In 2016, students from the Natural History Museum at Kansas University found a relatively small dinosaur pelvis in the Hell Creek formation in Montana. It was the end of their dig season and they didn't have the time to unearth the rest of the remains. But the fossil was worth the wait; when they returned last summer, they found part of a jaw, teeth and skull segment of what appears to be a 66.5-million-year-old juvenile T. rex, reports Laura Geggel at LiveScience.
In the last 100 years of excavations, fewer than five juvenile T. rex have come out of Hell Creek, which boasts an impressive assemblage of ancient remains. And the latest find may be the best so far. "[T]his is probably the most preserved and most complete,” Kyle Atkins-Weltman, who is helping to prepare the fossil at KU tells Geggel. “This is a 1-in-100-million specimen.”
While the researchers believe the fossil is of a juvenile T. rex, they aren't yet one hundred percent certain. “The teeth suggest it’s a Tyrannosaurus rex; however, there is still more work to be done,” David Burnham, preparator of vertebrate paleontology at the KU Biodiversity Institute, says in a press release. “Because a young T. rex is so rare, there are only a few that have been found over the years, so it’s difficult to discern what are changes due to growth or if the differences in the bones reflect different species."
The researchers are comparing the University's older T. rex and a previously found young T. rex they have on loan to help in the identification efforts, Burnham says. One of the biggest challenges of this analysis, however, is the transformation the creatures undergo as they age. Dinosaurs, in particular T. rex, changed quite a bit during their childhood and adolescence, making it difficult to connect the fossils of young dinosaurs to their adult counterparts.
As Geggel reports, this particular fossil appears to be younger than an 11-year-old T. rex pulled out of Hell Creek but older than a 3-year-old fossil. It’s likely, if this is a T. rex, that it was six to eight years old at the time of its death.
But the find also revives a long-simmering controversy in paleontology. In 1988, eminent paleontologist Robert Bakker announced that the skull of what was believed to be a juvenile T. rex dug up in Montana decades before was actually a new type of tiny T. rex-like dinosaur, dubbing it Nanotyrannus, Brian Switek reported for Smithsonian.com in 2015.
In 1999, Thomas Carr, a paleontologist at Carthage College in Wisconsin, challenged that claim. After reexamining the skull, he argued that it was just a juvenile T. rex. In 2015, he and his team revisited the Nanotyrannus, comparing its features to the skull of an 11-year-old T. rex pulled out of Hell Creek. Again, Carr concluded the supposed Nanotyrannus skull was just a very young T. rex.
There are a few distinctive features to help link the young and old creatures. For example, young T. rex have features like a wide forehead, a narrow snout and forward-facing eye sockets, Carr tells Geggel. All of these features are present in the contentious fossil.
The identity of the latest fossil is still unconfirmed. And as Burnham tells Geggel, he and his colleagues are keeping "an open mind."
"If it turns out to be Nanotyrannus, we're OK with that. If it turns out to be the best small T. rex in the world, we're happy with that as well," he says. The KU teams hopes to return to the same spot where they found the skull this summer to see if they can dig up more of the fossil, which is estimated to be around 17 feet long.