#tyranosaurs

Even the giant Tyrannosaurus rex had humble beginnings. A new fossil shows that an early ancestor was only about the size of a deer. Its discovery helps fill a 70-million-year gap in the evolution of giant tyrannosaurs such as T. rex.

Lindsay Zanno is a paleontologist at North Carolina State University in Raleigh. She and her colleagues dug for 10 years around Emery County in Utah. They were searching for clues to solve a longtime dino mystery: When and how did tyrannosaurs get their famous bulk?

Early tyrannosaurs were much smaller. Teeth from petite species have been found in rocks in North America dating to around 150 million years ago. At that time, in the Late Jurassic Period, large allosaurs topped the food chain. The next time tyrannosaurs showed up in the North American fossil record was 70 million years later, during the Cretaceous Period. By then, they had become the colossal top predators best known today.

Explainer: How a fossil forms

Zanno and her team were looking for clues to what happened in between when they found a long, thin leg bone. It dated to around 96 million years ago. They determined the fossil came from a new species of tyrannosaur. It’s the oldest one known from the Cretaceous. They named the species Moros intrepidus, or “omen of doom.”

M. intrepidus is one of the smallest tyrannosaurs from the Cretaceous. Analyses of the fossil leg show it would have stood about 1.2 meters (4 feet) tall at the hip. It probably weighed about 78 kilograms (172 pounds). That’s about the size of a mule deer. The find was described February 21 in Communications Biology.

The bone’s long, thin shape suggests M. intrepidus was a swift runner. Later titanic tyrannosaurs were likely far less speedy.