Description and Classification

Wannanosaurus yansiensis is the only known species within this genus. It was a very small dinosaur, estimated to be only about 60 centimeters (2 feet) in length, making it significantly smaller than most other dinosaurs. As a pachycephalosaur, or “bone-headed” dinosaur, Wannanosaurus possessed a thickened skull roof, although its dome was relatively flat and wide compared to the highly domed skulls of later pachycephalosaurs like Pachycephalosaurus or Stegoceras. Its eye sockets were quite large for its skull size.

Wannanosaurus is classified within the group Pachycephalosauria. Due to its flat skull and other features, it is considered a basal, or primitive, member of this group. Some scientists have suggested that the known specimen might be a juvenile of a larger pachycephalosaur species. However, the fusion of certain bones in its skull has led others to believe it represents a small-bodied adult species. It shared characteristics with other early pachycephalosaurs like Homalocephale.

Distinguishing Features

Wannanosaurus can be identified by several unique characteristics:

• Extremely small adult size for a pachycephalosaur, one of the smallest known.
• A relatively flat and broad skull roof (frontoparietal bone) with slight ornamentation, rather than a thick, high dome.
• Large openings on the top rear of the skull (supratemporal fenestrae) that were not closed over by bone, a feature often seen in juvenile or more primitive pachycephalosaurs.
• Teeth present in the front of the upper jaw (premaxilla), a trait common in basal pachycephalosaurs but lost in more advanced forms.
• Long hind limbs relative to its overall body size.

Paleoenvironment and Diet

Wannanosaurus lived in what is now eastern China during the late Campanian stage of the Late Cretaceous. The deposits where its fossils were found suggest an inland environment, possibly with floodplains and varied vegetation. Given its small size and pachycephalosaurian dentition, Wannanosaurus was likely herbivorous. It probably fed on low-growing plants, such as ferns, cycads, seeds, and fruits. It might have also eaten insects occasionally. Its small stature would have meant it foraged close to the ground.

Significance and Ongoing Research

The discovery of Wannanosaurus is important because it represents one of the smallest known pachycephalosaurs, providing insights into the diversity of body sizes within this group. Its primitive features help paleontologists understand the early evolution of pachycephalosaurs and how their characteristic thickened skulls developed. The ongoing debate about whether Wannanosaurus is a distinct adult species or the juvenile form of another pachycephalosaur highlights the challenges in interpreting dinosaur fossils, especially when dealing with limited remains or individuals that died before reaching full size.

Further study of Wannanosaurus and comparison with other pachycephalosaur fossils, including juvenile specimens of genera like Stegoceras or Prenocephale, could help clarify its status and its exact position within the pachycephalosaur family tree. Its existence contributes to a broader understanding of dinosaur ecosystems in Asia during the Late Cretaceous period.