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Trierarchuncus; Temporal range: Late Cretaceous, 66.2–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Alvarezsauridae
Subfamily:	†Parvicursorinae
Genus:	†Trierarchuncus; Fowler et al., 2020
Species:	†T. prairiensis
Binomial name
	†Trierarchuncus prairiensis; Fowler et al., 2020

Trierarchuncus (meaning "captain hook," after its single-clawed hands) is a monotypic genus of alvarezsaurid theropod which includes a single species, Trierarchuncus prairiensis, which is known from fossils found in deposits of the Hell Creek Formation in Montana.^[1]^[2] It is the youngest known alvarezsaurid and one of the last non-avian dinosaurs, going extinct during the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago.^[3]^[1]^[4]

Discovery and naming

The first remains were discovered in Montana in 1980 and it was informally named as the "Hell Creek alvarezsaur" by Hutchinson and Chiappe (1998).^[5] The then unnamed species was not mentioned again until it was mentioned briefly in the 2018 Society of Vertebrate Paleontology abstract book. The species and genus were scientifically described by Denver Fowler and colleagues in 2020 based on three claw phalanges from MD-I-2, including the holotype MOR 6622, the distal end of a radius and fragmentary metatarsal.^[1] New manual unguals from individuals of different maturity were referred to this taxon by Freimuth & Wilson (2021).^[6]

The first part of the generic name, Trierarch, means "triarch" (the title of captain of the trireme in classical Greece), while the second, uncus, is translated from Latin as "hook"; it can thus be translated as "captain hook", although its describers do not explicitly make the connection with the Peter Pan character. The specific name means "from the prairie" and refers to the plains of eastern Montana where the remains were discovered.^[1]

Description

Trierarchuncus was described by Fowler and colleagues based upon several differences, especially on the three phalanges, the radius and the metatarsals. Trierarchuncus is known from its arms, feet and toes. Like its relatives, Trierarchuncus would have been feathered, with short arms bearing one clawed digit, a bird-like head and long legs.^[1] Ontogenetic change based on unguals of different maturity indicate allometric growth.^[6]

Classification

Cladogram according to Fowler et al., 2020, with clade names added by definition:^[1]

Alvarezsauroidea

Aorun

Haplocheirus

	Tugulusaurus

	Xiyunykus

	Shuvuuia

	Ceratonykus

	Kol

	Linhenykus

	Mononykus

	Trierarchuncus

	Parvicursor

	Albinykus

	Xixianykus

Paleoecology

Trierarchuncus represents one of the few known alvarezsaurids from North America.^[5]^[2]^[1] At 66 million years old, it is the youngest known alvarezsaurid and is one of the youngest non-avian dinosaurs in general, like its contemporaries Tyrannosaurus and Triceratops. Additionally, another Hell Creek alvarezsaurid, called "Ornithomimus" minutus, is known, but Fowler et al. did not refer it to Trierarchuncus.^[1]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h Denver W. Fowler; John P. Wilson; Elizabeth A. Freedman Fowler; Christopher R. Noto; Daniel Anduza; John R. Horner (2020). "Trierarchuncus prairiensis gen. et sp. nov., the last alvarezsaurid: Hell Creek Formation (uppermost Maastrichtian), Montana". Cretaceous Research. 116: Article 104560. doi:10.1016/j.cretres.2020.104560. S2CID 225630913.
^ ^a ^b "Dinosaur distribution (Late Cretaceous; North America; Montana)." Weishampel, et al. (2004). Page 584.
^ Fortey, Richard (1999). Life: A natural history of the first four billion years of life on Earth. Vintage. pp. 238–260. ISBN 978-0-375-70261-7.
^ "Badlands - Research - 2020 new alvarezsaurid species". Dickinson Museum Center. Retrieved 2020-07-17.
^ ^a ^b Hutchinson and Chiappe, (1998). The first known alvarezsaurid (Theropoda: Aves) from North America. Journal of Vertebrate Paleontology. 18(3), 447–450.
^ ^a ^b Freimuth, William J.; Wilson, John P. (2021). "New manual unguals of Trierarchuncus prairiensis from the Hell Creek Formation, Montana, and the ontogenetic development of the functional alvarezsaurid hand claw". Cretaceous Research. 119: Article 104698. doi:10.1016/j.cretres.2020.104698. S2CID 228830237.