RALEIGH, NC — The discovery of pint-sized dinosaur — at least compared to the widely known Tyrannosaurus rex believed to to weigh as much as 50 pounds and stand as tall as a six-story building — is closing a 70-million-year gap in scientists’ understanding of how the tyrant dinosaur T. rex evolved and came to rule in North America.
A team of researchers led by North Carolina State University paleontologist Lindsay Zanno discovered fossils of the diminutive relative of Moros intrepidus — which means “harbinger of doom” after the Greek god Moros — in Utah. The small tyrannosaur lived about 96 million years ago in a lush delta area of what is now Utah during the Cretaceous period. It stood about three or four feet tall at its hip.
“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,” according to Zanno, the head of paleontology at the North Carolina Museum of Sciences and the lead author of a paper describing the research.
“Early in their evolution, tyrannosaurs hunted in the shadows of archaic lineages such as allosaurs that were already established at the top of the food chain,” Zanno said in a news release.
Medium-sized, primitive tyrannosaurs dating back to the Jurassic period about 150 million years ago have been found throughout North America. But by the time the Cretaceous period rolled around about 81 million years ago, they had evolved into the ginormous apex predators most people associate with dinosaurs.
The lack of fossil records between the two time periods has stymied paleontologists.
“When and how quickly tyrannosaurs went from wallflower to prom king has been vexing paleontologists for a long time,” says Zanno. “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 nearly a decade hunting for dinosaur fossils in rocks deposited during the Late Cretaceous period before they found teeth and hind leg from the new tyrannosaur. The fossils were found in the same area where Zanno and her team had previously discovered Siats meekerorum, a giant meat-eating carcharodontosaur that lived during the same period.
Zanno thinks the Moros was about 7 years old when it died, and it was nearly full-grown.
Though small — about the size of a mule deer — the tyrannosaur was nothing to mess with in its day.
“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.
“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 says. “We now know it took them less than 15 million years to rise to power.”
The discovery also answered questions about the origin of T. rex’s lineage in North America. When scientists placed Moros within the family tree of tyrannosaurs, they discovered that its closest relatives were from Asia.
“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 says. “Moros signals the establishment of the iconic Late Cretaceous ecosystems of North America.”
The research, which was published in the journal Communications Biology, was supported in part by Canyonlands Natural History Association. Lecturer Terry Gates, postdoctoral research scholar Aurore Canoville and graduate student Haviv Avrahami from NC State, as well as the Field Museum’s Peter Makovicky and Ryan Tucker from Stellenbosch University, contributed to the work.