Wannanosaurus (meaning "Wannan lizard", named after the location where it was discovered) is a genus of basal pachycephalosaurian dinosaur from the Campanian-age Upper Cretaceous Xiaoyan Formation, about 80 million years ago (mya) in what is now Anhui, China. The type species, Wannanosaurus yansiensis, was described by Lian-Hai Hou in 1977. It is known from a single partial skeleton, including a partial skull roof and lower jaw, an upper leg and lower leg, part of a rib, and other fragments. Because it has a flat skull roof with large openings, it has been considered primitive among pachycephalosaurs. Sometimes it has been classified as a member of the now-deprecated family Homalocephalidae, now thought to be an unnatural assembly of pachycephalosaurians without domed skulls. Although its remains are from a very small individual, with a femur length of ~8 centimeters (3.1 in) and an estimated overall length of less than 1 meter (3.3 ft), the fused bones in its skull suggest that it was an adult at death. Like other pachycephalosaurians, it was probably herbivorous or omnivorous, feeding close to the ground on a variety of plant matter, and possibly insects as well.