A significant discovery in paleontology has emerged from a complete tyrannosaur skeleton, which challenges long-held beliefs about the growth of Tyrannosaurus rex. This fossil, part of the renowned “Dueling Dinosaurs” specimen found in Montana, features two dinosaurs engaged in a fierce battle: a Triceratops and a smaller tyrannosaur. Researchers have now confirmed that this smaller dinosaur is not a juvenile T. rex, as previously thought, but rather a fully grown Nanotyrannus lancensis.
“This fossil doesn”t just settle the debate. It flips decades of T. rex research on its head,” stated Lindsay Zanno, an associate research professor at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences. Zanno is also a co-author of the study published in Nature.
Utilizing growth rings, spinal fusion data, and developmental anatomy, the research team revealed that the specimen was approximately 20 years old and physically mature at the time of its death. Distinct skeletal characteristics, such as larger forelimbs, an increased number of teeth, fewer tail vertebrae, and unique skull nerve patterns, suggest these features were established early in development and are biologically incompatible with T. rex.
“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” added James Napoli, an anatomist at Stony Brook University and co-author of the study. “It”s not just unlikely – it”s impossible.”
The implications of this finding are substantial. For years, paleontologists relied on Nanotyrannus fossils to model the growth and behavior of T. rex. This new evidence indicates that those studies were based on fundamentally different species and suggests that multiple tyrannosaur species coexisted in the ecosystems leading up to the asteroid impact.
As part of their research, Zanno and Napoli analyzed over 200 tyrannosaur fossils. They identified one skeleton that was previously believed to be a juvenile T. rex but demonstrated distinct differences from the Dueling Dinosaurs Nanotyrannus lancensis. This led to the classification of the fossil as a new species, named N. lethaeus, a reference to the River Lethe in Greek mythology, symbolizing how this species was hidden in plain sight for decades.
The confirmation of Nanotyrannus as a distinct species indicates a greater diversity of predatory dinosaurs during the last million years of the Cretaceous period than previously recognized. This discovery may also imply that other small-bodied dinosaur species have been misidentified.
“This finding enriches our understanding of the competitive dynamics during the final days of the dinosaurs,” Zanno remarked. “While T. rex was a dominant predator, it did not reign supreme without rivals. Alongside it was Nanotyrannus, a more agile and swift hunter.”
This study, published in Nature, received support from the State of North Carolina, NC State University, the Friends of the North Carolina Museum of Natural Sciences, and the Dueling Dinosaurs Capital Campaign.
