Hesperosaurus
| Key | Value |
|---|---|
| Name Meaning | “western lizard” |
| Location | United States (Wyoming, Montana) |
| Time Period | c. 156 million years ago (Late Jurassic) |
| Length | 20 ft (6 m) |
| Weight | 3.5 tons (3,175 kg) |
| Locomotion | Quadruped |
| Diet | Herbivore |
| Described | 2001 (Carpenter, Miles & Cloward) |
| Geological Formation(s) | Morrison |
| Valid Species | Hesperosaurus mjosi (type) |
Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Stegosauria > Stegosauridae > Stegosaurinae
Overview: As far as North American stegosaurs go, Stegosaurus itself is by far the most famous of them. Hesperosaurus, on the other hand, is quite obscure and poorly known to the general public, though like Stegosaurus, it too is known from the Morrison Formation. In its case, however, fossils come from its deeper layers, so Hesperosaurus predated most species of Stegosaurus. Remains of Hesperosaurus were first discovered in what is now Wyoming, in the mid-1980’s. At first, the bones were referred to Stegosaurus, but some key differences would soon be noted. In 2001, it was given its own name as a distinct genus, meaning “western lizard”, referring to it having been discovered in the western United States. Other specimens of Hesperosaurus would be found afterward, with the genus being based on more than a dozen individual specimens, though it’s rarer than Stegosaurus. Some studies place Hesperosaurus as a species of Stegosaurus, but most see them as distinct.
As far as their basic anatomy goes, Hesperosaurus and Stegosaurus were very similar, being high-backed herbivores bearing two rows of plate-like osteoderms running down their backs, as well as a set of spikes on the end of the tail (the “thagomizer”). The plates were probably used for visual display, while the thagomizer would’ve been used defensively. Hesperosaurus possessed some of its own unique traits. Its dorsal plates were usually wider, but less tall than those seen on its more famous relative, as well as less robust at the base. Its back didn’t have the same tall profile either, while the skull and jaws were a bit deeper than those of Stegosaurus. Another point of difference was their necks, with that of Stegosaurus being much longer. The two were close relatives, sharing the same subfamily, or Stegosaurinae, within the family Stegosauridae. Hesperosaurus may’ve lived alongside dinosaurs like Gargoyleosaurus, Brontosaurus, Haplocanthosaurus, and Allosaurus.