Trierarchuncus

The first remains were discovered in Montana in 1980 and it was informally named as the "Hell Creek alvarezsaur" by Hutchinson and Chiappe (1998). 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. Potential specimens identified as cf. T. prairiensis (YPM VP 56916, YPM VP 57236 and YPM VP 57402) have been recovered from the Lance Formation in Wyoming. ==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. Ontogenetic change based on unguals of different maturity indicate allometric growth. ==Classification==

Cladogram according to Fowler et al., 2020, with clade names added by definition: {{clade|style=font-size:90%;line-height:90% ==Paleoecology==

; Trierarchuncus in light pink Trierarchuncus represents one of the few known alvarezsaurids from North America. 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. == References ==