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Tiny tyrannosaur fossil discovery changes the dinosaur timeline

CNN logo CNN 2/21/2019 By Ashley Strickland, CNN
a dinosaur in a body of water: Moros intrepidus © Courtesy Jorge Gonzalez Moros intrepidus

Tyrannosaurus rex wasn't always the king of the dinosaurs. Before they became towering predators, tyrannosaurs started out much smaller, and a newly discovered fossil is helping fill the gap between those two extremes.

The fossil findings are detailed in a study published Thursday in Communications Biology.

The dinosaur fossil was found in Utah, where it lived 96 million years ago in a lush delta during the Cretaceous period. It's been named Moros intrepidus, which means "harbinger of doom." The dinosaur lived at the end of the allosaurs' reign at the top of the food chain and before Tyrannosaurus rex arrived.

It's now the oldest tyrannosaur from the Cretaceous period found in North America.

Medium-size tyrannosaur fossils have been found from the Jurassic period, about 150 million years ago. And then, about 81 million years ago during the Cretaceous, tyrannosaurs grew into giant predators and replaced allosaurs as the top of the food chain.

So what happened in between? Moros is helping researchers fill that 70 million-year gap, as well as provide a portrait of tyrannosaur lineage in North America. Moros links the earliest, smaller tyrannosaurs to Tyrannosaurus rex.

"With a lethal combination of bone-crunching bite forces, stereoscopic vision, rapid growth rates, and colossal size, tyrant dinosaurs reigned uncontested for 15 million years leading up to the end-Cretaceous extinction -- but it wasn't always that way," said Lindsay Zanno, lead study author and paleontologist at North Carolina State University, in a statement. "When and how quickly tyrannosaurs went from wallflower to prom king has been vexing paleontologists for a long time. The only way to attack this problem was to get out there and find more data on these rare animals."

Zanno and her team spent a decade searching for fossils from the Late Cretaceous period. They recovered teeth and a hind limb consisting of a femur, a tibia and parts of a foot belonging to Moros in the same area where Zanno found the fossil of a giant carnivorous carcharodontosaur.

But Moros stood between 3 and 4 feet tall. The dinosaur they found was 7 years old when it died, a nearly full-grown adult that would have weighed around 172 pounds. The elongated leg and foot bones indicated that it would be a great runner.

"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."

This allowed Moros to be a survivor as the environment shifted and changed. For 15 million years, tyrannosaurs were restricted to this smaller size before evolving into giants (about 12 feet tall and 11,000 to 15,500 pounds) over a 16 million-year period.

"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."

Moros is most closely related to tyrannosaurs from Asia, which helped the researchers trace the dinosaurs' lineage. This means Moros crossed the Alaskan land bridge during the Early Cretaceous to reach North America.

"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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