Researchers claim to have finally identified a small-bodied dinosaur that fueled decades of paleontological contention, with significant consequences for everyone’s favorite extinct carnivore—the Tyrannosaurus rex.

Scientists have long debated whether a skull unearthed in 1946 in Montana’s approximately 65.5-million-year-old Hell Creek Formation was a young T. rex or a newly discovered species, which researchers named Nanotyrannus lancensis. A complete skeleton also from the Hell Creek Formation and unearthed in 2006, however, may have settled the matter. According to a study published today in the journal Nature, these two small dinosaurs represent a distinct species—a conclusion that is sure to give the paleontological community serious whiplash.

Rewriting T. rexes?

That’s because “a sizeable body of our current understanding of T. rex and the ecosystem it inhabited hinges upon the identification of small tyrannosauroids from the Hell Creek Formation as ‘teenage’ T. rex,” the researchers wrote in the paper. “If this taxonomic hypothesis is incorrect, so too is much of what we know about one of our planet’s most thoroughly studied extinct vertebrates.”

The newly described specimen was about 20 years old and physically mature at the time of its death. According to the researchers, N. lancensis’ larger forelimbs, additional teeth, fewer tail vertebrae, and unambiguous skull nerve patterns would have been biologically impossible in a T. rex.

“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” James Napoli, an anatomist at Stony Brook University and co-author of the study, said in a North Carolina State University statement. “It’s not just unlikely—it’s impossible.”

The team estimates that an adult N. lancensis would have weighed at most around 1,540 pounds (around 700 kilograms), whereas the T. rex would have maxed out at around 14,770 to 18,080 lbs (6,700 to 8,200 kg). Computer models also indicate that their bones grew differently.

In other words, Napoli and his co-author Lindsay Zanno, a paleontologist from the North Carolina Museum of Natural Sciences, conclude that both the original skull from 1946 and the newly analyzed specimen from 2006 are Nanotyrannus lancensis, bolstering the validity of both the distinct genus and species. Furthermore, they reclassify another small-bodied skeleton, nicknamed Jane and previously identified as a young T. rex, as another member of the same genus—Nanotyrannus lethaeus sp. nov (with “sp. nov.” meaning nova species in Latin, indicating a newly declared species).

Thriving until the end

More broadly, Napoli and Zanno argue that the coexistence of at least three predatory dinosaurs (N. lancensis, N. lethaeus, and T. rex) within the last million years of the Cretaceous period suggests that this region featured rich dinosaur diversity right before the infamous Chicxulub asteroid wiped most of them out. This bolsters another recent study suggesting that North America’s dinosaurs were diverse and thriving until the very end.

Steve Brusatte, a vertebrate paleontologist at the University of Edinburgh who was not involved in the study, says that the existence of Nanotyrannus is “proven beyond a reasonable doubt now.” He’s not convinced about every aspect of the paper, however, including the identification of N. lethaeus.

However, “the biggest nagging doubt I have is that this new study reinterprets not just a few smaller tyrannosaur skeletons to belong to Nanotyrannus, but argues that basically every small tyrannosaur skeleton from the latest Cretaceous of western North America is Nanotyrannus. So where are the juvenile T. rexes?” he explains in a statement emailed to Gizmodo.

To which he added: “If so many adults were fossilized, then there should be juveniles too—and that argument from basic probability helped underpin the conventional wisdom that the smaller tyrannosaurs were juvenile T. rexes. So I’m not yet ready to proclaim every smaller tyrannosaur skeleton to be Nanotyrannus.”

In other words, the matter isn’t settled, and further research is needed to confirm the paper’s findings. However, the study certainly makes it more likely that other specimens previously identified as teenage T. rexes are also a different species.