Scientists have discovered a new species of tyrannosauroid—the group of dinosaurs that includes Tyrannosaurus rex. The finding offers new insights into how massive predators like T. rex evolved, transforming the tyrannosauroid tree of life, according to a study published today.

In the early 1970s, researchers unearthed two partial tyrannosauroid skeletons in the Bayanshiree Formation, a geological formation in Mongolia. Researchers originally believed the fossils were from a previously discovered tyrannosauroid species from China, Alectrosaurus olseni.

Paleontologists hailed the discovery of A. olseni as significant and said a comparative phylogenetic analysis was long overdue, the authors wrote in the study.

Recently, an international team of researchers led by paleontologists Jared Voris and Darla Zelenitsky from the University of Calgary finally reexamined the fossils. After comparing the morphology of the tyrannosauroid remains to similar species, they assigned the fossils to a new genus and species: Khankhuuluu mongoliensis. Their findings are published in Nature.

Khankhuuluu was a medium-sized predator—roughly the size of a grizzly bear—with a skull up to 27 inches long (70 centimeters) and a femur measuring nearly 26 inches (67 cm).

The iconic T. rex was just one member of a group of fearsome predatory dinosaurs known as Eutyrannosaurians. These dinosaurs—characterized by their enormous size, powerful jaws, short arms, and long legs—dominated the landscape around 66 million years ago.

However, how these massive creatures evolved from their smaller, ganglier ancestors remains unclear. This gap exists because of a scarcity of fossils from “mid-grade” tyrannosauroids, species that represent an evolutionary middle ground between the group’s earliest, smallest members and the colossal T. rex. According to the study, Khankhuuluu mongoliensis fits this middle ground in terms of size and shares features with both the massive Tyrannosaurini and the small, shallow-snouted Alioramini clades that came later.

In the tyrannosauroid evolutionary tree, K. mongoliensis is positioned just outside of the Eutyrannosaurians, having branched off about 90 million years ago during the Late Cretaceous period, the study suggests. The specimen is described as one of the “best-represented mid-grade tyrannosauroids” and serves as an “exemplar ancestral form for the clade,” the authors wrote. Importantly, Khankhuuluu has features that characterize juvenile eutyrannosaurians, but not adults, including a slender build and longer forearms than legs.

These findings highlight the importance of heterochrony—changes in the timing of developmental processes, such as delaying skull development—in tyrannosauroid evolution, indicating that Eutyrannosaurians went through a period of accelerated or prolonged growth as they matured.

Based on the geographic distribution and morphological similarities between Khankhuuluu and other tyrannosauroids, the study proposes that mid-sized dinosaurs like Khankhuuluu initially spread from Asia to North America. There, they evolved into several types of Eutyrannosaurians, including T. rex, that remained in North America as they diversified in both form and ecological role. Later, tyrannosauroids migrated back to Asia, giving rise to the Alioramini and Tyrannosaurini clades.

In yet another dispersal, Tyrannosaurini dispersed back to North America, where they evolved into the gigantic Tyrannosaurus species we know and love. They dominated as apex predators for 2 million years, until the end-Cretaceous mass extinction.