Nanotyrannus fossils are juvenile Tyrannosaurus rex rather than a distinct small-bodied tyrannosaur.

The status of Nanotyrannus has been argued over for decades. The small, lightly built skull originally received its own genus, but by the late 1990s and 2000s many paleontologists interpreted the known specimens as young Tyrannosaurus rex. That explanation was strengthened by studies showing how dramatically tyrannosaur skulls and body proportions changed during growth.

The key question was maturity. A genuinely juvenile T. rex should still be growing toward the anatomy of an adult T. rex. In 2025, a detailed skeleton described as nearly fully grown retained a suite of features associated with Nanotyrannus and followed a different growth trajectory. A separate study examined microscopic growth structure preserved inside bone from the holotype. It likewise found an animal much closer to skeletal maturity than the juvenile interpretation required.

Those independent results shifted the taxonomic case. The small-bodied fossils are now better supported as a distinct tyrannosaur that shared its environment with T. rex, although future specimens can continue to refine the exact relationships within the group.