Fossil skull thought to be teen T. rex was actually different dinosaur

By Stephen Beech

A fossil skull found 83 years ago originally thought to be a teenage Tyrannosaurus rex actually comes from a separate species of tyrannosaur, according to new research.

Scientists, who analyzed more than 200 tyrannosaur fossils, now say that Nanotyrannus was a "swifter and leaner" predator who lived alongside the King of the Dinosaurs around 70 million years ago.

The skull was found in the Hell Creek Formation of Montana in the United States in 1942.

When scientists classified it as the new species Nanotyrannus lancensis in 1988, the skull sparked decades of debate over whether the fossils were a teenage version of T. rex, or a distinct species.

But the new study reveals that throat bones accurately record maturity in dinosaurs, definitely showing Nanotyrannus was a fully grown, entirely distinct species from T. rex.

The findings, published in the journal Science, also reveal new insights into how the giant predators achieved such terrifying sizes so quickly.

American researchers examined the throat bone’s microscopic structure, comparing it to those of living birds, crocodiles, and extinct dinosaurs to establish that Nanotyrannus, while smaller, was a fully grown and distinct predator in an ecosystem more diverse than previously imagined.

Slightly less than half the size of their massive adult cousins, the research team say Nanotyrannus competed with juvenile T. rex for prey in Late Cretaceous North America.

Study lead author Dr. Christopher Griffin, of Princeton University in New Jersey, said: "The identity of the holotype specimen was the key piece in this debate.

"Discovering that this small skull was actually fully grown shows definitively that it is different from Tyrannosaurus rex.”

The team examined thin slices of fossilized hyoid bone under a microscope, measuring the organisation of the tissues to gauge the age of the animal in life and understand how quickly it grew.

Since no one had previously shown that hyoid bones preserve a useful record of an animal's growth, the researchers needed to prove that the throat bone could be a reliable gauge for maturity.

Dr. Griffin brought together a team of experts to build a database comparing the hyoids of several species, including living lizards, crocodiles, and birds to extinct dinosaurs.

He said: “To show that hyoid microstructure would work to test maturity status in Nanotyrannus, we first had to compile strong support for this method across many groups of living reptiles and extinct dinosaurs."

Dr. Zach Morris, from the Dinosaur Institute at the Natural History Museum of Los Angeles County, oversaw the sampling and analysis.

He said: “The growth series in our Dino Hall was critical to demonstrating that the hyoid in Tyrannosaurus showed the same kind of growth record as long bones.

“Having a growth series that had already been histologically analysed meant that we could compare the growth record in the hyoid and the growth record in the long bones and see that they show consistent signals even in these uniquely giant predators."

Dr. Morris says the findings paint a different picture of Late Cretaceous North America, which until recently saw T. rex as the lone apex predator before the mass extinction of dinosaurs.

He said: “It is remarkable that our study matches findings from other independent lines of evidence, including an analysis published last month, demonstrating that multiple species of tyrannosaurs lived alongside one another.

"It shows that we need to re-evaluate what we think these ecosystems looked like."

Dr. Morris added: “I am fascinated by the ways in which changes during development give rise to the skeletal features which distinguish dinosaurs, birds, crocodylians, and other vertebratess.

“This project was an exciting collaboration to study developmental patterns in the fossil record directly.”

Dr. Nate Smith, Dinosaur Institute curator, said: “This study also highlights the incredible potential of unique museum collections like our T. rex growth series, which not only inform the public but also provide rich ground for new scientific discoveries.”