New ‘miniature T rex’ rewrites the history of the world’s largest predator

A new specimen of one of the most controversial dinosaur species has the potential to upend decades of T rex research.

Nanotyrannus, the “miniature T rex,” has been at the center of one of the fiercest debates in paleontology. Scientists have long wondered whether Nanotyrannus is a distinct species or simply a young T rex.

Controversy was sparked in 1999 when the only known Nanotyrannus fossil belonged to a juvenile. Since then, more complete fossils have failed to produce conclusive answers because they were also found to be juvenile.

But the debate over the identity of Nanotyrannus could finally be settled. A new fossil specimen, described in the journal Natureis the irrefutable proof that the researchers were looking for: an adult Nanotyrannus.

Known as the dueling dinosaurs, this fossil preserves a nearly complete Nanotyrannus and Triceratops buried together. They appear frozen in combat (it remains to be seen whether they were actually fighting when they were buried in terrestrial sediments).

Although the fossil was discovered in Montana, USA, in 2006, it was privately owned until the North Carolina Museum of Natural Sciences purchased it in 2020. Now available to scientists, the true nature of this remarkable fossil can be revealed for the first time.

Researchers confirmed that Nanotyrannus is a distinct miniature type of tyrannosaur by demonstrating that this specimen belonged to a nearly full-grown adult. The age and maturity of dinosaurs can be assessed by examining the inside of their bones.

Dinosaurs grew in faster and slower growth cycles that produced distinct layers of bone. When opened and examined under a microscope, these marks can be counted like the rings of a tree.

Using this method, researchers were able to determine that the dueling dinosaurs Nanotyrannus was at least 14 years old when it died. The researchers also found that its growth rate had slowed significantly in its later years, indicating that this individual had almost reached its maximum size.

So how big was this miniature T rex? Nanotyrannus is only about a tenth the size of an adult T rex. However, as one of the largest predators to ever walk the Earth, the T rex would make most animals appear small. The dueling dinosaur Nanotyrannus is more than four meters long and weighs more than 700 kg, as much as some of the largest polar bears.

Other Nanotyrannus specimens are even larger. The nearly complete skeleton known as Jane, discovered in 2001 also in Montana, is estimated to weigh more than a ton, larger than any terrestrial predator alive today.

Researchers found enough differences in the shape of bones in the skulls of the dueling fossil dinosaurs and the larger Jane to separate them into two different species: Nanotyrannus lancensis and the new name Nanotyrannus lethaeus.

Besides small size, another characteristic that researchers used to distinguish Nanotyrannus from T rex is the number of teeth. Despite its much smaller mouth, Nanotyrannus could undoubtedly have a powerful bite with its more than 60 teeth. The T rex had 40 to 50 teeth in its jaws.

The teeth themselves are also different. Nicknamed “deadly bananas,” the T rex’s teeth are curved and serrated like steak knives. These unique teeth are perfect for slicing flesh and could crush bones. In contrast, some Nanotyrannus teeth are straight, chisel-shaped, and without serrations, more closely resembling those of other types of carnivorous dinosaurs.

T rex had tiny arms, the source of many dinosaur jokes and impressions. Nanotyrannus does not share this feature. Its arms are almost the same size as those of the T rex, although they belong to an animal ten times smaller.

Nanotyrannus is overall more slender than the large T rex, with proportionately longer legs to match its longer arms. The foot bones of the small dueling dinosaurs Nanotyrannus are almost as long as those of the largest known T rex. Research suggests that while the T rex couldn’t do anything faster than a walk, Nanotyrannus was well adapted to running and pursuing prey.

In addition to confirming the existence of this long-controversial species of dinosaur, this discovery upends decades of research on T rexes. Much of what scientists think about the life and growth of the T rex is based on the assumption that many fossils of lanky “adolescent” tyrannosaurs grew up to become the well-known bulky giants. In light of this discovery, scientists must re-evaluate the life of a young T rex and return to the search for true juvenile T rex fossils.

It has been widely believed that the T rex was the only dinosaur of its type living in the United States during the Last Dinosaur Era. Some researchers have proposed that the T rex was so dominant that other large predators could not live alongside it, with its own juveniles outcompeting all other medium-sized predators. This total domination is now called into question.

The T rex has now been joined by not one but two species of smaller carnivorous dinosaurs, hinting at a more complex and diverse ecosystem that thrived in the United States toward the end of the dinosaur reign.

Although arguably the best-known and best-studied dinosaur, the T rex and its close relatives continue to surprise us from beyond the grave. There is still much to learn about the largest and fiercest predators to ever exist on Earth.

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