Paleontologists have unveiled a remarkable new species of tyrannosauroid dinosaur from the Cretaceous period: a small relative of Tyrannosaurus rex. The discovery is described in the journal Communications Biology.
Medium-sized, primitive tyrannosaurs have been found in North America dating from the Jurassic period (around 150 million years ago).
By the Cretaceous period (81 million years ago) North American tyrannosaurs had become the enormous, iconic apex predators we know and love.
The fossil record between these time periods has been a blank slate, preventing paleontologists from piecing together the story behind the ascent of tyrannosaurs in North America.
“When and how quickly tyrannosaurs went from wallflower to prom king has been vexing paleontologists for a long time,” said Dr. Lindsay Zanno, a paleontologist at North Carolina State University and head of paleontology at the North Carolina Museum of Sciences.
“The only way to attack this problem was to get out there and find more data on these rare animals.”
The newly-discovered tyrannosaur, named Moros intrepidus, lived about 96 million years ago in the lush, deltaic environment of what is now Utah.
It is the oldest Cretaceous tyrannosaur species yet discovered in North America, narrowing a 70-million-year gap in the fossil record of tyrannosauroid dinosaurs on the continent.
“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,” Dr. Zanno said.
“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.”
With a 3.9-foot (1.2 m) length and 78-kg mass, Moros intrepidus ranks among the smallest Cretaceous tyrannosauroids.
Dr. Zanno and colleagues estimate that the individual was over seven years old when it died, and that it was nearly full-grown.
“Moros intrepidus was lightweight and exceptionally fast,” Dr. 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.”
The bones of the new tyrannosaur also revealed the origin of T. rex’s lineage on the North American continent.
When the scientists placed Moros intrepidus 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,” Dr. Zanno said.
“Moros intrepidus signals the establishment of the iconic Late Cretaceous ecosystems of North America.”
Lindsay E. Zanno et al. 2019. Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record. Communications Biology 2, article number: 64; doi: 10.1038/s42003-019-0308-7