Ahvaytum Bahndooiveche

THE Earliest Dinosaur in the Northern Hemisphere

When most people imagine the dawn of the dinosaurs, they picture the Triassic landscapes of South America. A tiny fossil from Wyoming is forcing scientists to redraw that map. In early 2025 researchers announced Ahvaytum bahndooiveche, a chicken‑sized creature that roamed what is now the western United States about 230 million years ago. Its discovery suggests that dinosaurs were already present in the northern hemisphere at a very early stage of their evolution, challenging the long‑held idea that they arose exclusively in Gondwana (the southern supercontinent).

The fossils, collected from the Otto Road Formation near Casper, Wyoming, consist of leg bones, vertebrae and parts of the pelvis. At roughly the size of a modern chicken, Ahvaytum would not have been an imposing animal—its long, slender tail and lightweight bones point to a nimble lifestyle. Yet its significance is enormous: it represents the oldest known dinosaur from Laurasia (the northern supercontinent that included North America and Eurasia). Such early northern dinosaurs were thought to be missing because suitable fossil deposits were rare or undiscovered.

Ahvaytum has an unusual mixture of features. Its ankle bones and pelvis resemble those of other early dinosaurs, but the proportions of its limb bones suggest it was adapted for speed. Scientists are still debating exactly where it fits on the dinosaur family tree—some analyses place it near the base of the group that eventually gave rise to theropods like T. rex. What’s clear is that Ahvaytum pushes back the timeline for dinosaurs in the northern hemisphere and shows that early dinosaurs diversified quickly and spread widely across Pangaea. The discovery underscores the importance of fieldwork in under‑explored regions: sometimes the biggest evolutionary surprises come from the smallest fossils.

Credit: O.C. Marsh / Public domain

Sources: ScienceDaily article on Ahvaytum bahndooiveche