Editing Troodon (section)

== History of discovery ==
===Early research===
[[File:Troodon formosus.jpg|thumb|left|1860 illustration of the ''T. formosus'' holotype tooth]] 
The name was originally spelled ''Troödon'' (with a [[Diaeresis (diacritic)|diaeresis]]) by [[Joseph Leidy]] in 1856, which was officially amended to its current status by Sauvage in 1876.<ref>{{Cite journal
|publisher=Société géologique de France
|url=https://books.google.com/books?id=aG27AAAAIAAJ&q=tro%25C3%25B6don&pg=PA770|journal=Bulletin de la Société géologique de France|date=1876
| last = Sauvage | first=H.-E.
| issue= 3e série 4
| pages= 434–444
| title=Notes sur les reptiles fossiles
|language=fr}}</ref> The [[type (zoology)|type specimen]] of ''Troodon'' has caused problems with classification, as the entire genus is based only on one 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.<ref name=Talos/>

The ''Troodon'' tooth was originally classified as a "lacertilian" ([[lizard]]) by Leidy, but reassigned as a [[megalosaurid]] dinosaur by [[Franz Nopcsa von Felső-Szilvás]] 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 time the family [[Pachycephalosauridae]] was known as [[Troodontidae]]. In 1945, [[Charles Mortram Sternberg]] rejected the possibility that ''Troodon'' was a pachycephalosaur thanks 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''.<ref name=sternberg1945>{{Cite journal | last1 = Sternberg | first1 = C. | year = 1945 | title = Pachycephalosauridae proposed for domeheaded dinosaurs, ''Stegoceras lambei'' n. sp., described | journal = Journal of Paleontology | volume = 19 | pages = 534–538 }}</ref>

===Naming of related species===
[[File:Troodontid teeth.jpg|thumb|right|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 [[Paraves|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| Polyodontosaurus grandis]]''. In 1951, Sternberg later recognized ''P. grandis'' as a possible synonym of ''Troodon'' and speculated that, since ''Stenonychosaurus'' had a "very peculiar [[pes (zoology)|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.<ref name=LS2017/>

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 [[Stenonychosaurus#The "Dinosauroid"|"dinosauroid"]].<ref name=russell1982>{{Cite journal | last1 = Russell | first1 = D. A. | last2 = Séguin | first2 = R. | year = 1982 | title = Reconstruction of the small Cretaceous theropod ''Stenonychosaurus inequalis'' and a hypothetical dinosauroid | url = https://archive.org/details/syllogeus37nati | journal = Syllogeus | volume = 37 | pages = [https://archive.org/details/syllogeus37nati/page/n8 1]–43 }}</ref> ''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'', ''Polyodontosaurus grandis'', and ''[[Pectinodon bakkeri]]'' as junior synonyms of ''Troodon formosus''. Currie also made Saurornithoididae a junior synonym of Troodontidae.<ref name=currie1987>{{Cite journal | last1 = Currie | first1 = P. | year = 1987 | title = Theropods of the Judith River Formation | journal = Occasional Paper of the Tyrrell Museum of Palaeontology | volume = 3 | pages = 52–60 }}</ref> In 1988, [[Gregory S. Paul]] went farther and included ''Saurornithoides mongoliensis'' in the genus ''Troodon'' as ''T. mongoliensis'',<ref name=paul1988b>{{cite book |last=Paul |first=G.S. |year=1988 |title=Predatory Dinosaurs of the World |location=New York |publisher=Simon and Schuster |pages=[https://archive.org/details/predatorydinosau00paul/page/398 398–399] |isbn=978-0-671-61946-6 |url-access=registration |url=https://archive.org/details/predatorydinosau00paul/page/398 }}</ref> 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 [[paleontology|paleontologists]] and all of the specimens once called ''Stenonychosaurus'' were referred to as ''Troodon'' in scientific literature through the early 21st century.

===Discussion of the one species model===

[[File:The Childrens Museum of Indianapolis - Troodon teeth.jpg|thumb|Teeth from South Dakota assigned to ''T. formosus'', with a [[US dime]] coin for scale, [[Children's Museum of Indianapolis]]]]
However, the concept that all Late Cretaceous North American troodontids belong to one single 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'').<ref name=currie2005>Currie, P. (2005). "Theropods, including birds." in Currie and Koppelhus (eds). ''Dinosaur Provincial Park, a spectacular ecosystem revealed, Part Two, Flora and Fauna from the park.'' Indiana University Press, Bloomington. Pp 367–397.</ref>

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'', 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]]''.<ref name=Talos/>

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.<ref name=Alberavenator>{{cite journal|last1=Evans|first1=D. C.|last2=Cullen|first2=T.M.|last3=Larson|first3=D.W.|last4=Rego|first4=A.|title=A new species of troodontid theropod (Dinosauria: Maniraptora) from the Horseshoe Canyon Formation (Maastrichtian) of Alberta, Canada|journal=Canadian Journal of Earth Sciences|volume=54|issue=8|date=2017|pages=813–826|doi=10.1139/cjes-2017-0034|bibcode=2017CaJES..54..813E|url=http://osf.io/b8mqe/}}</ref> 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]]''.<ref name=LS2017>{{cite journal|last1=van der Reest|first1=A. J.|last2=Currie|first2=P. J.|title=Troodontids (Theropoda) from the Dinosaur Park Formation, Alberta, with a description of a unique new taxon: implications for deinonychosaur diversity in North America|journal=Canadian Journal of Earth Sciences|volume=54|issue=9|date=2017|pages=919–935|doi=10.1139/cjes-2017-0031|bibcode=2017CaJES..54..919V|hdl=1807/78296|url=https://tspace.library.utoronto.ca/bitstream/1807/78296/1/cjes-2017-0031.pdf|hdl-access=free}}</ref> 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".<ref>{{cite journal |first1=D. J. |last1=Varricchio |first2=M. |last2=Kundrát |first3=J. |last3=Hogan |title=An Intermediate Incubation Period and Primitive Brooding in a Theropod Dinosaur |journal=Scientific Reports |volume=8 |date=2018 |issue=1 |pages=12454 |doi=10.1038/s41598-018-30085-6 |pmid=30127534 |pmc=6102251 |bibcode=2018NatSR...812454V }}</ref> This conclusion by Varricchio was agreed upon by Sellés and colleagues in their 2021 description of ''[[Tamarro]]''.<ref>{{cite journal |last1=Sellés |first1=A. G. |last2=Vila |first2=B. |last3=Brusatte |first3=S.L. |last4=Currie |first4=P.J. |last5=Galobart |first5=A. |title=A fast-growing basal troodontid (Dinosauria: Theropoda) from the latest Cretaceous of Europe |journal=Scientific Reports |year=2021 |volume=11 |issue=1 |page=4855 |doi=10.1038/s41598-021-83745-5|pmid=33649418 |pmc=7921422 |bibcode=2021NatSR..11.4855S |doi-access=free }}</ref> 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.<ref name=":0">{{Cite journal|last1=Cullen|first1=Thomas M.|last2=Zanno|first2=Lindsay|last3=Larson|first3=Derek W.|last4=Todd|first4=Erinn|last5=Currie|first5=Philip J.|last6=Evans|first6=David C.|date=2021-06-30|title=Anatomical, morphometric, and stratigraphic analyses of theropod biodiversity in the Upper Cretaceous (Campanian) Dinosaur Park Formation|url=https://cdnsciencepub.com/doi/abs/10.1139/cjes-2020-0145|journal=Canadian Journal of Earth Sciences|volume=58|issue=9|pages=870–884|language=en|doi=10.1139/cjes-2020-0145|url-access=subscription}}</ref> 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 insufficient justification to accept a proposed lumping of taxa lacking overlapping diagnostic materials.<ref name=":0" />

In 2025, Varricchio and colleagues proposed the troodontid material (MOR 553, "a collection of elements representing multiple individuals of differing ontogenetic stages") from the Jack's Birthday Site of the [[Two Medicine Formation]] as the neotype of ''T. formosus'' and considered ''[[Stenonychosaurus]]'' as a possible junior synonym of the former. While the specimen is not from the Judith River Formation, the authors considered the neotype proposal to be appropriate, since both the Judith River and Two Medicine Formation are "contemporary and confluent". However, because the holotype is not lost or destroyed, in accordance with the [[International Code of Zoological Nomenclature]] (ICZN) article 75.5, the authors are preparing a petition to the ICZN for a formal neotype designation.<ref>{{Cite journal|last1=Varricchio |first1=D. J. |last2=Hogan |first2=J. D. |last3=Gardner |first3=J. D. |year=2025 |title=Troodontid specimens from the Cretaceous Two Medicine Formation of Montana (USA) and the validity of ''Troodon formosus'' |journal=Journal of Paleontology |pages=1–22 |doi=10.1017/jpa.2024.67 |doi-access=free }} [[File:CC-BY icon.svg|50px]]  Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/  Creative Commons Attribution 4.0 International License].</ref>