A Major Dinosaur Discovery

North America’s oldest dinosaur was the size of a chicken. Gabriel Ugueto

How and when did dinosaurs first emerge and spread across the planet more than 200 million years ago? That question has been a source of debate among paleontologists faced with fragmented fossil records. The mainstream view has held that the reptiles emerged on the southern portion of the ancient supercontinent Pangea, called Gondwana, millions of years before spreading to the northern half, named Laurasia.

But now, a newly described dinosaur whose fossils were uncovered by UW–Madison paleontologists is challenging that narrative, with evidence that the reptiles were present in the northern hemisphere millions of years earlier than previously known.

The UW team has been analyzing the fossil remains since they were discovered in 2013 in present-day Wyoming, an area that was near the equator on Laurasia. The creature has been named Ahvaytum bahndooiveche, and its fossils are estimated to be around 230 million years old.

“We have, with these fossils, the oldest equatorial dinosaur in the world — it’s also North America’s oldest dinosaur,” says Dave Lovelace PhD’12, a research scientist at the UW Geology Museum who co-led the work with graduate student Aaron Kufner ’16, MS’21, PhDx’25. (See our story Fabulous Fossils.)

It took years of careful work by Lovelace and his colleagues to analyze the fossils, establish them as a new dinosaur species, and determine their estimated age.

“It was basically the size of a chicken but with a really long tail,” says Lovelace. “We think of dinosaurs as these giant behemoths, but they didn’t start out that way.”

Lovelace and his colleagues performed high-precision radioisotopic dating of rocks in the formation that held the dinosaur’s fossils. They also found an early dinosaur-like track in even older rocks, demonstrating that dinosaurs or their cousins were already in the region a few million years earlier.

“We’re showing that the ideas we’ve held for so long — supported by the fragmented evidence we had — weren’t quite right,” Lovelace says. “We now have this piece of evidence that shows dinosaurs were here in the northern hemisphere much earlier than we thought.”

Published in the Spring 2025 issue