. That same decade Jack Horner reported the discovery of Troodon nests in Montana. Interest in the life history of Troodon continued in the 1990s with a study of its growth rates based on histological sections of fossils taken from a bonebed in Montana and the apparent pairing of eggs in Troodon nests. This decade also saw the first potential report of European troodontid remains, although this claim has been controversial. A single mysterious tooth from the Late Jurassic Morrison Formation of the United States was described as the oldest known troodontid remains, although this has also been controversial. In the 2000s, several new kinds of troodontid were named, like Byronosaurus and Sinovenator.Tochisaurus
Tochisaurus (meaning "Ostrich lizard") is a genus of small troodontid theropod dinosaur from the Late Cretaceous Period of Mongolia. The type (and only named) species is Tochisaurus nemegtensis.
In 1948, a Soviet-Mongolian expedition found the remains of a small theropod in the Gobi Desert near Nemegt. In 1987 the find was reported by Sergei Kurzanov and later that year discussed by Halszka Osmólska who suggested it could represent a specimen of the troodontid Borogovia.Later Osmólska understood it was a species new to science. It was formalized by Kurzanov and Osmólska in 1991 as Tochisaurus nemegtensis. The generic name is derived from Mongolian toch', "ostrich", in reference to the fact that the foot, like with that bird, is functionally didactyl, i.e. has only two weight-bearing toes. The specific name refers to the Nemegt.Its holotype fossil, PIN 551-224, was found in a layer of the Nemegt Formation, dating from the early Maastrichtian, about 69 million years old. It consists solely of the (left) metatarsus, the first discovered of an Asian troodontid. The first metatarsal is missing. The top of the fossil shows some damage that was originally somewhat inexpertly restored.Tochisaurus is a bipedal dinosaur. The metatarsus has a length of 242 millimetres, showing it was a relatively large troodontid. The second metatarsal, 222 millimetres long, is very reduced and narrow. The joint surface on top of the metatarsus is sloped forward and downward.Based on the partial fossils, Tochisaurus is thought to have been a member of the Troodontidae.Troodon
Troodon ( TROH-ə-don; Troödon in older sources) is a former wastebasket taxa and a potentially dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Cretaceous period (about 77 mya). It includes at least one species, Troodon formosus, known from Montana. Discovered in October 1855, T. formosus was among the first dinosaurs found in North America, although it was thought to be a lizard until 1877. Several well-known troodontid specimens from the Dinosaur Park Formation in Alberta were once believed to be members of this genus. However, recent analyses in 2017 have found the genus to be undiagnostic and referred some of these specimens to the genus Stenonychosaurus (long believed to be synonymous with Troodon) and others to the newly created genus Latenivenatrix.
The genus name is Greek for "wounding tooth", referring to the teeth, which were different from those of most other theropods known at the time of their discovery. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet.Troodontinae
Troodontinae is a subfamily of troodontid dinosaurs. The subfamily was first used in 2017 for the group of troodontids descended from the last common ancestor of Gobivenator mongoliensis and Zanabazar junior.Urbacodon
Urbacodon ("URBAC tooth") is a genus of troodontid dinosaur, a type of small carnivore. It lived in Uzbekistan during the early Late Cretaceous Period, about 95 million years ago.
On 9 September 2004, a lower jaw of a small theropod was uncovered by Anton Sergeevich Rezwiy near Itemir in the IT-01 quarry.The type species, Urbacodon itemirensis, was named by Alexandr Averianov and Hans-Dieter Sues in 2007. The first part of the generic name Urbacodon is an acronym, honouring the Uzbek, Russian, British, American and Canadian scientists who participated in its discovery. This acronym was combined with a Greek ὀδών, odon, "tooth". The specific name refers to the provenance from Itemir.The name was based on the holotype ZIN PH 944/16, a single left dentary with preserved replacement teeth from the Cenomanian Dzharakuduk Formation. Averianov and Sues also identified teeth and other material, earlier described by Lev Nesov, as a Urbacodon sp. from the nearby Turonian Bissekty Formation.The holotype dentary of U. itemirensis is 79.2 millimetres long (3.12 in) and has 32 tooth positions. It is rather straight in top view. The teeth are closely packed but between the front twenty-four teeth and the rear eight teeth, a distinctive gap is present, a diastema. This is a unique trait but was not formally designated as an autapomorphy because it might be the result of individual variation. Urbacodon resembles Byronosaurus and Mei but differs from most other Troodontidae in that its teeth lack serrations. Urbacodon is distinguished from Byronosaurus by a less vascularized lateral dentary groove and more bulbous anterior tooth crowns, and from Mei by considerably larger size.Averianov and Sues viewed Urbacodon as more plesiomorphic than Troodon and Saurornithoides in having a straight dentary with fewer teeth, but did not attempt to place it on a cladogram. In 2010, a cladistic analysis showed it as a close relative of Byronosaurus and Xixiasaurus.Xiaotingia
Xiaotingia is a genus of anchiornithid theropod dinosaur from early Late Jurassic deposits of western Liaoning, China, containing a single species, Xiaotingia zhengi.Zanabazar junior
Zanabazar is an extinct genus of troodontid theropod dinosaur from the Late Cretaceous of Mongolia. The genus was originally named by Rinchen Barsbold as a species of Saurornithoides, S. junior. In 2009 it was reclassified as its own genus, named after the first spiritual figurehead of Tibetan buddhism, Zanabazar. The holotype, GIN 100–1, includes a skull, vertebrae, and right hindlimb. Zanabazar was one of the most derived troodontids, and the second largest after Troodon.