For decades, paleontologists have argued about size. Specifically, whether or not certain fossils found in Montana belonged to a young Tyrannosaurus rex or a completely separate, smaller species. Researchers at North Carolina State University and Ohio University now say they can settle the debate. Tyrannosaurus rex did, in fact, have a diminutive relative in North America. While Nanotyrannus lancensis was comparatively small, a study published on October 30 in Nature makes it clear that they remained a fearsome Cretaceous era predator.
Tyrannosaurus rex earned its “Tyrant King” status partly due to its enormous size. An adult T. rex could easily grow over 42-feet-long and weigh more than nine tons.Even juveniles weighed in at around 4,000 pounds. With that in mind, it’s understandable why a small skull discovered in 1946 at Montana’s fossil-rich Hell Creek Formation has been such a controversial specimen. Initially described as a species of Gorgosaurus, some paleontologists later asserted it actually belonged to an entirely separate Tyrannosaur. This initial categorization was met with pushback by an opposing camp of researchers, who claimed the skull simply possessed a combination of immature tyrannosaurid and T. rex characteristics. The truth had larger implications—if Nanotyrannus was actually its own species, that would end T. rex’s reign as the only widely accepted tyrannosauroid roaming North America during the late Cretaceous.
“In the years since, Nanotyrannus has become a hot-button issue, and the debate has often been acrimonious,” paleontologists Lindsay Zanno and Lawrence Witmer explained in their study.
During an expedition back to the Hell Creek Formation, Zanno and Witmer’s team discovered an “exceptionally preserved, skeletally complete tyrannosauroid specimen.” After a close examination of the approximately 67-million-year-old remains, the study’s authors concluded that their find—as well as the 1946 skull—belong to Nanotyrannus.
The key piece of evidence came from the new specimen’s age. A morphological analysis of its bones indicates they were nearly fully grown. If they were a juvenile, then their skeleton would still be developing. Additional modelling also led the paleontologists to believe Nanotyrannus displayed different bone-growth trajectories than T. rex, further supporting the argument. With this in mind, the study authors reexamined the previously discovered specimens, and determined the Nanotyrannus genus possessed at least two distinct species: Nanotyrannus lancensis and Nanotyrannus lethaeus.
In contrast to the T. rex, an adult Nanotyrannus only grew to a length of around 18 feet and weighed about 1,550 pounds. Where its larger cousin evolved for brute strength with thick legs and a bone-crushing bite, Nanotyrannus took a different approach to hunting prey. Its longer legs and strong arms made it both agile and fast, allowing it to quickly ambush its targets.
The confirmation of Nanotyrannus’ existence means that Tyrannosaurus likely had some company for at least around one million years leading up to the Cretaceous-Paleogene extinction event. This further supports the theory that dinosaurs weren’t already on the decline prior to their demise.If anything, the diversity suggests that they were doing just fine.
“Our results undermine a nearly uniform consensus among theropod specialists and rectify a significant taxonomic error underpinning decades of research,” the authors wrote. “The impact is beyond our ability to summarize here.”
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