For four decades, a frequently acrimonious debate has raged in paleontological circles about the correct taxonomy for a handful of rare fossil specimens. One faction insisted the fossils were juvenile Tyrannosaurus rex; the other argued that they represented a new species dubbed Nanotyrannus lancensis. Now, paleontologists believe they have settled the debate once and for all due to a new analysis of a well-preserved fossil.
The verdict: It is indeed a new species, according to a new paper published in the journal Nature. The authors also reclassified another specimen as a second new species, distinct from N. lancensis. In short, Nanotyrannus is a valid taxon and contains two species.
“This fossil doesn’t just settle the debate,” said Lindsay Zanno, a paleontologist at North Carolina State University and head of paleontology at North Carolina Museum of Natural Sciences. “It flips decades of T. rex research on its head.” That’s because paleontologists have relied on such fossils to model the growth and behavior of T. rex. The new findings suggest that there could have been multiple tyrannosaur species and that paleontologists have been underestimating the diversity of dinosaurs from this period.
Our story begins in 1942, when the fossilized skull of a Nanotyrannus, nicknamed Chomper, was excavated in Montana by a Cleveland Museum of Natural History expedition. Originally, paleontologists thought it belonged to a Gorgosaurus, but a 1965 paper challenged that identification and argued that the skull belonged to a juvenile T. rex. It wasn’t until 1988 that scientists proposed that the skull was actually that of a new species, Nanotyrannus. It’s been a constant back-and-forth ever since.
As recently as 2020, a highly influential paper claimed that Nanotyrannus was definitively a juvenile T. Rex. Yet a substantial number of paleontologists still believed it should be classified as a distinct species. A January 2024 paper, for instance, came down firmly on the Nanotyrannus side of the debate. Co-authors Nicholas Longrich of the University of Bath and Evan Saitta of the University of Chicago measured the growth rings in Nanotyrannus bones and concluded the animals were nearly fully grown.
Dueling dinosaurs
Credit: N.C. State University/CC BY-NC-ND Lindsay Zanno of North Carolina State University, who also heads paleontology at the North Carolina Museum of Natural Sciences, with the “dueling dinosaurs” fossil.
Furthermore, there was no evidence of hybrid fossils combining features of both Nanotyrannus and T. rex, which one would expect if the former were a juvenile version of the latter. Longrich and Saitta had also discovered a skull bone, archived in a San Francisco museum, that did belong to a juvenile T. rex, and they were able to do an anatomical comparison. They argued that Nanotyrannus had a lighter build, longer limbs, and larger arms than a T. rex and likely was smaller, faster, and more agile.