Hesperosaurus: Unveiling the Mysteries of the Western Lizard

When we delve into the realm of ancient creatures, few spark as much intrigue as the dinosaurs. Among these prehistoric giants, Hesperosaurus stands out, not just for its impressive arsenal of armored ornamentation, but also for the mysteries it carries from the Late Jurassic Period. Discovered relatively recently, this herbivorous behemoth offers a fascinating glimpse into a world long gone. Its name, meaning “Western Lizard,” embodies its geographical roots, painting a picture of a time when massive reptiles dominated an American West that was truly Wild in a way it has not been since.

Hesperosaurus Key Facts

Hesperosaurus Origins, Taxonomy and Timeline

Hesperosaurus, whose name translates to “Western Lizard”, owes its nomenclature to the Greek words ‘hesperos’ (western) and ‘sauros’ (lizard/reptile). This name signifies its discovery in the western United States, a time in which the western-most regions of North America were under a large body of water called the Sundance Sea, and one could have traveled from the USA to Africa on foot, if they felt so inclined.  

Belonging to the Stegosauria group, Hesperosaurus is a proud member of the larger group of ornithischian dinosaurs known as Thyreophora – the “shield bearers”. These dinosaurs, which also includes the club-tailed Ankylosauria, are famous for the diverse array of body armor sported by the various members of the group. The type species for the genus is Hesperosaurus mjosi.

The era in which Hesperosaurus flourished is set in the Late Jurassic Period. More specifically during the Kimmeridgian Epoch, dating to approximately 157.3 to 152.1 million years ago. This era, marked by significant geological and climatic changes, provided the backdrop for the evolution and existence of Hesperosaurus.

Discovery & Fossil Evidence

The tale of Hesperosaurus’ discovery is as intriguing as the dinosaur itself. It all began in 1985, on the ranch of S.B. Smith in Johnson County, Wyoming. Here, fossil hunter Patrick McSherry unearthed the remains of what initially appeared to be a specimen of Stegosaurus. The challenging nature of the excavation, due to the hard rock matrix encasing the specimen, led McSherry to seek assistance from Ronald Mjos and Jeff Parker of Western Paleontological Laboratories, Inc. Their collaboration with paleontologist Dee Hall of Brigham Young University was pivotal in the subsequent analysis and preservation of the find.

Initially, the discovery was thought to be a specimen of the well-known Stegosaurus. However, as the remains were meticulously prepared back in the lab, a revelation unfolded. The researchers recognized that the fossils differed in certain aspects to the well-known Stegosaurus, representing instead a species previously unknown to science.

The type species, Hesperosaurus mjosi, was formally named and described in 2001 by Kenneth Carpenter, Clifford Miles and Karen Cloward. The genus name, derived from the Greek word ‘hesperos’ meaning ‘western’, pays homage to its discovery in the western United States. The specific name honors Ronald Mjos for his significant contributions to the collection and preparation of the holotype, and for facilitating a cast display at the Denver Museum of Natural History.

Holotype Fossil Evidence

The holotype, initially cataloged as HMNH 001 but now identified as FPDM-V9674 for the Japanese institute where it is currently housed, was discovered in the Windy Hill Member, a section of the lower Morrison Formation. Dating back to the early Kimmeridgian, about 156 million years ago, this find represented the oldest known American stegosaur at the time. 

The holotype comprises a nearly complete skull and a substantial portion of the skeleton. This includes disarticulated skull elements, parts of the lower jaws, a hyoid, numerous vertebrae from different body sections, ribs, a shoulder blade, a complete pelvis, ossified tendons, and several neck and back plates. Notably, the skeleton was partly articulated and showed signs of healed fractures, indicating it belonged to an aged individual. This level of preservation has provided invaluable insights into the anatomy, age, and possibly the lifestyle of Hesperosaurus.

Hesperosaurus Size and Description

Hesperosaurus, a member of the stegosaurid family, was distinguished by its unique physical characteristics. The most striking feature was its array of long, low plates running along its back in two parallel rows. These features – known as osteoderms – represent bony outgrowths of the skin, and are the most striking feature of thyreophoran dinosaurs. The plates had asymmetrical bases, suggesting they were aligned in two, possibly alternating, rows.

Hesperosaurus‘ tail was adorned with a “thagomizer”, consisting of two pairs of spikes. The front pair was thicker, while the rear pair was thinner and extended more horizontally. Perhaps the most obvious point of difference with the closely related Stegosaurus lay in the shape of Hesperosaurus’ distinctive back plates, which appeared to have been proportionally longer with a more rounded peak, compared to the more sharply pointed backplates of its celebrity cousin and possible descendant (although the jury is still out on whether these represent different sexual morphs).

A 2012 study using CAT-scans revealed fascinating details about these plates and spikes. The plates had thin, dense outer walls with a thick, spongy interior. This structure was the result of a growth process that remodeled the bone over time. The spikes were even more robust, with thicker walls and smaller cavities in their spongy core. These features were likely supplied with blood through large vessels running along their length, hinting at their potential role in the dinosaur’s physiology.

Fossilized impressions of skin

In 2010, a study of a second, partially complete specimen dubbed “Victoria” shed light on Hesperosaurus’ soft tissue, with impressions in the surrounding sediment and natural casts providing a glimpse into the texture of its skin. The lower trunk flank revealed rows of small, hexagonal scales, while higher up, larger scales formed rosette structures. Interestingly, an impression of the underside of a back plate showed a smooth surface with parallel ridges, likely representing the horn sheath of the plate. This discovery is significant as it suggests that stegosaur back plates had a defensive function, possibly providing Hesperosaurus with sharp edges for protection.

Size and Weight of Type Species

Hesperosaurus was a large stegosaur, measuring an impressive 21.0 feet in length and weighing around 3.9 tons. Some particularly large individuals may have tipped the scales at approximately 5.5 tons. This size positioned Hesperosaurus as a formidable presence in its Jurassic environment, likely deterring predators and dominating its habitat.

Contemporary Dinosaurs

Hesperosaurus was a sturdy, armored dinosaur, known for its impressive plates and spikes, was a sight to behold. But life in the Jurassic wasn’t just about looking tough; it was about survival, and for Hesperosaurus, this meant navigating a world filled with both awe-inspiring and daunting contemporaries.

Imagine a typical day for Hesperosaurus, peacefully munching on low-lying vegetation such as cycads. Nearby, the colossal Apatosaurus, larger than our spiky friend, would be sweeping its long neck through higher branches, likely targeting food just beyond a stegosaurs’ modest reach. Diplodocus, another gigantic herbivore, might have been a common sight, its size dwarfing Hesperosaurus, yet their coexistence was relatively peaceful, each sticking to their own dietary preferences. Camarasaurus, though not as large as Apatosaurus or Diplodocus, still towered over Hesperosaurus, but again, their paths crossed without much conflict, thanks to the varied levels of vegetation they each fed on.

However, not all interactions were so benign. The formidable Allosaurus, a fearsome predator of the time, posed a significant threat. Larger and more agile than Hesperosaurus, Allosaurus was a constant reminder of the dangers lurking in the Jurassic forests. While Hesperosaurus was not at the top of the food chain, its armor and spikes were not just for show; they were crucial for defense against predators like Allosaurus. In fact, a perforated allosaur tail vertebra was discovered in which the hole proved an exact match for a stegosaur thagomizer spike!

In this dynamic ecosystem, Hesperosaurus played a unique role. Nonetheless, stegosaurs ultimately proved the shortest-lived of the major dinosaur groups. They went extinct not long after the Jurassic-Cretaceous transition about 145 million years ago. This roughly coincided with the decline of cycad plants that the group may have been specialized feeders of.

Interesting Points about Hesperosaurus

Hesperosaurus in its Natural Habitat

Hesperosaurus thrived in an environment that was both challenging and bountiful. During the Late Jurassic Period, the landscape was a mosaic of lush forests, open plains, and meandering rivers, creating a habitat rich in resources but also fraught with dangers. The climate was warmer and more humid than today, supporting a diverse range of vegetation, from towering conifers to ferns and cycads.

As a herbivore, this dinosaur’s diet primarily consisted of plants, such as moss and cycads, which it likely browsed at low to medium heights. Its four-legged stance, and beak-like mouth were perfectly adapted for this feeding habit. Hesperosaurus, while not the fastest dinosaur, was likely adept at navigating its environment, avoiding predators, and seeking out the best grazing spots.

The social behavior of Hesperosaurus is still a matter of speculation. Whether it was a solitary wanderer or moved in herds remains a question. However, considering the social patterns of other dinosaurs, it’s plausible that Hesperosaurus had some level of social interaction, perhaps during mating seasons or while raising young.

Frequently Asked Questions

Sources

The information in this article is based on various sources, drawing on scientific research, fossil evidence, and expert analysis. The aim is to provide a comprehensive and accurate overview of Hesperosaurus. However, please be aware that our understanding of dinosaurs and their world is constantly evolving as new discoveries are made.

Article last fact-checked: Joey Arboleda, 11-29-2023

Featured Image Credit: Nobu Tamura, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons