Researchers have uncovered the remains of a new species of dinosaur from the same family as the iconic Tyrannosaurus rex, casting new light on the evolutionary history of these animals. Unlike its giant relative, the new species—named Moros intrepidus (meaning “Harbinger of Doom”)—was unusually small for its kind, according to a study published in the journal Communications Biology.
The fossils—which were found in Emery County, Utah—suggest that the dinosaur would have stood only three or four feet tall at the hip when fully grown, and weighed around 170 pounds, said a team from the North Carolina Museum of Natural Sciences who authored the study.
By contrast, tyrannosaurus rex was one of the largest carnivorous dinosaurs to ever live, measuring around 40 feet in length and between 15-20 feet high. It could have weighed up to 9 tons, according to one study published in the journal PLOS ONE. But despite being much smaller, Moros would still have been a skilled hunter.
“Moros was lightweight and exceptionally fast,” lead author Lindsay Zanno said in a statement. “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.”
According to the researchers, not only are the fossils 15 million years older than any others belonging to the tyrannosauroid family that have been found in North America, but the dinosaur is also among the smallest of its kind from the Cretaceous Period (145 to 66 million years ago.)
The latest discovery helps to fill a 70-million-year-gap in the North American fossil record and improves our understanding of how small tyrannosaurids evolved into huge apex predators, such as T. rex.
“With a lethal combination of bone-crunching bite forces, stereoscopic vision, rapid growth rates, and colossal size, tyrannosauroids reigned uncontested for 15 million years leading up to the end-Cretaceous mass extinction event [which was caused by an asteroid that struck the Earth 66 million years ago]—but it wasn’t always that way,” Zanno said.
“Yet, for the vast majority of their more than 100 million-year-long evolutionary history, tyrannosauroids were small bodied, subordinate hunters, evolving in the shadow of archaic lineages that were already established at the top of the food chain,” the authors wrote in the study.
To understand how this transition from large to small occurred, researchers need to look at the fossil record. However, our knowledge has been limited by the lack of North American tyrannosauroid fossils dating from the 70-million-year “dark period,” which begins just before the start of the Cretaceous Period. This is despite the fact that researchers have uncovered several such specimens from this time in Asia.
In light of their findings, the authors suggest that tyrannosauroids remained small for a period of around 15 million years, before growing to sizes as large as T. rex over another period of 16 million years.
“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.”