Huayangosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Huayangosauridae

Genus:Huayangosaurus

Species:taibaii

Introduction

Huayangosaurus lived during the Middle Jurassic period (174–163 million years ago) and shared the lush continental landscape of what is now modern-day Asia with a multitude of dinosaur species. The name Huayangosaurus taibaii comes from Huayang, the ancient name for the Chinese province of Sichuan, and from Tai Bai, the famed Chinese poet from Sichuan.

Huayangosaurus is one of the smallest stegosaur species to have ever been found and was related to the larger Kentrosaurus from Africa and the largest-sized Stegosaurus from America. Considered a primitive and possibly basal species of stegosaur, this dinosaur lived some 20 million years before its better-recognized and much larger cousin, Stegosaurus. The discovery of this species highlighted the early phylogeny of stegosaurs, and provided a clearer idea of the evolutionary path of these species.

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Under the Linnaean system, Huayangosaurus falls under the Ornithischia (or bird-hipped) order and is further classified as falling under the Thyreophora suborder, the members of which are armored, as well as under the Stegosauria infraorder, the members of which are quadrupedal and feature bony plates that run along their backs. Two subfamilies are recognized in the classification of the stegosaurian species: the Huayangosaurinae and the Stegosaurinae. The Huayangosaurinae include a single genus, Huayangosaurus, are anatomically primitive, and contain the ancestor(s) of the Stegosaurinae subfamily, which consists of species such as Stegosaurus, Kentrosaurus, Lexovisaurus, and Tuojiangosaurus.

A cladistic analysis also places Huayangosaurus into the Thyreophora clade, and further into the Stegosauria clade. The possession of the armor scutes (or scales) seen in both Huayangosaurus and the ankylosaur species has provided evidence for the existence of a common ancestor between stegosaur species and ankylosaurs. Paleontologists consider these species, both of which are grouped in the Ornithischia and Thyreophora groups/clades, to be closely related.

Anatomy

Although Huayangosaurus is considerably smaller than the more-recent Stegosaurus, it still possesses the characteristic spikes on its tail and rows of alternating triangular plates along its back. While all armored stegosaurs possessed bony plates and scales known as osteoderms, their arrangement differed species to species and the possession of a sideways row of scutes across the trunk in Huayangosaurus appears to be unique among stegosaurs, although similar to those seen on the more-primitive, armored dinosaurs. Huayangosaurus also had two pairs of long tail spines at the tip of its tail and possessed a single sharp parascapular spine over each shoulder. While these shoulder spikes were present in other species such as Kentrosaurus and Tuojiangosaurus, they were absent in the famous Stegosaurus.

With the exception of Huayangosaurus, all other stegosaur species possessed markedly shorter forelimbs than hind limbs. While this is considered a shared, derived feature of these dinosaurs, the more-basal Huayangosaurus species actually possessed limbs of relatively equal length. Another apparently unique anatomical feature of Huayangosaurus in comparison to other stegosaurs is the possession of small postorbital horns located above the eye, which may indicate sexual dimorphism, a condition in which males and females of a species have different physical characteristics.

With their small wedge-shaped skull and narrow beaked jaw, Huayangosaurus's head and dentition were well suited to an herbivorous diet. While all stegosaur species appear to possess cheek-teeth, Huayangosaurus also possessed premaxillary teeth, later lost in more recent stegosaur species.

Intelligence

Although new techniques in the study of intelligence have suggested that dinosaur species were not as dim as first thought, the small brain size of Huayangosaurus places it at the lower end of the dinosaur intelligence scale, with an EQ level falling between 0.5 and 0.6. Additionally, as Huayangosaurus is considered the basal species for all stegosaurs, it was likely to be among the least intelligent of stegosaurs.

Reproduction and Population

It is difficult to interpret with accuracy the reproductive behavior and population numbers of prehistoric, extinct creatures. Fossil finds of stegosaur species, however, are broadly distributed across Europe, Africa, North America, and Asia.

Current fossil evidence shows that stegosaur species were oviparous (egg-laying) dinosaurs, possibly laying several eggs at a time. While fossilized nesting sites exist for the more recent stegosaur species, suggesting possible post-hatching parental care, such sites are rare. Paleontologists believe, however, that Huayangosaurus probably exhibited some form of parental care after hatching. Researchers also consider it likely that the body armor present on species such as stegosaurs would not have developed until after the young had hatched, allowing the young embryo to fit comfortably rolled up inside the egg.

During the rare process of fossilization, most of the elements required to accurately identify gender are destroyed. Current paleontological theory suggests, however, that some dinosaur species would have exhibited sexual dimorphism, such as wider pelvic regions in females to allow for the passage of eggs, or perhaps larger ornamental crests and armor in males to attract mates.

Diet

The small wedge-shaped skull, narrow-beaked jaw, and low-to-the-ground position of Huayangosaurus's head indicate that feeding was restricted to low-lying gymnosperms, such as ferns, horsetails, cycads, and mosses. Although current theory suggests that the elongated hind limbs of all other stegosaur species, such Stegosaurus, may have been used to reach higher foliage in trees, researchers believe this was unlikely in Huayangosaurus due to its smaller stature and shorter hind limbs.

The generalized feeding pattern of the herbivorous stegosaur species involved the cropping and plucking of foliage with toothless, beak-like mouths and chewing and grinding with their rear cheek-teeth. The possession of premaxillary teeth in Huayangosaurus, which current evidence shows were lacking in all other species of stegosaurian dinosaurs, aided this feeding type behavior.

Behavior

Difficulties in determining prehistoric behavior often lead to scientific postulations based on the behavior of extant animals that share a similar ecological niche to those extinct species. Current paleontological evidence from fossilized bone-beds and trackways suggests that, like many of today's herbivore species, some dinosaur species exhibited herding behavior, most likely as a protective measure. Although the very large Stegosaurus may have had less predation concern than the much smaller Huayangosaurus, evidence found in North America of stegosaur herding behavior is quite strong and indicates that these species lived in non-segregated herds consisting of both juveniles and adults (unlike sauropod herbivores that appeared to live in age-segregated herds). Extrapolating from this, scientists believe many of the stegosaur species, including Huayangosaurus, exhibited herding behavior.

Paleontologists debate whether the function of the dermal plates on stegosaur species is behavioral or physiological. The most common and well-supported theories state the plates were defensive structures used to protect or defend against predator attack, or were thermoregulatory structures used to help control body temperature (and possibly even “blush” to help attract mates). As the plates of Huayangosaurus are considerably smaller and sharper than the large plates of species such as Stegosaurus, however, some researchers have cast doubt on whether the former's plates would have been effective as temperature regulators.

Habitat and Other Life Forms

Huayangosaurus taibaii lived during the Middle Jurassic period (174–163 million years ago), with fossil evidence indicating that it shared the continental Asian landscape with other herbivore species, such as the sauropods Protognathosaurus, Shunosaurus, Datousaurus, and Omeisaurus, and ornithopod Xiaosaurus. Some Chinese paleontologists contend that, despite its impressive plated armor, Huayangosaurus was also preyed upon by carnivorous species such as Gasosaurus, a large tetanuran dinosaur that lived during the same time period and whose remains have also been found in the same area as Huayangosaurus.

No polar ice caps existed during the Jurassic period (201–145 million years ago), leading to much higher sea levels and a warmer climate than seen today. The tropical climate of the Middle Jurassic created an abundance of lush green landscapes. Although angiosperms (flowering plants) had yet to evolve, many terrestrial areas were covered with gymnosperms (seed-bearing plants) such as ferns, cycads, and mosses, and large tree species such as Ginkgoes and conifers. During the Middle Jurassic, the supercontinent Pangaea had already commenced its separation into the supercontinents of Laurasia, which included most of the continents that would later form the Northern Hemisphere, and Gondwana, which contained most of the land that would later form the Southern Hemisphere. This resulted in an increase of large flooded areas and distribution of temperate and tropical forests, favored by Huayangosaurus.

Research

Dong Zhiming, a paleontologist from the Beijing's Institute of Vertebrate Paleontology and Paleoanthropology, first discovered Huayangosaurus taibaii in 1982 in the Sichuan province of China.

Despite being one of the least famous stegosaurians in terms of public recognition, Huayangosaurus is actually one of the best-known examples, with the fossils of at least nine individual dinosaurs considered among the finest examples of any stegosaur fossils collected to date. For more than a century, scientists based the cranial framework of stegosaur species on one almost complete Stegosaurus stenops skull found in western America in 1877. The 1982 discovery of a complete Huayangosaurus taibaii skull, however, helped clarify the morphology and phylogeny of stegosaurs, particularly in regards to classifying relationships among stegosaurian species.

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