Huayangosaurus

Huayangosaurus was a primitive, small stegosaur that lived in lush forested floodplains of Middle Jurassic China. Measuring about four metres long, it was notably smaller and more basal than later relatives such as Stegosaurus. It possessed a broader, shorter skull that still retained teeth at the front of the upper jaw, an ancestral feature lost in later stegosaurs which possessed entirely toothless horny beaks. Huayangosaurus carried two rows of paired, spike-like vertical plates running down its back, alongside shoulder spines and a four-spined thagomizer on its tail for defense against local megalosauroid predators. Its forelimbs were almost as long as its hind legs, giving it a less hunched silhouette than later stegosaurids. It represents the best-known early stage of stegosaurian evolution.

It retained primitive premaxillary teeth at the front of its mouth, unlike later stegosaurs.

Where and when

Huayangosaurus lived in the Middle Jurassic, from 168 to 161 million years ago (Bathonian-Callovian age). Its remains come from China, in the Lower Shaximiao Formation.

The numbers

Length 4.3 m (estimates run from 4 to 4.5 m). Mass about 980 kg. Hip height about 1.4 m. That is 2.5 human-lengths, or 0.38 of a double-decker bus. These are estimates from near-complete skeleton; fields marked estimated by the site are length, mass, hip height.

Compare Huayangosaurus with a bus Place on the timeline