Hesperosuchus

a mother Hesperosaurus defends its child against an Allosaurus

Type Species: Hesperosaurus mjosi  

Classification: Dinosauria – Ornithischia – Thyreophora – Stegosauria – Stegosauridae

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

The infamous Stegosaurus wasn’t the only stegosaur roaming the Morrison during the Kimmeridgian; its slightly smaller cousin Hesperosaurus was present, too (though it was likely more closely related to Dacentrurus of Europe). Hesperosaurus grew up to twenty to twenty-two feet in length – making it slightly smaller than Stegosaurus, which could grow up to thirty feet long – and weighed two to three tons. Like Stegosaurus it had two rows of plates running along its back; asymmetrical bases imply that the rows were staggered rather than symmetrical. Hesperosaurus’ plates were wider than those of Stegosaurus but not as tall, and its skull was deeper than that of its Morrison kin. Like Stegosaurus, Hesperosaurus had a four-spiked thagomizer on its tail; the front pair was thicker than the latter pair, and the latter were more horizontally directed towards the rear. Hesperosaurus’ tail spikes angled slightly backwards, pointing away from the body, implying that they were used in defense against predators such as Allosaurus and Torvosaurus. At least one Allosaurus specimen shows damage to its vertebrae that seems delivered by a stegosaur’s tail spike. 

CAT-scans on Hesperosaurus’ plates show that they had thin but dense outer walls and were filled with thick but spongy bone. This bone shows signs of being remodeled during the growth process. The plate bones were supplied with blood via long and wide arterial vessels. CAT-scans of the tail spikes show that they have thicker walls than the plates, and the interior spongy hollows are smaller. Each spike had a single artery that ran along the longitudinal axis. Skin impressions from Hesperosaurus give us a glimpse of what it looked like enfleshed: a part of the lower flank preserves rows of small, hexagonal, non-overlapping convex scales; higher on the flank are rosette structures with large central scales. Impressions of the back plates show no scales but a smooth surface with low parallel ridges; these likely represent the horn sheath covering the plate, and if so, these represent the first direct proof of such plate sheaths in stegosaurs. The horn sheath on the plates indicate that they had a defensive function, as the horn layer would not only strengthen the bony plate but give it sharp cutting edges. Simultaneously, the display function seems plausible, as these plates would likely be brightly colored in real life. Horn sheaths are a blow to those who believe the plates served a thermoregulatory function, as the sheaths would’ve hampered the plates’ ability to thermoregulate; however, the theory cannot be wholly rejected, for some cattle and ducks use horns and beaks to dump excess heat despite horn coverings.