Troodon

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Troodon
Temporal range: Late Cretaceous (Campanian),77.5–76.5  Ma
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Troodon formosus.jpg
Illustration of the T. formosus holotype tooth
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: Troodontidae
Subfamily: Troodontinae
Genus: Troodon
Leidy, 1856
Type species
Troodon formosus
Leidy, 1856

Troodon ( /ˈtr.ədɒn/ TROH-ə-don; Troödon in older sources) is a former wastebasket taxon and dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late 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 genus Latenivenatrix . Thus, it is still controversial if Troodon is valid, but the general consensus is that it no longer is true.

Contents

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. [1]

History of discovery

Early research

The name was originally spelled Troödon (with a diaeresis) by Joseph Leidy in 1856, which was officially amended to its current status by Sauvage in 1876. [2] The type specimen of Troodon has caused problems with classification, as the entire genus is based only on a single tooth from the Judith River Formation. Troodon has historically been a highly unstable classification and has been the subject of numerous conflicting synonymies with similar theropod specimens. [3]

The Troodon tooth was originally classified as a "lacertilian" (lizard) by Leidy, but reassigned as a megalosaurid dinosaur by Nopcsa in 1901 (Megalosauridae having historically been a wastebin taxon for most carnivorous dinosaurs). In 1924, Gilmore suggested that the tooth belonged to the herbivorous pachycephalosaur Stegoceras , and that Stegoceras was in fact a junior synonym of Troodon (the similarity of troodontid teeth to those of herbivorous dinosaurs continues to lead many paleontologists to believe that these animals were omnivores). The classification of Troodon as a pachycephalosaur was followed for many years, during which the family Pachycephalosauridae was known as Troodontidae. In 1945, Charles Mortram Sternberg rejected the possibility that Troodon was a pachycephalosaur due to its stronger similarity to the teeth of other carnivorous dinosaurs. With Troodon now classified as a theropod, the family Troodontidae could no longer be used for the dome-headed dinosaurs, so Sternberg named a new family for them, Pachycephalosauridae. [4]

Comparison of troodontid teeth; A is the T. formosus holotype Troodontid teeth.jpg
Comparison of troodontid teeth; A is the T. formosus holotype

The first specimens assigned to Troodon that were not teeth were both found by Sternberg in the early 1930s, in the Dinosaur Park Formation of Alberta. The first was named Stenonychosaurus inequalis by Sternberg in 1932, based on a foot, fragments of a hand, and some tail vertebrae. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of early paravians. Sternberg initially classified Stenonychosaurus as a member of the family Coeluridae. The second, a partial lower jaw bone, was described by Gilmore (1932) as a new species of lizard which he named Polyodontosaurus grandis . Later, in 1951, Sternberg recognized P. grandis as a possible synonym of Troodon, and speculated that since Stenonychosaurus had a "very peculiar pes" and Troodon "equally unusual teeth", they may be closely related. Unfortunately, no comparable specimens were available at that time to test the idea. In a recent revision of the material by van der Reest & Currie, Polyodontosaurus was determined to be a nomen dubium, not fit for synonymy with other taxa. [5]

A more complete skeleton of Stenonychosaurus was described by Dale Russell in 1969 from the Dinosaur Park Formation, which eventually formed the scientific foundation for a famous life-sized sculpture of Stenonychosaurus accompanied by its fictional, humanoid descendant, the "dinosauroid". [6] Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with Saurornithoides , it formed the family Saurornithoididae. Based on differences in tooth structure, and the extremely fragmentary nature of the original Troodon formosus specimens, saurornithoidids were thought to be close relatives while Troodon was considered a dubious possible relative of the family. Phil Currie, reviewing the pertinent specimens in 1987, showed that supposed differences in tooth and jaw structure among troodontids and saurornithoidids were based on age and position of the tooth in the jaw, rather than a difference in species. He reclassified Stenonychosaurus inequalis as well as Polyodontosaurus grandis and Pectinodon bakkeri as junior synonyms of Troodon formosus. Currie also made Saurornithoididae a junior synonym of Troodontidae. [7] In 1988, Gregory S. Paul went farther and included Saurornithoides mongoliensis in the genus Troodon as T. mongoliensis, [8] but this reclassification, along with many other unilateral synonymizations of well known genera, was not adopted by other researchers. Currie's classification of all North American troodontid material in the single species Troodon formosus became widely adopted by other paleontologists, and all of the specimens once called Stenonychosaurus were referred to as Troodon in the scientific literature through the early 21st century.

Teeth from South Dakota assigned to T. formosus, with a US dime coin for scale, Children's Museum of Indianapolis The Childrens Museum of Indianapolis - Troodon teeth.jpg
Teeth from South Dakota assigned to T. formosus, with a US dime coin for scale, Children's Museum of Indianapolis

Dissolution of the one species model

However, the concept that all Late Cretaceous North American troodontids belong to one species began to be questioned soon after Currie's 1987 paper was published, including by Currie himself. Currie and colleagues (1990) noted that, while they believed the Judith River troodontids were all T. formosus, troodontid fossils from other formations, such as the Hell Creek Formation and Lance Formation, might belong to different species. In 1991, George Olshevsky assigned the Lance formation fossils, which had first been named Pectinodon bakkeri but later synonymized with Troodon formosus to the species Troodon bakkeri, and several other researchers (including Currie) have reverted to keeping the Dinosaur Park Formation fossils separate as Troodon inequalis (now Stenonychosaurus inequalis). [9]

In 2011, Zanno and colleagues reviewed the convoluted history of troodontid classification in Late Cretaceous North America. They followed Longrich (2008) in treating Pectinodon bakkeri as a valid genus, and noted that it is likely the numerous Late Cretaceous specimens currently assigned to Troodon formosus almost certainly represent numerous new genera, but that a more thorough review of the specimens is required. Because the holotype of T. formosus is a single tooth, this renders Troodon a nomen dubium . [3]

In 2017, Evans and colleagues further discussed the undiagnostic nature of the holotype of Troodon formosus and suggested that Stenonychosaurus be used for troodontid skeletal material from the Dinosaur Park Formation. [10] Later in the same year, Aaron J. van der Reest and Currie came to a similar conclusion as Evans and colleagues, and also split much of the material assigned to Stenonychosaurus into a new genus: Latenivenatrix . [5] In 2018, Varricchio and colleagues disagreed with Evans and colleagues, citing that Stenonychosaurus had not been used in the thirty years since Currie and colleagues synonymized it with Troodon, and they indicated that "Troodon formosus remains the proper name for this taxon". [11] This conclusion by Varricchio was agreed upon by Sellés and colleagues in their 2021 description of Tamarro . [12] Varricchio's comments were later addressed by Cullen and colleagues in their 2021 review of Dinosaur Park Formation biodiversity, where they noted that while Stenonychosaurus has indeed not been used for 30 years, Currie's original hypothesis of subjective synonymy (based on tooth and jaw morphology) was never directly tested, and given that later research found that teeth were not diagnostic below the family level in troodontids, Currie's original hypothesis is therefore not supported by the available data, regardless of the amount of time since it was originally proposed. [13] They suggested that the description of more complete skeletal material (i.e. containing dental, frontal, and postcranial elements) that can be tied to the holotype could allow the direct testing of the synonymy hypothesis, but re-affirmed that for now, given the lack of supporting evidence, the synonymy of Troodon and Stenonychosaurus cannot be maintained, and that merely remaining untested for 30 years is not sufficient justification to accept a proposed lumping of taxa lacking overlapping diagnostic materials. [13]

Classification

Troodon is considered to be one of the most derived members of its family. Along with Zanabazar , Saurornithoides and Talos it forms a clade of specialized troodontids. [3]

Below is a cladogram of Troodontidae by Zanno et al. in 2011. [3]

Troodontidae

Sinovenator changii

Sinovenator changii

Mei long

IGM 100/44

Sinornithoides youngi

Talos sampsoni

Byronosaurus jaffei

Talos sampsoni

Talos sampsoni

Saurornithoides mongoliensis

Zanabazar junior

Troodon formosus

Paleobiology

Hypothetical restoration based on related animals Troodon (cropped).jpg
Hypothetical restoration based on related animals

One study was based on multiple Troodon teeth that have been collected from Late Cretaceous deposits from northern Alaska. These teeth are much larger than those collected from more southern sites, providing evidence that northern Alaskan populations of Troodon grew to larger average body size, hinting at Bergmann's rule. This study also provides an analysis of the proportions and wear patterns of a large sample of Troodon teeth. It proposes that the wear patterns of all Troodon teeth suggest a diet of soft foods - inconsistent with bone chewing, invertebrate exoskeletons, or tough plant items. This study hypothesizes a diet primarily consisting of meat. [14]

In 2011, another derived troodontid, Linhevenator , was described from Inner Mongolia, China. It was noted by the authors as having relatively short and robust forelimbs, along with an enlarged second pedal ungual akin to that of the dromaeosaurids, in comparison to more basal troodontids. It was proposed that derived troodontids had convergently evolved dromaeosaurid-style relatively large second pedal unguals, likely as an adaptation relating to predation. The authors noted that it is plausible that this may be applicable to other derived troodontids, including Troodon, although this is currently uncertain due to a paucity of sufficient remains of the latter genus. [15]

Paleoecology

Restored skeleton of an unnamed Alaskan species, Perot Museum Alaskan troodont Perot Museum.jpg
Restored skeleton of an unnamed Alaskan species, Perot Museum

The type specimen of Troodon formosus was found in the Judith River Formation of Montana. The rocks of the Judith River Formation are equivalent in age with the Oldman Formation of Alberta, [16] which has been dated to between 77.5 and 76.5 million years ago. [17]

In the past, remains have been attributed to the same genus as the Judith River Troodon from a wide variety of other geological formations. It is now recognized as unlikely that all of these fossils, which come from localities hundreds or thousands of miles apart, separated by millions of years of time, represent a single species or genus of troodontid dinosaurs. Further study and more fossils are needed to determine how many species of Troodon existed. It is questionable that, after further study, any additional species can be referred to Troodon, in which case the genus would be considered a nomen dubium. [3]

A species of Troodon is known from the Prince Creek Formation, a fossil site from Alaska that dates from the latest Campanian to Maastrichtian of the late Cretaceous. [18] Based on the presence of Gypsum and Pyrite in the rocks, it suggests that the formation was bordered by a large body of water. It seems based on the presence of pollen fossils, the dominant plants were trees, shrubs, herbs, and angiosperms. The temperature ranged from possibly 2-12°C, which roughly correlates to 36-54°F, and based on Alaska's position in the late Cretaceous, the area faced 120 or so days of winter darkness. [19] This maniraptoran lived alongside many other reptiles like the centrosaurine Pachyrhinosaurus perotorum , a species of the saurolophine hadrosaur Edmontosaurus , the pachycephalosaurin Alaskacephale gangloffi , an azhdarchid pterosaur, and the tyrannosaurine Nanuqsaurus hoglundi . It also lived alongside the metatherian mammal Unnuakomys hutchisoni. [20] Based on the amount of teeth found, this troodontid was the most common theropod of the formation, making up 2/3 of all specimens, a stark contrast to more southern deposits in Montana where troodontids only comprise 6% of all theropod remains. [18] This, along with evidence that Troodon was more abundant during cooler intervals such as the early Maastrichtian, may indicate that Troodon favored cooler climates. [21]

Size estimates of North American theropods compared to a 1 metre scale bar; Nanuqsaurus hoglundi (A), Tyrannosaurus rex or T. imperator specimen FMNH PR2081 "Sue" (B), T. rex specimen AMNH 5027 (C), Daspletosaurus torosus (D), Albertosaurus sarcophagus (E), Troodon formosus (F), and the Alaskan Troodon (G). Nanuqsaurus size.tif
Size estimates of North American theropods compared to a 1 metre scale bar; Nanuqsaurus hoglundi (A), Tyrannosaurus rex or T. imperator specimen FMNH PR2081 "Sue" (B), T. rex specimen AMNH 5027 (C), Daspletosaurus torosus (D), Albertosaurus sarcophagus (E), Troodon formosus (F), and the Alaskan Troodon (G).

Additional specimens currently referred to Troodon come from the upper Two Medicine Formation of Montana. Troodon-like teeth have been found in the lower Javelina Formation of Texas and the Naashoibito Member of the Ojo Alamo Formation in New Mexico. [22] [23]

See also

Related Research Articles

<i>Daspletosaurus</i> Genus of tyrannosaurid dinosaur from Late Cretaceous period

Daspletosaurus is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus Daspletosaurus contains two species. Fossils of the earlier type species, D. torosus, have been found in Alberta, while fossils of the later second species, D. horneri, have been found only in Montana. A possible third species, also from Alberta, awaits formal identification and another possible species D. degrootorum, also exists, but it may belong to the separate genus Thanatotheristes instead. Daspletosaurus is closely related to the much larger and more recent tyrannosaurid Tyrannosaurus rex. Like most tyrannosaurids, Daspletosaurus was a multi-tonne bipedal predator equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.

Troodontidae Extinct family of bird-like dinosaurs

Troodontidae 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, 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.

<i>Stenonychosaurus</i> Theropod dinosaur

Stenonychosaurus is a genus of troodontid dinosaur from the Late Cretaceous Dinosaur Park Formation of Alberta, Canada, as well as possibly the Two Medicine Formation. The type and only species, S. inequalis, was named by Charles Mortram Sternberg in 1932, based on a foot, fragments of a hand, and some caudal vertebrae from the Late Cretaceous of Alberta. S. inequalis was reassigned in 1987 by Phil Currie to the genus Troodon, which was reverted by the recognition of Stenonychosaurus as a separate genus from the possibly dubious Troodon in 2017 by Evans et al. and also later in the same year by Van der Reest and Currie.

<i>Saurornitholestes</i> Extinct genus of dinosaurs

Saurornitholestes is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Canada (Alberta) and the United States.

<i>Saurornithoides</i> Extinct genus of dinosaurs

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 derived from the Greek stems saur~ (lizard), ornith~ (bird) and eides (form), referring to its bird-like skull.

<i>Byronosaurus</i> Extinct genus of dinosaurs

Byronosaurus is a genus of troodontid dinosaur from the Late Cretaceous Period of Mongolia.

<i>Orodromeus</i> Extinct genus of dinosaurs

Orodromeus is a genus of herbivorous orodromine thescelosaurid dinosaur from the Late Cretaceous of North America. Only one species is known, the type species Orodromeus makelai.

<i>Zapsalis</i> Extinct genus of dinosaurs

Zapsalis is a genus of dromaeosaurine theropod dinosaurs. It is a tooth taxon, often considered dubious because of the fragmentary nature of the fossils, which include teeth but no other remains.

<i>Paronychodon</i> Extinct genus of dinosaurs

Paronychodon was a theropod dinosaur genus. It is a tooth taxon, often considered dubious because of the fragmentary nature of the fossils, which include "buckets" of teeth from many disparate times and places but no other remains, and should be considered a form taxon.

<i>Pectinodon</i> Genus of bird-like dinosaur

Pectinodon is a genus of troodontid dinosaurs from the Late Cretaceous period (66 mya). It currently contains a single valid species, Pectinodon bakkeri, known only from teeth.

Polyodontosaurus is a potentially dubious genus of troodontid dinosaur named in 1932 by Charles W. Gilmore for a left dentary from the Dinosaur Park Formation. It had been considered a synonym of Stenonychosaurus or Troodon for a significant time, before being declared a nomen dubium. The only known species is the type, P. grandis.

<i>Xixiasaurus</i> Genus of dinosaur

Xixiasaurus is a genus of troodontid dinosaur that lived during the Late Cretaceous Period in what is now China. The only known specimen was discovered in Xixia County, Henan Province, in central China, and became the holotype of the new genus and species Xixiasaurus henanensis in 2010. The names refer to the areas of discovery, and can be translated as "Henan Xixia lizard". The specimen consists of an almost complete skull, part of the lower jaw, and teeth, as well as a partial right forelimb.

Continuoolithus is an oogenus of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs are similar to the eggs of oviraptorid dinosaurs, but have a distinctive type of ornamentation.

<i>Linhevenator</i> Extinct genus of dinosaurs

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Talos is an extinct genus of carnivorous bird-like theropod dinosaur, an advanced troodontid which lived during the late Cretaceous period in the geographic area that is now Utah, United States.

Timeline of troodontid research Events in the history of paleontology

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.

Timeline of pachycephalosaur research The progression of pachycephalosaur research over time

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.

<i>Daliansaurus</i> Extinct genus of dinosaurs

Daliansaurus is a genus of small troodontid theropod dinosaur, measuring approximately 1 metre long, from the Early Cretaceous of China. It contains a single species, D. liaoningensis, named in 2017 by Shen and colleagues from a nearly complete skeleton preserved in three dimensions. Daliansaurus is unusual in possessing an enlarged claw on the fourth digit of the foot, in addition to the "sickle claw" found on the second digit of the feet of most paravians. It also has long metatarsal bones, and apparently possesses bird-like uncinate processes. In the Lujiatun Beds of the Yixian Formation, a volcanically-influenced region with a cold climate, Daliansaurus lived alongside its closest relatives - Sinovenator, Sinusonasus, and Mei, with which it forms the group Sinovenatorinae.

<i>Liaoningvenator</i> Extinct genus of dinosaurs

Liaoningvenator 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.

<i>Latenivenatrix</i> Extinct genus of reptiles

'Latenivenatrix' was a genus of troodontid known from one species, 'L. mcmasterae', described in 2017 from remains formerly identified as Troodon. In 2021, it was found to be a junior synonym of Stenonychosaurus inequalis.

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