Tyrannosaurs are among the most intensively studied and best-known dinosaurs. Despite this, their relationships and systematics are highly controversial. An ongoing debate concerns the validity of
Nanotyrannus lancensis, interpreted either as a distinct genus of small-bodied tyrannosaur or a juvenile of
Tyrannosaurus rex. We examine multiple lines of evidence and show that the evidence strongly supports recognition of
Nanotyrannus as a distinct species for the following reasons: 1. High diversity of tyrannosaurs and predatory dinosaurs supports the idea that multiple tyrannosaurids inhabited the late Maastrichtian of Laramidia; 2.
Nanotyrannus lacks characters supporting referral to
Tyrannosaurus or Tyrannosaurinae but differs from
T.
rex in >150 morphological characters, while intermediate forms combining the features of
Nanotyrannus and
T.
rex are unknown; 3. Histology shows specimens of
Nanotyrannus showing (i) skeletal fusions, (ii) mature skull bone textures, (iii) slow growth rates relative to
T. rex, (iv) decelerating growth in their final years of life, and (v) growth curves predicting adult masses of ~1500 kg or less, showing these animals are subadults and young adults, not juvenile
Tyrannosaurus; 4. growth series of other tyrannosaurids, including
Tarbosaurus and
Gorgosaurus, do not show morphological changes proposed for a
Nanotyrannus–Tyrannosaurus growth series, and deriving
Tyrannosaurus from
Nanotyrannus requires several changes inconsistent with known patterns of dinosaur development; 5. Juvenile
T.
rex exist, showing diagnostic features of
Tyrannosaurus; 6. Phylogenetic analysis suggests that
Nanotyrannus may lie outside Tyrannosauridae. Tyrannosaur diversity before the K-Pg extinction is higher than previously appreciated. The challenges inherent in diagnosing species based on fossils mean paleontologists may be systematically underestimating the diversity of ancient ecosystems.
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