Trierarchuncus

Trierarchuncus is a small alvarezsaurid theropod known from Montana's Hell Creek Formation, representing the geologically youngest known alvarezsaur in the world. Thriving in the latest Maastrichtian stage, it survived until the catastrophic asteroid impact that ended the age of the dinosaurs. Known principally from distinct manual unguals and limb elements, Trierarchuncus provides rare proof that alvarezsaurs inhabited North American ecosystems up to the very end of the Cretaceous. The hook-like claws found at various growth stages show progressive changes, becoming more robust and heavily furrowed for tendon attachments as the animal matured. Like its Asian and South American cousins, it was a tiny, cursorial biped that presumably used its specialized, heavy foreclaws to rip open rotting wood and insect nests to feed on colonial insects.

It is the youngest known alvarezsaurid in the fossil record, living right up to the K-Pg extinction.

Where and when

Trierarchuncus lived in the Late Cretaceous, from 67 to 66 million years ago (Maastrichtian age). Its remains come from United States, in the Hell Creek Formation.

The numbers

Length 0.9 m (estimates run from 0.8 to 1 m). Mass about 1.9 kg. Hip height about 0.35 m. That is 0.5 human-lengths, or 0.08 of a double-decker bus. These are estimates from fragmentary remains; fields marked estimated by the site are length, mass, hip height.

Compare Trierarchuncus with a bus Place on the timeline