Trierarchuncus

The first remains ware discovered in Montana in 1980 and it was informally known as the "Hell Creek alvarezsaur".[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.

The first part of the generic name, Trierarch, means "triarch" (the title of captain of the trireme in classical Greece); the second, uncus, is translated from Latin as "hook", thus forming Captain Hook, a reference to the villainous hook-handed pirate of Peter Pan. The specific name means "from the prairie" and refers to the plains of eastern Montana where the remains were discovered.[1]

Trierarchuncus was described by Fowler 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]

Trierarchuncus is estimated to be around 54 centimetres (21 in) tall and 140-150 centimeters (55-59 inches) in length when fully grown.[1]

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

Hell Creek fauna; Trierarchuncus in violet

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]