The most definitive evidence comes from a microscopic feature called an external fundamental system—a series of tightly packed growth rings in bone—that suggests the fossilized animal had finished growing, explains study co-author James G. Napoli, a paleontologist at Stony Brook University. Zanno and Napoli argue this confirms Nanotyrannus as a distinct genus alongside T. rex in the ecosystem of the fossil-rich Hell Creek Formation site in Montana, which spanned the last million years of the Cretaceous.