Hesperosaurus

Hesperosaurus (meaning "western lizard", from Classical Greek ἕσπερος (hesperos) "western" and σαυρος (sauros) "lizard") is an herbivorous stegosaurian dinosaur from the Kimmeridgian epoch of the Jurassic period, approximately 156 million years ago. Fossils of Hesperosaurus have since 1985 been found in the state of Wyoming and Montana in the United States of America. The type species Hesperosaurus mjosi was named in 2001. It is from an older part of the Morrison Formation, and so a little older than other Morrison stegosaurs. Several relatively complete skeletons of Hesperosaurus are known. One specimen preserves the first known impression of the horn sheath of a stegosaurian back plate.

Hesperosaurus was six to seven metres long and two to three tonnes in weight. It was a member of the Stegosauridae, quadrupedal plant-eaters protected by vertical bony plates and spikes. It was closely related to Stegosaurus and was similar to it in having two rows of, possibly alternating, plates on its back and four spikes on its tail end. The plates on its back were perhaps not as tall, but were longer. It possibly had a deeper skull than Stegosaurus .

Hesperosaurus is a large stegosaurid. In 2010, Gregory S. Paul estimated its length at 6.5 metres, the weight at 3.5 tonnes.[10]

Skeleton in front view
In 2001 Carpenter provided a diagnosis. Due to his conclusion that Hesperosaurus were rather basal, in it many comparisons were made with the basalmost known stegosaurian Huayangosaurus,[1] that lost their relevance once it became clear that the phylogenetic position was in fact quite derived. In 2008 Maidment indicated three autapomorphies: the possession of eleven back vertebrae; the fourth sacral not being fused to the sacrum; back plates that are longer (from front to rear) than tall. Maidment also provided some traits in which Hesperosaurus was more basal than Stegosaurus armatus. In the atlas, even in adult specimens the neural arches are not fused to the intercentrum. The postzygapophyses, the rear joint processes, of the rear neck vertebrae do not prominently protrude upwards. In the back vertebrae, the neural arches, above the level of the neural canal, are not especially lengthened to above. At the hip region ossified tendons are present. The ribs are expanded at their lower ends. The neural spines of the tail vertebrae are not bifurcated. The lower end of the pubic bone is expanded (spoon-shaped in side view).[5] To Carpenter this differential diagnosis was problematic because he considered Stegosaurus armatus, the type species of Stegosaurus, a nomen dubium and rejected Maidment's lumping of all North-American Stegosaurus material into a single species, the great variability of which making it difficult to establish any differences with Hesperosaurus. He considered Stegosaurus stenops, the name historically given to several well-preserved specimens, a separate species and provided a new differential diagnosis of Hesperosaurus compared to S. stenops. The antorbital fenestra is large instead of very small. The maxilla is short and deep, half as tall as long, instead of having a height a third of the length. The basisphenoid of the lower braincase is short instead of long. Thirteen neck vertebrae are present instead of ten. Thirteen dorsal (back) vertebrae are present instead of seventeen. The middle dorsals have a basal form in possessing a low neural arch rather than a high one. The cervical ribs have expanded lower ends. In the front tail vertebrae, the tops of the neural spines are rounded instead of bifurcated. The front edge of the shoulderblade is indented instead of running parallel to the rear edge. The front blade of the ilium diverges strongly sideways instead of weakly. The rear blade of the ilium has a knob-shaped expansion at the rear end. The front end of the prepubic process has an upward expansion. The plates of the hip and tail base are oval and low instead of high and triangular.

Restoration
The various published descriptions of Hesperosaurus contradict each other because of changes and differences in interpretation. Originally, Carpenter reconstructed the disarticulated skull elements into a very convex head, modelling it on the shape of Huayangosaurus .[1] The discrepancies in the vertebral count are caused by applying different criteria to the problem whether (and which) cervicodorsal vertebrae should be considered part of the neck or the back. The exact shape of the plates is hard to determine due to erosion. Paul considered the neck plates to be low, but the back plates as taller.[10] Also the Aathal specimens are as yet undescribed. A complete description of the entire material is in preparation by Octávio Mateus.[4]

The number of maxillary teeth were twenty per side, lower than the number with Stegosaurus. Carpenter described them as similar to the teeth of Stegosaurus, though somewhat larger. Peter Malcolm Galton in 2007 established some differences: there are rough vertical ridges present on the upper part of the crown, one per denticle; the fine grooves on the tooth surface are weakly developed.