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A complete tyrannosaur skeleton has just ended one of paleontology\u2019s longest-running debates\u2014whether Nanotyrannus is a distinct species, or just a teenage version of Tyrannosaurus rex.<\/p>\n
The fossil, part of the legendary \u201cDueling Dinosaurs\u201d specimen unearthed in Montana, contains two dinosaurs locked in prehistoric combat: a Triceratops and a small-bodied tyrannosaur.<\/p>\n
That tyrannosaur is now confirmed to be a fully grown Nanotyrannus lancensis\u2014not a teenage T. rex, as many scientists once believed.<\/p>\n
\u201cThis fossil doesn\u2019t just settle the debate. It flips decades of T. rex research on its head,\u201d says Lindsay Zanno, associate research professor at North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences, and coauthor of the study in Nature<\/a>.<\/p>\n Using growth rings, spinal fusion data, and developmental anatomy, the researchers demonstrated that the specimen was around 20 years old and physically mature when it died. Its skeletal features\u2014including larger forelimbs, more teeth, fewer tail vertebrae, and distinct skull nerve patterns\u2014are features fixed early in development and biologically incompatible with T. rex.<\/p>\n \u201cFor Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,\u201d says James Napoli, anatomist at Stony Brook University and coauthor of the study. \u201cIt\u2019s not just unlikely\u2014it\u2019s impossible.\u201d<\/p>\n The implications are profound. For years, paleontologists used Nanotyrannus fossils to model T. rex growth and behavior. This new evidence reveals that those studies were based on two entirely different animals\u2014and that multiple tyrannosaur species inhabited the same ecosystems in the final million years before the asteroid impact<\/a>.<\/p>\n As part of their research, Zanno and Napoli examined over 200 tyrannosaur fossils. They discovered that one skeleton, formerly thought to represent a teenage T. rex, was slightly different than the Dueling Dinosaurs\u2019 Nanotyrannus lancensis. They named this fossil a new species of Nanotyrannus, dubbed N. lethaeus. The name references the River Lethe from Greek mythology\u2014a nod to how this species remained hidden in plain sight and \u201cforgotten\u201d for decades.<\/p>\n Confirmation of the validity of Nanotyrannus means that predator diversity in the last million years of the Cretaceous was much higher than previously thought, and hints that other small-bodied dinosaur species might also be victims of mistaken identity.<\/p>\n \u201cThis discovery paints a richer, more competitive picture of the last days of the dinosaurs,\u201d Zanno says. \u201cWith enormous size, a powerful bite force, and stereoscopic vision, T. rex was a formidable predator, but it did not reign uncontested. Darting alongside was Nanotyrannus\u2014a leaner, swifter, and more agile hunter.\u201d<\/p>\n Support for the research came from the State of North Carolina, NC State University, the Friends of the North Carolina Museum of Natural Sciences, and the Dueling Dinosaurs Capital Campaign.<\/p>\n