Tyrannosaurid skull anatomy is well understood as complete skulls are known for all genera but "Alioramus", which is known only from partial skull remains. "Tyrannosaurus", "Tarbosaurus", and "Daspletosaurus" had skulls that exceeded in length. Adult tyrannosaurids had tall, massive skulls, with many bones fused and reinforced for strength. At the same time, hollow chambers within many skull bones and large openings (fenestrae) between those bones helped to reduce skull weight. Many features of tyrannosaurid skulls were also found in their immediate ancestors, including tall premaxillae and fused nasal bones.