Editing Troodon (section)

===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>