Debate surrounding the classification of the small theropod dinosaur, Nanotyrannus, has reached a definitive conclusion: it is not a juvenile Tyrannosaurus rex but a distinct species. This landmark finding, reported on October 30, 2023, in the journal Nature, stems from the analysis of a remarkably preserved skeleton from the Hell Creek formation in Montana, dating back 67 million years.
The fossil, part of the well-known Dueling Dinosaurs specimen, features a small tyrannosaur intertwined with a horned ceratopsian dinosaur. This rare entombment provides crucial insights into the life of N. lancensis, marking it as the first identified adult specimen of this species. Researchers, led by paleontologists Lindsay Zanno of North Carolina State University and James Napoli of Stony Brook University, have presented compelling evidence to separate Nanotyrannus from T. rex.
The controversy surrounding Nanotyrannus began in the early 1940s with the discovery of a small skull initially classified as Gorgosaurus. In 1988, the fossil was reclassified as a new species, N. lancensis. However, skeptics argued that the skull belonged to a juvenile T. rex, leading to ongoing debate. As additional fossils were unearthed from the Hell Creek Formation, many were similarly attributed to young specimens of T. rex.
Recent investigations have changed this narrative. The newly analyzed fossil measured approximately six meters in length. Growth ring analysis of its leg and arm bones indicated the specimen was skeletally mature, distinguishing it from juvenile T. rex, which can grow up to 14 meters long. Notably, this specimen also includes the first preserved tail and arm bones of N. lancensis, which reveal significant anatomical differences from T. rex.
As Zanno explained, “The arm of our Nanotyrannus is already a bit bigger than a T. rex arm.” Additional differences include the number of vertebrae in the tail and unique cranial nerve structures, which remain consistent regardless of growth. This new evidence allows researchers to refine their understanding of the evolutionary history of these formidable predators.
Another significant finding involved a previously debated specimen known as Jane. Analysis suggested Jane was a juvenile, but with the new insights into N. lancensis, researchers have reclassified Jane as a young Nanotyrannus, proposing a new species named N. lethaeus. This name draws from Greek mythology, referencing the River Lethe, which was said to induce forgetfulness, highlighting how Jane’s identity had eluded researchers for decades.
While some, like Holly Ballard, a paleontologist at Oklahoma State University, acknowledge the findings, she remains skeptical about classifying Jane as a new species of Nanotyrannus. Ballard notes that Jane was larger than N. lancensis even in its juvenile form.
The study’s implications extend beyond the classification of a new species. Zanno notes that the findings challenge existing understandings of T. rex regarding locomotion, diet, and growth, suggesting that decades of research may need to be reevaluated.
Adding to the intrigue, Holtz remarked on the significance of the findings, stating, “If Nanotyrannus is real, and it looks like it is, we now once again do not know what a teen T. rex looks like.” A potential new fossil, currently being prepared at the Denver Museum of Nature & Science, may provide further evidence on the adolescent form of T. rex.
These discoveries underscore the diversity and complexity of dinosaur life just before the cataclysmic event that marked the end of the Cretaceous Period. The findings of N. lancensis and its distinctions from T. rex contribute to a deeper understanding of the ecological dynamics that existed within the Hell Creek ecosystem.
