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Tiny Tyrannosaur Discovery Shows T. Rex Had a Really Late Growth Spurt

Moros intrepidus was just four feet tall.

When an asteroid wiped out the dinosaurs 66 million years ago, the mighty Tyrannosaurus rex stood about 12 feet tall. But there were smaller tyrannosaurs that roamed the Earth before it. A newly discovered specimen has revised the timeline of when short tyrannosaurs became the “tyrant lizard king” we know as T. rex.

Paleontologists have discovered Moros intrepidus, a tiny new species of tyrannosaur in Utah who lived as far back as 96 million years ago. The dino was six or seven years old when it died. Even though it was almost a full-grown adult, it still only stood three or four feet tall, according to a study published February 21, 2019 in Communications Biology.

Moros was lightweight and exceptionally fast,” said Lindsay Zanno, a paleontologist at North Carolina State University and the paper’s lead author, in a press release. “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.”

Moros intrepidus

This little reptile helps us understand how tyrannosaurs evolved between the Jurassic period (201.3 million to 145 million years ago) and the Cretaceous period (145 million to 66 million years ago). Scientists know that tyrannosaurs were smaller during the Jurassic period, but it’s still a mystery as to when they grew into giant, T. rex-sized creatures.

The Utah dino shows that the transition from small to large tyrannosaurs happened later than we thought. It is the oldest tyrannosaur from the Cretaceous period discovered in North America, living just 16 million years before T. rex giants took over. That means tyrannosaurs had a pretty fast growth spurt in a relatively short amount of time. And it also means that paleontologists are eager to find more dino remains from this period to see how this actually happened.

“When and how quickly tyrannosaurs went from wallflower to prom king has been vexing paleontologists for a long time,” said Zanno in the press release. “The only way to attack this problem was to get out there and find more data on these rare animals."

As a nod to the fact that the little tyrannosaur in Utah was soon taken over by larger creatures, paleontologists have given it a decidedly emotive genus name: Moros, which the paleontologists write is Greek for “the embodiment of impending doom.”

But the Utah dino’s story isn’t all doom and gloom. Its full species name, Moros intrepidus, signals the “intrepid” way it seems to have migrated from Asia to North America via a land bridge.

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