Trierarchuncus prairiensis gen. et sp. nov., the last alvarezsaurid: Hell Creek Formation (uppermost Maastrichtian), Montana

Abstract

The enigmatic Alvarezsauria (Dinosauria: Theropoda) are characterized by extremely short forelimbs with a single functional digit bearing a large, robust ungual. Alvarezsauria are first recorded from the Jurassic of China, but are otherwise mostly known from the Cretaceous of South America and Asia, including a number of relatively complete skeletons. North America has yielded only a fragmentary skeleton from the lower Maastrichtian Horseshoe Canyon Formation, Alberta, and a pubis, partial ischium, and metatarsal from the upper Maastrichtian Hell Creek Formation, Montana, and Lance Formation, Wyoming. Here we describe new alvarezsaurid material (Trierarchuncus prairiensis gen. et sp. nov.) from the Hell Creek Formation, Montana, comprising a metatarsal III, distal end of a radius, and three manual digit (MD)-I unguals, which form a progressive size series. One MD-I ungual is the most complete yet described, and is much more curved than typically depicted for Alvarezsauridae. Manual D-I unguals are of particular interest as they undergo a number of changes within the clade, including enclosure of the ventral blood vessel groove, development of a ventral sulcus, and increased robusticity and rugosity. Comparison among the new specimens suggests that these features also develop ontogenetically, which may have taxonomic implications. Stratigraphic data shows that alvarezsaurids occur through most of the ~85 m thick Hell Creek Formation, with the uppermost specimen occurring ~10 m below the upper contact with the Fort Union Formation. As such these are the youngest known alvarezsaurid remains and demonstrate that the clade survived at least until ~1–200 kyrs before the Cretaceous–Paleogene mass extinction.