Dominant Dinosaur That Lived Before T. rex Discovered

Everyone knows that Tyrannosaurus rex was the biggest and baddest thing around during the age of the dinosaurs. But what else was out there? What was the biggest thing before the T. rex? Scientists at the Field Museum and collaborators have uncovered the bones of another large predator in the Cedar Mountain Formation in Utah – one that would have filled the role as top predator of its time and kept T. rex’s ancestors in check! 

Despite its large size, this new dinosaur is not a close relative of the T. rex, or any other predator that dominated North America during the last 20 million years of the age of the dinosaurs. In fact, it belongs to a group of predatory dinosaurs whose fossils have to this day only been found on other continents, such as Asia, Australia, and South America. 

“This is the first evidence that these animals existed in North America,” said Pete Makovicky, Curator of Dinosaurs at the Field Museum. “Until a few years ago, these dinosaurs were not known from northern continents, so we thought that the dinosaurs that roamed northern continents were distinct from those that lived on southern ones due to continental drift. As it turns out, finding closely related dinosaurs on multiple continents suggests that these animals were better able to cross barriers like spreading oceans than we previously thought.”

This new dinosaur was over 30 feet long when it was alive, and weighed up to four tons! Scientists have named the dinosaur Siats meekerorum, in reference to a cannibalistic monster from Native American mythology and the John Caldwell Meeker family for their support of paleontological research at the Field Museum.  

The fossil record shows that dinosaurs other than tyrannosaurs (T. rex and its immediate kin) were the dominant predators in North America 145 million years ago, while T. rex and its relatives held that role around the end of the Cretaceous period, 85-65 million years ago. But because of a sparse fossil record in between, when and why that switch occurred has been a mystery until now.

However, Siats is around 100 million years old, which is right in the middle of this gap in the fossil record. Siats was doubtless the top predator of its day due to its large size, leading scientists to believe that the tyrannosaurs only became the dominant group of predators when that competition died out. Scientists found small Tyrannosaur fossils in the same rocks where Siats was found, supporting the idea that Tyrannosaur was excluded from the top predator niche.

Today, new dinosaurs are being uncovered at a faster rate than ever – about 20 a year! These new discoveries are quickly shedding light on the gap in the fossil record, and enabling scientists to understand the history and role of the dinosaurs during their time on earth.

Makovicky notes, "We have made more exciting discoveries including two new species of dinosaur, which will further illuminate how North American Cretaceous dinosaurs are related to those on other continents." These new finds await preparation and description, and more fieldwork is planned for 2014.

Fieldwork was conducted under permits through the Bureau of Land Management and supported by the Field Museum. Research was done in collaboration with Lindsay Zanno, Director of Paleontology at the North Carolina Museum of Natural Sciences.