Huayangosaurus is a tiny bit of a mystery. Some sources state that there are spikes held horizontally at the shoulder (and most illustrations go with this model) while some say they are held
horizontally at the hips. Gregory Paul has drawn skeletals with and without spikes attached at the shoulders as well. The thing that gets me here, however, is that I have yet to find a displayed skeleton with a prominent spike at either position. There is, in some instances, a visible extension of the scapula which may or may not constitute the core of this rear facing and not entirely horizontal spike. If that is what the idea of the spike is based off of, I accept that completely, but I do not see the giant spikes jutting out enormously that we see in
illustrations. In fact, the spike looks tiny and even with an outer keratin sheath while alive the spike would not have added to the frontal girth of the animal much. Any thorn can hurt, as most anyone can attest to, but this certainly does not look as dangerous as it does when illustrated. The skull of
H. taibaii is very basal in its likeness to what we think of when we hear the name Stegosaurus. Their skull almost looks more like that of a nodosaur or ankylosaur at the angle presented above than it does like any other stegosaur. It's not a case of wrong head wrong body though, that is simply how far back down the line Huayangosaurus is on the stegosaur family tree.
The plates are also smaller than other stegosaurs, though not small and eccentric like in the African Kentrosaurus, and this gives the animal the look of being a juvenile Stegosaurus. However, we know quite well that this is not the case, that it is in fact just the smaller (15 feet as opposed to 30 feet) adult of Huayangosaurus. Such an evolutionary pattern makes sense in a world in which the predators were evolving larger stronger frames over time to grapple with the larger sauropods. Had the stegosaur family not followed suit by bulking up and growing longer and taller, they would have been too easy to pick off for predators such as Allosaurus and Yangchuanosaurus. Gasosaurus, a 13 foot long predator of Huayangosaurus, attests to this proportionality battle between predator and prey (another prey item at the time in China which was large and heavily populating the land was Shunosaurus at 31 feet and 3 tons which is small for a sauropod).
One more thing that is key to identifying Huayangosaurus is the tail. Unlike Stegosaurus, which had its tail spikes oriented more toward the lateral line of the tail and extremely rear facing, the spikes of Huayangosaurus are more central and vertical. A small bony club nodule also ends the tail of Huayangosaurus whereas later stegosaurs do not possess this bony nodule. This is one more instance of evidence of the divergence in evolution of stegosaurs from their cousins the nodosaurs and ankylosaurs.Personally, I am much more of a marginocephalian (horns, frills, and domed skulls) person typically, but looking at stegosaurs and ankylosaurs a lot since I have started this blog has made me really come to love their families (also known as the thyreophora) as well.
The plates are also smaller than other stegosaurs, though not small and eccentric like in the African Kentrosaurus, and this gives the animal the look of being a juvenile Stegosaurus. However, we know quite well that this is not the case, that it is in fact just the smaller (15 feet as opposed to 30 feet) adult of Huayangosaurus. Such an evolutionary pattern makes sense in a world in which the predators were evolving larger stronger frames over time to grapple with the larger sauropods. Had the stegosaur family not followed suit by bulking up and growing longer and taller, they would have been too easy to pick off for predators such as Allosaurus and Yangchuanosaurus. Gasosaurus, a 13 foot long predator of Huayangosaurus, attests to this proportionality battle between predator and prey (another prey item at the time in China which was large and heavily populating the land was Shunosaurus at 31 feet and 3 tons which is small for a sauropod). 
One more thing that is key to identifying Huayangosaurus is the tail. Unlike Stegosaurus, which had its tail spikes oriented more toward the lateral line of the tail and extremely rear facing, the spikes of Huayangosaurus are more central and vertical. A small bony club nodule also ends the tail of Huayangosaurus whereas later stegosaurs do not possess this bony nodule. This is one more instance of evidence of the divergence in evolution of stegosaurs from their cousins the nodosaurs and ankylosaurs.Personally, I am much more of a marginocephalian (horns, frills, and domed skulls) person typically, but looking at stegosaurs and ankylosaurs a lot since I have started this blog has made me really come to love their families (also known as the thyreophora) as well.
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