Tiny T. rex relative among earliest Cretaceous tyrannosaurs in N. America

What scientists describe as a miniature tyrannosaur roamed the continent about 96 million years ago, and predated the ferocious beast most know as T. rex.

By Brooks Hays
The newly discovered Moros intrepidus was roughly the size of modern mule deer. Photo by Jorge Gonzalez/NC State
The newly discovered Moros intrepidus was roughly the size of modern mule deer. Photo by Jorge Gonzalez/NC State

Feb. 22 (UPI) -- Scientists have discovered a new tyrannosaur species, a miniature T. rex that roamed North America some 96 million years ago.

The new species, Moros intrepidus, is the oldest Cretaceous tyrannosaur yet found in North America.


The early fossil record has provided evidence of a diversity of medium-sized, primitive tyrannosaurs dating to the Jurassic period, around 150 million years ago. And 80-million-year-old Cretaceous deposits have offered plenty of fossil evidence of large tyrannosaurs.

Until now, paleontologists have learned little of the 70-million-year gap separating the two fossil-rich deposits. The newly discovered species could help scientists better understand how T. rex and its ferocious peers became apex predators.

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"When and how quickly tyrannosaurs went from wallflower to prom king has been vexing paleontologists for a long time," Lindsay Zanno, paleontologist at North Carolina State University, said in a news release. "The only way to attack this problem was to get out there and find more data on these rare animals."

Zanno and her colleagues found the new fossil evidence, including teeth and a hind limb, hiding in the rocks of Utah's Upper Cedar Mountain Formation -- deposits dating to the beginning of the Late Cretaceous.


Researchers described the Moros intrepidus fossil in the journal Communications Biology.

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Unlike the tyrannosaurs that came later, like T. rex, the newly discovered species was relatively small. The mini tyrannosaur was roughly the size of a modern mule deer, measuring just three to four feet tall from the hip.

Despite, Moros intrepidus's diminutive size, scientists claim the species was still a fearsome predator.

"Moros was lightweight and exceptionally fast," Zanno said. "These adaptations, together with advanced sensory capabilities, are the mark of a formidable predator. It could easily have run down prey, while avoiding confrontation with the top predators of the day."

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According to Zanno, the new species offers proof that tyrannosaurs had all the tools to become apex predators. All that was lacking was size.

"Although the earliest Cretaceous tyrannosaurs were small, their predatory specializations meant that they were primed to take advantage of new opportunities when warming temperatures, rising sea-level and shrinking ranges restructured ecosystems at the beginning of the Late Cretaceous," Zanno said. "We now know it took them less than 15 million years to rise to power."

When scientists situated Moros intrepidus on the tyrannosaur family tree, they determined the species' relatives hail from Asia. Research last year showed the land bridge connecting Siberia and Alaska served as a migrational superhighway for dinosaurs -- just as it did millions of years later for early humans.


"T. rex and its famous contemporaries such as Triceratops may be among our most beloved cultural icons, but we owe their existence to their intrepid ancestors who migrated here from Asia at least 30 million years prior," Zanno said. "Moros signals the establishment of the iconic Late Cretaceous ecosystems of North America."

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