Troodont
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Troodontidae is a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of bird-like
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
to
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
ian lineages. More recent fossil discoveries of complete and articulated specimens (including specimens which preserve
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and an exa ...
s,
eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...
,
embryos An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, and complete juveniles), have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like ''
Sinovenator ''Sinovenator'' (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period (geology), Period. Discovery and naming Two specimens of a troodontidae, troodontid were described in 2002 in paleo ...
'', demonstrate striking anatomical similarities with ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'' and primitive
dromaeosaurids Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Gree ...
, and demonstrate that they are relatives comprising a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
called
Paraves Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosauridae, dromaeosaurids, troodontidae, troodontids, Anchiornithidae, anchiornithids, and possibly the scansor ...
.


Evolution

The oldest definitive troodontid known is ''
Hesperornithoides ''Hesperornithoides'' (meaning "western bird form"; nicknamed "Lori") is a genus of troodontid theropod dinosaur that lived in North America during the Late Jurassic period. Discovery Although several Troodontid teeth were found, with the trood ...
'' from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
of
Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...
. The slightly older '' Koparion'' of
Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...
is only represented by a single tooth, and small maniraptoran teeth from the
Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...
of
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
were identified as those of indeterminate troodontids in 2023. Over the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, troodontids radiated throughout western
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
,
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
, and
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
, suggesting a mostly
Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...
n distribution for the group. However, in 2013, a single diagnostic tooth from the latest Cretaceous (
Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
) Kallamedu Formation of southern
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
was identified as a troodontid, suggesting that troodontids either also inhabited
Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...
or managed to disperse to India from elsewhere prior to its separation as an island continent. 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 Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...
. 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.


Description

Troodontids are a group of small, bird-like,
gracile Gracility is slenderness, the condition of being gracile, which means slender. It derives from the Latin adjective ''gracilis'' (masculine or feminine), or ''gracile'' ( neuter), which in either form means slender, and when transferred for examp ...
maniraptora Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to ''Ornithomimus velox''. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, ...
ns. All troodontids have unique features of the skull, such as large numbers of closely spaced
teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
in the lower jaw. Troodontids have sickle-claws and
raptorial In biology (specifically the anatomy of arthropods), the term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod leg, arthropod's foreleg that make it function for the grasping of prey whi ...
hands A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "han ...
, and some of the highest non- avian
encephalization quotient Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed and predicted brain mass for an animal of a given size, based on nonlinear regre ...
s, 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 Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Gree ...
. 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 ''Borogovia'' is a troodontid theropod dinosaur genus which lived during the Late Cretaceous Period, in what is now Mongolia. It contains one species, ''Borogovia gracilicrus.'' Discovery and naming In 1971, a Polish-Mongolian expedition ...
'', 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 Binocular vision is seeing with two eyes. The Field_of_view, field of view that can be surveyed with two eyes is greater than with one eye. To the extent that the visual fields of the two eyes overlap, #Depth, binocular depth can be perceived. Th ...
. 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
owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...
s. The specialization of the ears may indicate that troodontids hunted in a manner similar to owls, using their hearing to locate small prey.


Diet

Although most
paleontologists Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
believe that they were
predatory Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...
s, the many small, coarsely serrated teeth, large denticle size, and U-shaped jaws of some species (particularly ''
Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...
'') suggest that some species may have been
omnivorous An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize ...
or
herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
. Some suggest that the large denticle size is reminiscent of the teeth of extant iguanine lizards. In contrast, a few species, such as ''
Byronosaurus ''Byronosaurus'' is a genus of Troodontidae, troodontid dinosaur from the Late Cretaceous Period (geology), Period of Mongolia. Discovery and naming In 1993, Michael Novacek, a member of an American Museum of Natural History expedition to the Go ...
'', had large numbers of needle-like teeth, which seem best-suited for picking up small prey, such as birds,
lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...
s and small
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s. 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. Analyses of
barium Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
/
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
and
strontium Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to ...
/calcium ratios, which are higher in carnivores due to
bioaccumulation Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance faster than it can be lost or eliminated by catabolism and excretion. T ...
, found low ratios in teeth of '' Stenonychosaurus'', suggesting that it had a diet ranging from mixed to plant-dominant omnivory. 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 A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often ...
lifestyle, though the study indicates that troodontids were still likely to have used the
ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...
s 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.


Classification

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
ornithischia Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek ...
ns and allied them with the
pachycephalosauria Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachyceph ...
n ''
Stegoceras ''Stegoceras'' is a genus of Pachycephalosauria, pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous Period (geology), period, about 77.5 to 74 million years ago (mya). The first specim ...
'' 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 Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from ...
, in a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
(natural group) known as
Deinonychosauria Deinonychosauria is a clade of paravian dinosaurs which lived from the Late Jurassic to the Late Cretaceous periods. Fossils have been found across the globe in North America, Europe, Africa, Asia, South America, and Antarctica,Case, J.A., Marti ...
, but this was by no means a consensus. Holtz (in 1994) erected the clade Bullatosauria, uniting
Ornithomimosauria Ornithomimosauria ("bird-mimic lizards") are theropod dinosaurs which bore a superficial resemblance to the modern-day ostrich. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia (now Asia, Europe and No ...
(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 China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
(such as ''
Sinovenator ''Sinovenator'' (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period (geology), Period. Discovery and naming Two specimens of a troodontidae, troodontid were described in 2002 in paleo ...
'' and ''Mei''), however, display strong similarities between Troodontidae, Dromaeosauridae and the primitive bird ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', 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 ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
''. This clade is together called
Paraves Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosauridae, dromaeosaurids, troodontidae, troodontids, Anchiornithidae, anchiornithids, and possibly the scansor ...
by Novas and Pol. The extensive cladistic analysis conducted by Turner ''et al.'', (2012) supported the monophyly of Troodontidae.


Taxonomy

* Family Troodontidae **'' Albertavenator'' **'' Almas'' **''
Geminiraptor ''Geminiraptor'' (meaning "twin seizer") is a genus of troodontid theropod dinosaur that lived in North America during the Early Cretaceous period. ''Geminiraptor'' was a small, ground-dwelling bipedal carnivorous paravian. The type species of ...
'' **''
Jianianhualong ''Jianianhualong'' (meaning "Jianianhua dragon") is a genus of troodontidae, troodontid theropoda, theropod dinosaur from the Early Cretaceous of China. It contains a single species, ''Jianianhualong tengi'', named in 2017 by Xu Xing (paleontolog ...
'' **''
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 ...
'' **''
Sinornithoides ''Sinornithoides'' (meaning "Chinese bird form") is a genus of troodontid theropod dinosaurs containing the single species ''Sinornithoides youngi''. ''S. youngi'' lived during the Early Cretaceous (Aptian/Albian stage, around 113 million years ...
'' **''
Talos In Greek mythology, Talos, also spelled Talus (; , ''Tálōs'') or Talon (; , ''Tálōn''), was a man of bronze who protected Crete from pirates and invaders. Despite the popular idea that he was a giant, no ancient source states this explicitl ...
'' **''
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 fou ...
'' **''
Xixiasaurus ''Xixiasaurus'' () is a genus of troodontid dinosaur that lived during the Late Cretaceous Period (geologic time), Period in what is now China. The only known specimen was discovered in Xixia County, Henan Province, in central China, and became ...
'' ** Subfamily Jinfengopteryginae ***'' Jinfengopteryx'' ***IGM 100/1128 ** Subfamily Sinovenatorinae ***''
Daliansaurus ''Daliansaurus'' (meaning "Dalian reptile") is a genus of small troodontid theropod dinosaur, measuring approximately long, from the Early Cretaceous of China. It contains a single species, ''D. liaoningensis'', named in 2017 by Shen and colleag ...
'' ***'' Mei'' ***''
Sinovenator ''Sinovenator'' (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period (geology), Period. Discovery and naming Two specimens of a troodontidae, troodontid were described in 2002 in paleo ...
'' ***'' Sinusonasus'' ** Subfamily Troodontinae ***''
Borogovia ''Borogovia'' is a troodontid theropod dinosaur genus which lived during the Late Cretaceous Period, in what is now Mongolia. It contains one species, ''Borogovia gracilicrus.'' Discovery and naming In 1971, a Polish-Mongolian expedition ...
'' ***''
Byronosaurus ''Byronosaurus'' is a genus of Troodontidae, troodontid dinosaur from the Late Cretaceous Period (geology), Period of Mongolia. Discovery and naming In 1993, Michael Novacek, a member of an American Museum of Natural History expedition to the Go ...
''? ***''
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 ...
'' ***'' Latenivenatrix'' ***'' Linhevenator'' ***'' Pectinodon'' ***'' Philovenator'' ***''
Saurornithoides ''Saurornithoides'' ( ) 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 deri ...
'' ***'' Stenonychosaurus'' ***''
Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...
'' ***'' Urbacodon'' ***''
Zanabazar Öndör Gegeen Zanabazar (born Eshidorji) was the first '' Jebtsundamba Khutuktu'' and the first ''Bogd Gegeen'' or supreme spiritual authority, of the Gelugpa (Yellow Hat) lineage of Tibetan Mahayana Buddhism in Mongolia. The son of a Mongol ...
''


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 A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
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 Coelurosauria (; from Greek, meaning "hollow-tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyra ...
, 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. Troodontinae is a
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
of
troodontid Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
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,'' but has been redefined to be the least inclusive clade containing '' Saurornithoides mongoliensis'' and ''
Troodon formosus ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...
,'' utilizing the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
of the clade. Below is a
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
of the Troodontinae as published by Aaron van der Reest and
Phil Currie Philip John Currie (born March 13, 1949) is a Canadian palaeontologist and museum curator who helped found the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta and is now a professor at the University of Alberta in Edmonton. In the ...
, in 2017.


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 ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...
''). A few troodont
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s, including specimens of '' Mei'' and ''
Sinornithoides ''Sinornithoides'' (meaning "Chinese bird form") is a genus of troodontid theropod dinosaurs containing the single species ''Sinornithoides youngi''. ''S. youngi'' lived during the Early Cretaceous (Aptian/Albian stage, around 113 million years ...
'', 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 ''Jianianhualong'' (meaning "Jianianhua dragon") is a genus of troodontidae, troodontid theropoda, theropod dinosaur from the Early Cretaceous of China. It contains a single species, ''Jianianhualong tengi'', named in 2017 by Xu Xing (paleontolog ...
'', 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 Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between one ...
eggs in the Djadokhta Formation of Mongolia. The nest is quite certainly that of an
oviraptorosaur Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or wi ...
, 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 ''Byronosaurus'' is a genus of Troodontidae, troodontid dinosaur from the Late Cretaceous Period (geology), Period of Mongolia. Discovery and naming In 1993, Michael Novacek, a member of an American Museum of Natural History expedition to the Go ...
'' 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.


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 Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from ...
and
tyrannosauridae Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to fifteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
. 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. Ba/Ca and Sr/Ca ratios found in the enamel of
Oldman Formation The Oldman Formation is a stratigraphic unit of Late Cretaceous (Campanian stage) age that underlies much of southern Alberta, Canada. It consists primarily of sandstones that were deposited in fluvial channel and floodplain environments. It was ...
troodontids indicate that they were mixed-feeding to plant-dominant omnivores.


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 ''Microraptor'' (Greek language, Greek, μικρός, ''mīkros'': "small"; Latin language, Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovere ...
'' and ''
Rahonavis ''Rahonavis'' is a genus of bird-like theropod from the Late Cretaceous (Maastrichtian, from about 72.1 to 66 mya) of what is now northwestern Madagascar. It is known from a partial skeleton ( UA 8656) found by Catherine Forster and colleagues ...
''.


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 ''
Hesperornithoides ''Hesperornithoides'' (meaning "western bird form"; nicknamed "Lori") is a genus of troodontid theropod dinosaur that lived in North America during the Late Jurassic period. Discovery Although several Troodontid teeth were found, with the trood ...
'' ("Lori") is a troodontid from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltston ...
, close to the time of ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
''. 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 A temporal paradox, time paradox, or time travel paradox, is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time travel or other foreknowledge of the future. While the notion of time travel to the futu ...
" cited by the few remaining opponents of the idea that birds are closely related to dinosaurs.


See also

*
Timeline of troodontid research This timeline of troodontid research is a chronological listing of events in the History of paleontology, history of paleontology focused on the troodontids, a group of bird-like theropod dinosaurs including animals like ''Troodon''. Troodontid r ...
*
Evolution of birds The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves. Birds are categorized as a Class (biology), biological class, Aves. For more than a century, the small theropod ...


References


External links

{{Taxonbar, from=Q132452 Dinosaur families