The popular image of a tyrannosaur is the colossal Tyrannosaurus rex, a dominant predator of ancient North America. The discovery of a much smaller, earlier relative, however, has provided new information on their evolution. This tiny ancestor, named Moros intrepidus, was a swift, deer-sized hunter that lived in the shadow of larger predators millions of years before T. rex would claim the throne.
The Discovery in Utah
Paleontologists led by Lindsay Zanno explored the Cedar Mountain Formation in Utah, an area rich in fossils from the dawn of the Late Cretaceous. In 2013, their decade-long search paid off when team members spotted limb bones protruding from a hillside. After years of excavation, the fossils were prepared for study.
The unearthed remains were sparse, consisting of a nearly complete right hind limb and a few teeth. From these fragments, including a femur, tibia, and metatarsals, the team conducted anatomical research. By comparing the bone structure to other known theropods, they confirmed it was a new tyrannosaur species, which they formally named Moros intrepidus in 2019. The discovery was significant because the fossils were found in the same area as Siats meekerorum, a massive carcharodontosaur, hinting at the ecosystem dynamics of the time.
Anatomy of a Miniature Predator
Moros intrepidus was a contrast to its famous, massive descendant. Standing only three to four feet tall at the hip and weighing an estimated 170 pounds (78 kg), it was roughly the size of a modern mule deer. Analysis of the bone microstructure indicated the individual discovered was likely six or seven years old and nearing its full adult size when it died.
Its build was slender and lightweight, designed for speed and agility rather than brute force. The foot bones were thin, with proportions more akin to fast-running ornithomimids than to later, heavy-set tyrannosaurs. This anatomy suggests Moros was a predator of small prey, able to run down targets while avoiding larger apex predators. The name Moros intrepidus reflects its role in its lineage; Moros is Greek for the “harbinger of doom,” and intrepidus is Latin for “intrepid,” referencing the rise of its descendants.
Solving a Paleontological Puzzle
The discovery of Moros intrepidus helped fill a major gap in the North American fossil record. Paleontologists had long noted a 70-million-year gap between small, primitive tyrannosaurs (150 million years ago) and giant apex predators like T. rex (81 million years ago). Dated to 96 million years old, Moros lands squarely within this period, making it the oldest Cretaceous tyrannosaur species found on the continent.
This finding reveals that for millions of years, tyrannosaurs remained small, secondary predators living in the shadows of dominant carnivores like the allosaurs. The fossils suggest that tyrannosaurs of this era were not yet the rulers of their ecosystems.
Phylogenetic analysis of the Moros fossils revealed its closest relatives were from Asia. This indicates that the ancestors of T. rex likely migrated to North America at least 30 million years before the giant tyrannosaurs came to dominate. Once larger predators like the allosaurs went extinct, these smaller tyrannosaurs were able to take over. In a short evolutionary span of about 10 to 15 million years, they rapidly increased in size, giving rise to the giants that ruled the end of the Cretaceous period.