Paleontologists investigated a remarkably preserved and nearly mature Tyrannosaurus rex skeleton found in the Hell Creek Formation of Montana. Their findings suggest that this specimen exhibits traits similar to the holotype of Nanotyrannus and confirm that this lineage is distinct from tyrannosaurus.

“This fossil turns decades of research on its head,” stated Dr. Lindsay Zanno, a researcher at North Carolina State University and the North Carolina Museum of Natural Sciences.

The researchers utilized tree ring data, spinal fixation, and developmental anatomy to show that the new specimen reached physical maturity at around 20 years.

Skeletal characteristics like larger forelimbs, more teeth, fewer caudal vertebrae, and distinctive skull neural patterns are established early in development and are biologically incompatible with tyrannosaurus rex.

“For Nanotyrannus to develop into a tyrannosaurus rex would contradict everything we know about vertebrate growth. That’s not just unlikely; it’s impossible,” remarked Dr. James Napoli, a researcher at Stony Brook University.

For years, paleontologists have incorrectly modeled tyrannosaurus growth and behavior using Nanotyrannus fossils.

This new evidence indicates that those studies were based on two entirely different species, suggesting that multiple tyrannosaurus species coexisted in the same ecosystem during the last million years before the asteroid impact.

As part of their study, Dr. Zanno and Dr. Napoli analyzed over 200 Tyrannosaurus fossils.

They found one skeleton, previously thought to represent a juvenile tyrannosaurus rex, that differed from the documented species Nanotyrannus lansensis, which is part of the renowned dueling dinosaur fossil.

The authors concluded that this specimen is a new species of Nanotyrannus, named Nanotyrannus letaeus.

The confirmation of Nanotyrannus implies that predator diversity during the final million years of the Cretaceous was far greater than previously understood, indicating that other small dinosaur species may also have been misidentified.

“This discovery provides a more complex and competitive view of the last days of the dinosaurs,” Dr. Zanno mentioned.

“With its massive physique, powerful bite, and stereoscopic vision, tyrannosaurus rex was indeed a fierce predator, but it did not dominate its environment without competition.”

“Darting together were Nanotyrannus — a slimmer, faster, more agile hunter.”

These findings were published in today’s edition of Nature.

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LE Zanno and JG Napoli. Nanotyrannus and tyrannosaurus coexisted at the end of the Cretaceous period. Nature published online October 30, 2025. doi: 10.1038/s41586-025-09801-6