Dino of the Week: Huayangosaurus

Type Species: Huayangosaurus taibaii

Classification: Dinosauria – Ornithischia – Thyreophora – Stegosauria - Huayangosauridae

Time Period: Middle Jurassic

Location: China 

Diet: Herbivore 

In 1979 the fossilized remains of twelve stegosaurs were discovered in China. These belonged to a new species that lived during the Bathonian and Callovian stages of the Middle Jurassic. Huayangosaurus looked a lot like its later cousin, Stegosaurus, though it was much smaller at only fifteen feet in length. Huayangosaurus lived twenty million years before Stegosaurus and is one of the earliest known stegosaurs (though Europe’s Lexovisaurus beats it to the punch by a few million years). Huayangosaurus’ primitive placement in the stegosaur ‘family tree’ is evidenced by several factors. First, this stegosaur had a broader skull than later stegosaurs, and its skull had a small opening in front of each eye and another small opening in each half of the lower jaw (these openings are absent in later stegosaurs); second, Huayangosaurus had fourteen teeth at the front of its snout (seven on each side), and later stegosaurs lacked these teeth; third, Huayangosaurus had long front limbs that were three-quarters longer than the back limbs, whereas later stegosaurs had forelimbs that were much shorter than the hind limbs; and fifth, the armor plates that ran in two rows along Huayangosaurus’ back were narrower and thicker than those adorning the backs of its later relatives.

a pair of Huayangosaurus with a herd of Omeisaurus

Huayangosaurus nevertheless resembles a ‘proper’ stegosaur. It had the distinctive double row of plates that characterize all stegosaurs; these plates began at its neck and ran along its back until it reached the hips, at which point the plates were replaced with four spikes. After this series of spikes, the plates resumed until they ended with two pairs of long spikes extending horizontally near the end of the tail. These spikes were undoubtedly defensive weapons against such predators as Gasosaurus and Monolophosaurus. Huayangosaurus, like the earlier Lexovisaurus, had two spikes that protruded either from the hips or the shoulders. The exact placement is unknown, with artistic depictions representing either location depending on artistic fancy. The current general consensus is that, as in Lexivosaurus, they were placed on the shoulders; in this case, they would serve as defensive weapons: if an assailant attacked from the front or from the forward side quadrants, Huayangosaurus could ‘thrust’ its spikes deep into the adversary’s hide. In a world without medicine, such penetrating attacks into an enemy’s organs could often be lethal. Only the most foolish of predators would attempt to make a meal out of the armed stegosaur.