Nanotyrannus

A puzzle of a skeleton

Nanotyrannus is known mostly from skull material, so what its full body looked like is not fully settled. It was a meat-eating theropod with a slender, low skull and small, blunt crests just in front of its eyes. Its jaws held small, blade-like teeth, most of them serrated on both edges, though the teeth right at the front of the upper jaw were smooth-edged. Whether it represents an adult animal or a growing juvenile of a different, larger species is exactly what scientists cannot agree on.

Its world

Nanotyrannus lived at the end of the Cretaceous, between about 72 and 66 million years ago, in what is now Montana, Wyoming, and South Dakota. Its fossils come from the Hell Creek and Lance formations, the same rock layers that preserve Tyrannosaurus, Triceratops, Ankylosaurus, Pachycephalosaurus, Edmontosaurus, and Torosaurus. Only 5 fossil finds have been assigned to Nanotyrannus so far, which is part of why paleontologists still argue over what the bones actually represent.

A genus in dispute

Bakker and his coauthors named Nanotyrannus in 1988, arguing that its small size and light build set it apart from Tyrannosaurus. Since then, scientists have disagreed sharply. Some argue that every Nanotyrannus specimen is simply a young, still-growing Tyrannosaurus, since young tyrannosaurs are known to have been slimmer and faster than adults of the same species. Others argue the skeletal differences are too consistent to be explained by growth alone, and that Nanotyrannus deserves to stand as its own genus. No single answer has settled the question, and both views still have supporters among paleontologists.