Hesperosaurus (Western lizard)
Hesperosaurus (Western lizard)
Named By : K. Carpenter, C. A. Miles & K. Cloward - 2001
Diet : Herbivore
Size : Estimated 6 meters long
Type of Dinosaur : Armoured Dinosaur
Type Species : H. mjosi (type)
Found in : USA - Wyoming - Morrison Formation
When it Lived : Late Jurassic, 154-142 million years ago
Hesperosaurus, which means “western lizard” in Classical Greek, is a herbivorous stegosaurian dinosaur that lived approximately 156 million year ago.
Since 1985, fossils of Hesperosaurus were found in Montana and Wyoming in the United States of America. 2001 was the year that Hesperosaurus mjosi, a type species, was officially named. It comes from the Morrison Formation’s older section, so it is a bit older than other Morrison Stegosaurs. There are several complete skeletons of Hesperosaurus. One specimen is the only known example of the horn sheath on a stegosaurian rear plate.
Hesperosaurus measured six to seven meters in length and weighed between two and three tonnes. It belonged to the Stegosauridae, which were quadrupedal plant-eaters. They had spikes and vertical bony plates. It was closely related with Stegosaurus, and had two rows of plates on its back, possibly alternating, and four spikes at its tail end. Although the plates on its back were not as tall, they were much longer. It may have had a deeper skull that Stegosaurus.
Patrick McSherry was a fossil hunter who discovered the remains of a stegosaur at S.B. Smith, Johnson County, Wyoming, discovered the remains of a Stegosaur. Because of the hard rock matrix, Smith had trouble securing the specimen. He sought the help of Ronald G. Mjos, and Jeff Parker, Western Paleontological Laboratories, Inc. They collaborated with Dee Hall, a paleontologist at Brigham Young University. It was initially assumed that it was an example of Stegosaurus. Clifford Miles discovered that the remains belonged to a new species while preparing them.
Clifford Miles and Karen Cloward named and described the type species Hesperosaurus mjosi in 2001. In reference to the location of the species in the west United States, the generic name derives from the Greek esperos (hesperos), which means “western”. Mjos was responsible for the collection and preparation of the holotype. He also made a cast of the holotype. This cast is displayed in the Denver Museum of Natural History with the inventory number DMNH 29431.
The holotype, HMNH 001 (later HMNS 14), was discovered in Windy Hill Member, stratigraphic Zone 1 of the lower Morrison Formation. It dates back to the early Kimmeridgian, approximately 156 million years ago. It was the oldest American stegosaur known in 2001. It includes a large portion of the skull and skeleton. It contains the skull’s disarticulated parts, the lower jaws at the rear, and the hyoid. Partially articulated, the skeleton is believed to belong to an elderly person due to healed fractures. It was retrieved by the Japanese Hayashibara Museum of Natural Science in Okayama.
The Howe-Stephens Quarry, Big Horn County, Wyoming was named after the Howe Ranch. It was once visited by Barnum Brown and later purchased by Press Stephens. From 1995, Hans Jacob Siber, a Swiss Palaeontologist, excavated stegosaur fossils. SMA 3074FV01 (also SMAM04), was the first skeleton. It was named “Moritz” in honor of Max und Moritz, who had previously been called “Max” by the Galeamopus Sauropod Skeleton. SMA 0018, also mistakenly called SMA V03, was discovered in 1996/97. It was named “Victoria” because of the sense of victory that the team experienced when they found Allosaurus “Big Al Two”, after the original “Big Al”, which had been taken as federal property. It is a complete skeleton, including a skull. Also preserved skin and horn sheath impressions. SMA L02 was the third specimen found in 2002. It was named “Lilly” for the two sisters Nicola Lillich, who helped with the excavations as volunteers. These specimens are part the Aathal Dinosaur museum’s collection in Switzerland. They were initially considered Stegosaurus Exemplars. In 2009, only “Moritz”, “Lilly”, and “Lilly,” were reclassified to cf. Hesperosaurus mjosi. Emanuel Tschopp and Nicolai Christiansen referred to “Victoria”, Hesperosaurus mjosi in 2010.
Carpenter originally believed that Hesperosaurus had been a basal stegosaur. Susannah Maidment, along with her colleagues, published a more detailed phylogenetic analysis in 2008. It was found to be a derived form that is closely related to Stegosaurus, Wuerhosaurus, and was then published by Maidment. Hesperosaurus should therefore be considered a species Stegosaurus. Hesperosaurus became Stegosaurus majosi, while Wuerhosaurus was renamed to Stegosaurus homheni. Carpenter rejected the synonymy of Hesperosaurus and Stegosaurus in 2010, stating that Hesperosaurus was sufficiently distinct from Stegosaurus for it to be called a separate genus. Christiansen, in 2010, affirmed the same conclusion. Raven and Maidment recognized Miragaia and Hesperosaurus in 2017 as distinct genera from Stegosaurus.
Additional specimens were discovered in 2015: a concentration at least five individuals was found at the JRDI 5ES Quarry, Montana near Grass Range and two individuals were found at the Meilyn Quarry, Como Bluff. A new H. mjosi specimen was discovered in Montana in 2018.
Hesperosaurus, a large stegosaurid, is also known as. Gregory S. Paul in 2010 estimated the length of Hesperosaurus at 6.5m (21.3 ft) and the weight at 3.85 tonnes (3.86 short tons).  Thomas Holtz, 2012, gave a shorter estimate of 5m (16 ft), and a range of weights of 454-907kg (1.000-2.000 lbs).
Carpenter presented a diagnosis in 2001. Carpenter provided a diagnosis in 2001. Hesperosaurus was deemed to be rather basal. Many comparisons were made to the Huayangosaurian Huayangosaurus. However, these were no longer relevant once it became apparent that the phylogenetic position of Hesperosaurus was quite derived. Maidment identified three autapomorphies in 2008: The possession of 11 back vertebrae, the fourth sacral not being fused with the sacrum and back plates that are taller (from the front to the rear) than they are tall. Maidment also identified traits where Hesperosaurus was less basal than Stegosaurus Armatus. The atlas shows that even adult specimens do not have the neural arches fused to their intercentrum. Postzygapophyses are the rear joint processes of the rear neck vertebrae. They do not protrude upwards. The neural arches above the neural canal are not particularly long in the back vertebrae. There are ossified tendon at the hip region. At their lower ends, the ribs expand. The neural spines at the tail vertebrae’s lower ends are not bifurcated. In side view, the lower end of a pubic bone is expanded (spoon-shaped). Carpenter found this differential diagnosis problematic as he considered Stegosaurus Armatus, the type Stegosaurus species, a nomen Dubium. He rejected Maidment’s classification of all North American Stegosaurus material into one species. The great variability makes it difficult to distinguish Hesperosaurus from the rest. He considered Stegosaurus staenops, the historical name given to several well preserved specimens, to be a distinct species and offered a new differential diagnosis for Hesperosaurus. The antorbital penestra is larger than it should be. Instead of being a third the length, the maxilla is shorter and deeper, with a height half as high as its length. The basisphenoid for the lower braincase has a shorter than usual length. Instead of the ten neck vertebrae, there are thirteen. Instead of the seventeen dorsal vertebrae, there are thirteen. The basal form of the middle dorsals is that they have a lower neural arch than their high counterparts. The lower ends of the cervical ribs are larger. The tops of the neural spinal spines in the front tail vertebrae are now rounded, instead of bifurcated. Instead of running parallel to its rear edge, the front edge of the shoulderblade has been indented. The front edge of the ilium’s shoulderblade is indented rather than running parallel to it. The rear edge of the ilium’s rear blade has a knob-shaped expansion. The prepubic process’s front has an upward expansion. The hip and tail bases plates are oval and lower than triangular.
Because of differences in interpretation and changes, the various published Hesperosaurus descriptions contradict each other. Carpenter originally reconstructed the skull pieces into a convex head and modeled it after Huayangosaurus. The discrepancies in vertebral count can be caused by different criteria being applied to the problem of whether (and which?) cervicodorsal vertebrae should belong to the neck or the spine. Due to erosion, it is difficult to determine the exact shape of these plates. Paul thought the neck plates were low while the back plates were taller. The Aathal specimens remain undescribed. Octavio Mateus is currently working on a complete description.
Twenty maxillary teeth per side were found, which was lower than that of Stegosaurus. Carpenter described them as being similar to Stegosaurus’s teeth, but slightly larger. Peter Malcolm Galton, 2007, identified some differences. There are two rough vertical ridges that are present on the crown’s upper portion; one per denticle. The fine grooves are very weakly developed on the tooth surface.