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A Juvenile Tyrannosaurus rex, or Not? A New Study Challenges an Ongoing Debate

New research may explain a debate decades in the making of two T. rexes that may be instead a smaller species.

By Elizabeth Gamillo
Jan 5, 2024 8:30 PMJan 8, 2024 8:07 PM
Low-Res Nanotyrannus versus baby T. rex
An artist representation of a Nanotyrannus attacking a teen T. rex. (Credit: Raul Martin)


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Fossils once thought to be teen Tyrannosaurus rex fossils may instead be a separate, smaller Tyrannosaurus species. When scientists reexamined the bones of the fossils, they found that the growth rings and simulated weights did not match those modeled for a T. rex, but instead were closer to a full grown Nanotyrannus lancensis.

The findings published in Fossil Studies showed that as the dinosaur's growth slowed, the rings found towards the outer edge of the bone were more closely packed together. This indicated that the dino's growth was slowing down and it was most likely full-grown. Young dinosaurs would still show rapid bursts of change in their bones at a young age.

"If they were young Trex they should be growing like crazy, putting on hundreds of kilograms a year, but we're not seeing that," said Nick Longrich, a paleontologist at the University of Bath in the U.K., in a statement.

Do the Fossils Belong to a Young T. rex, or Not?

The new research challenges a different study from 2020 in Science Advances that suggests that the fiercely debated fossil status was not a separate species but a teenager T. rex close to starting a rapid shift in growth.

To gather the age of the elusive dinosaur fossils and to see if they belong to a young T. rex or an entirely different dinosaur, researchers of the new study looked at growth rings inside the Nanotyrannus bones. Through computer modeling, the paleontologists could simulate that at this bone growth rate, the animals would have reached anywhere between 900 and 1500 kilograms at total growth, or about 15 percent the size of a T. rex, according to a press release.  

"We tried modeling the data in a lot of different ways, and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young Trex," said Longrich.

The researchers also argue that if Nanotyrannus were a baby T. rex, it would have shown more distinctive features like those seen in a full-grown T. rex. 

"In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. And Nanotyrannus just doesn't look anything like a Trex," said Longrich in a press release.

Read More: Is the T. Rex Three Different Species?

Why Is N. lancensis Considered a Juvenile T. rex?

'Casper', T. rex skull on display at the Berlin Natural History Museum. (Credit: BERK OZDEMIR / Shutterstock.com)

Some experts argue the fossil is a juvenile T. rex because they say that the skulls of the N. lancensis are similar to the skull features of other juvenile tyrannosaurid species.

In the early 2000s, experts excavated well-preserved limbs and skulls of two Nanotyrannus specimens. From these bones, other researchers argue that the Nanotyrannus fossil, dubbed 'Jane' by the Burpee Museum in Illinois, was around 11 years old when she died.

Like tree rings, dinosaur bones form concentric rings in their bone's cross-section. By looking at the rings, paleontologists can read them to determine the dinosaur's age when it died.

Thin section from the Tibia of fossil specimen BMRP 2002.4.1 'Jane' (Credit: Holly N. Woodward et al. ,Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy “Nanotyrannus” and supports ontogenetic niche partitioning in juvenile Tyrannosaurus.Sci. Adv.6,eaax6250(2020).DOI:10.1126/sciadv.aax6250)

From analyzing growth rings found in the femur and tibia bones, the team concluded Jane was a juvenile T. rex, as a T. rex is full-grown at around 20 years old, according to the Cleveland Museum of Natural History. The other specimen was dubbed 'Petey.'

Read More: What a Tyrannosaurus Rex Skull Tells Us About Its Intelligence

When Was N. lancensis Discovered?

An artist's representation of Nanotyrannus. (Credit: Catmando/Shutterstock)

In the 1940s, paleontologists first discovered the skull of N. lancensis within the Lance Formation in Montana. Since then, it has been hotly debated whether the fossil is its own separate genus or a small, young T. rex.

The fossil has a history of reclassification. The first time was when it was discovered in 1942. Paleontologists first thought it was a Gorgosaurus, but it was later renamed in 1988 to Nanotyrannus, meaning pygmy or small, according to the Australian Museum.

Read More: How Many Tyrannosaurus Rex Walked the Earth?

What Are Some Similarities and Differences Between N. lancensis and T. rex?

(Credit: Nick Longrich)

Nanotyrannus's skull is squarish, with a slender muzzle and sharp teeth. According to the Australian Museum, the fossil also features more extended eye sockets. The head is similar to that of a T. rex, with differences of size in length and a smaller jaw.

Both specimens would have lived in the same western part of the U.S. during the Late Cretaceous Period.

Read More: The Tyrannosaurus rex May Have Had More Brains Than You Think

So, What's The Verdict? 

More specimens are needed for researchers to gather more evidence and data to conclude whether these fossils are a baby T. rex or not. Until then, the baby T. rex debate will continue.

A juvenile T. rex specimen dubbed 'Chomper' is being auctioned for 20 million, according to the New York Times. Specimens like these could be the key to settling the debate, but only if museums can use them for study. 

Read More: How T. rex Came to Rule the World

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