A study published in Science on December 4, 2025, has provided direct evidence that the Cleveland skull—the holotype of Nanotyrannus lancensis—represents a fully mature adult dinosaur. The research was led by Dr. Christopher Griffin of Princeton University and Dr. Caitlin Colleary of the Cleveland Museum of Natural History (CMNH). The findings address a long-standing question in paleontology about whether Nanotyrannus is a separate species or a juvenile Tyrannosaurus rex.
The Cleveland skull has been debated by scientists since its discovery in 1942 in Montana’s Hell Creek Formation by CMNH curator Dr. David Dunkle. It was first classified as a new species of Gorgosaurus by Smithsonian paleontologist Charles Gilmore in 1945. In 1988, Dr. Michael Williams and colleagues at CMNH re-examined the specimen and suggested it belonged to a new genus, Nanotyrannus, based on its features and fused skull bones. This view was questioned as more juvenile T. rex fossils were found in later years.
A key issue in the debate was the lack of additional bones from the same individual, making it hard to determine the age of the Cleveland skull. Dr. Colleary and Dr. Griffin’s team addressed this by studying a small hyoid bone from the throat, which had been stored with the skull. They used a histological method to examine thin slices of the hyoid bone under a microscope, looking for growth markers that show skeletal maturity…