Stegoceras

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Pachycephalosauridae

Genus:Stegoceras

Species:Stegoceras validum

Introduction

Stegoceras was one of the smallest pachycephalosaur (thick-headed reptile) dinosaurs and the first to be described. It was closely related to other pachycephalosaurids, such as Pachycephalosaurus, Prenocephale, and Stygimoloch. (It is not, however, to be confused with the similarly named Stegosaurus, which was a large thyreophoran, or armored and plate-backed, dinosaur of the Jurassic period.) Stegoceras's most characteristic feature was its hard, thick skull that gave it the appearance of a having a domed head. In fact, Stegoceras and other pachycephalosaurs are often called “bonehead,” “helmet head,” or “dome head” dinosaurs.

Stegoceras has served as a model for other pachycephalosaurs because of the amount of the fossil remains compared to its relatives. Pachycephalosaur fossils are extremely rare, and only one partial Stegoceras skeleton has been unearthed (in present-day Alberta, Canada). Two other pachycephalosaur skeletons have also been discovered, both in present-day Mongolia.

Classification

Scientists often disagree as to the most accurate method of classifying living organisms, including dinosaurs. The most common classification system is the Linnaean system, a rank-based system that separates animals into different categories based on shared traits. An alternate system, called cladistics, is a method of classification that seeks to draw relationships between species.

In the Linnaean system, Stegoceras belongs to the Pachycephalosauridae family. (It, in fact, shares the same classifications with the more familiar Pachycephalosaurus, but differentiates at the genus level.) Stegoceras was bipedal (two-legged), and herbivorous (plant eating), and possessed traits common to dinosaurs of the order Ornithischia and the suborder Cerapoda. These traits include bird-like hips, skull roofs with rounded or dome shapes, and an expanded area of bone near the end of the tail that contained a chamber. Fossil evidence for many dinosaurs in the Pachycephalosauridae family classification has been dated from the Cretaceous period, which roughly spanned 145 to 66 million years ago.

Cladistic or phylogenetic classification is an alternate system that paleontologists use to classify dinosaurs according to their common ancestry. Considered a somewhat fluid system, discoveries can sometimes cause differences in opinion between paleontologists based on how they read the data. For instance, cladistic analysis carried out in the 1980s revealed similarities between pachycephalosaurids and certain species of ceratopsian dinosaurs, which were large herbivorous dinosaurs from the Late Cretaceous that were known for their armored necks and horned skulls. This analysis suggested that ceratopsians were the closest relatives of the pachycephalosaurs.

However, not all paleontologists accept this notion, partly because ceratopsians date from the Middle Jurassic period, approximately 25 million years before the arrival of the first known pachycephalosaurs. In addition, the immediate ancestors of the pachycephalosaurs have yet to be unearthed. Cladistics, nonetheless, remain helpful to scientists in understanding the similarities and differences between and among species, as well as to develop theories about how different species evolved.

In 2006, a domed specimen thought to be a juvenile pachycephalosaur of uncertain species with discovered in New Mexico. The specimen was later classified by experts as a juvenile Stegoceras validum in 2006; however, in 2011, the specimen was reconsidered an adult and classified by some as a new Stegoceras species, Stegoceras novemexicanum. This classification was highly debated by experts, who argued that the specimens were indeed juvenile and did not display enough unique features to designate a new species.

Anatomy

Like all pachycephalosaurs, Stegoceras had a very thick and well-developed domed skull roof, measuring up to 8 centimeters (3 in) thick. A bony edging or shelf rimmed the dome around the rear of the head that curved around the eyes toward the snout. This bony edging grew as a Stegoceras matured, creating a hat-like appearance.

The animal's front limbs were small and not strong enough to support its body weight. Stegoceras had much stronger rear limbs than those of other similar dinosaurs, suggesting that Stegoceras was not a quick animal but, rather, a deliberate mover. The neck of Stegoceras was thick and strong, and it had a long tail that was evenly tapered. Bony tendons ran in a crisscross pattern from the back through the tail along the upper sides, adding strength and rigidity, suggesting that the tail was not flexible.

At the base of the tail, Stegoceras had an enlarged or expanded chamber, the function of which has yet to be determined. Paleontologists have offered several ideas, however, ranging from an enlarged nerve cord or muscle supports for tail spikes, to a chamber for storing body fat or glycogen, a feature found in some present-day birds.

Intelligence

Stegoceras, as with most other pachycephalosaurs, had a skull that was about 20 times thicker than the skull of a similarly sized ornithischian. Within this thick skull was a modest-sized brain, suggesting a higher level of intelligence. In addition, digital studies of cranial imprints and endocasts of pachycephalosaurids suggest that the brains of pachycephalosaurs occupied more space in the skull cavity than in other ornithischian dinosaurs.

Reproduction and Population

The limited fossil evidence from Stegoceras offers little information regarding its population numbers, gender differences, and mating and reproduction habits. Paleontologists have analyzed dinosaur hip anatomy and concluded that females, regardless of species, had a wider frame for producing and hatching eggs. As such, it is likely that the female Stegoceras had a wider hip structure than the male.

Other evidence indicates that most dinosaurs were oviparous, meaning that they laid eggs (normally several eggs at a time). Some dinosaurs laid their eggs in nests that were eventually abandoned, leaving the eggs vulnerable to predators. Some species’ nests and eggs were safely hidden or buried; fossil evidence proves that other species’ eggs were covered and kept warm by both parents.

Paleontologists have discovered some of the most important clues to dinosaur nesting habits at Egg Mountain, in present-day Montana, which would have been within Stegoceras's habitat. Evidence discovered here suggests that different dinosaur species used the same nesting ground year after year. Scientists discovered this same pattern with the unearthing of sauropod nests and eggs in Patagonia, Argentina. Until fossil evidence is found, these assumptions will provide some basis for our understanding of the Stegoceras.

Diet

Similar to other pachycephalosaurs,Stegoceras had several different types of teeth in its jaw. Teeth held in the front of the upper jaw were cone-shaped and probably used to nip plants. Side teeth on both the upper and lower jaws were small, weak leaf-shaped and not capable of chewing the fibrous plants that most herbivore dinosaurs ate. Although the teeth appear to have been enameled on both sides, Stegoceras most likely ate soft, low- growing vegetation, as well as seeds and some fruits. Some paleontologists contend, however, that the variety of teeth may also have led Stegoceras to eat small animals or eggs, as well.

Characteristic of ornithischian (bird-hipped) dinosaurs, Stegoceras had larger cheeks that could hold food in its mouth while chewing. In contrast, saurischian (lizard-hipped) dinosaurs had no cheek pouches and swallowed foods whole. In addition, Stegoceras's narrow snout also leads paleontologists to think that Stegoceras was a choosy eater that engaged in very specialized feeding patterns.

Behavior

Because the majority of the fossil evidence of Stegoceras (and other pachycephalosaurs) is made up of dome-shaped skulls, debate has focused on the purpose of such oddly shaped heads. Common thought into the late twentieth century was that Stegoceras engaged in head-butting contests over territory or mates, similar to the behavior seen in present-day bighorn sheep. Although still inconclusive, recent structural analyses of the skull fossils suggest to some paleontologists that Stegoceras could not have effectively engaged in head-butting displays. Based on their findings, these scientists argue that Stegoceras lost protective bone material in the skull as it matured, making head butting a potentially life-threatening behavior. Some paleontologists advance a theory that Stegoceras engaged in flank-butting contests rather than head butting. Further research in this area, however, is needed.

Additional examinations of pachycephalosaur skulls reveal the presence of Sharpey's fibers, which are typically found in bone that has an external head covering. Some paleontologists suggest that Stegoceras may have had some form of head ornamentation, such as the comb of a rooster or a horny covering found on some dinosaur species, such as Triceratops. This ornamentations, scientists suggest, would have been used in communication or recognition and perhaps mating displays.

Habitat and Other Life Forms

The habitat of the Early Cretaceous period was similar to that of the Jurassic period. Lush jungles of gymnosperms (non-flowering, seed-bearing plants) were abundant. However, angiosperms, or flowering plants, first developed during the Cretaceous and were common by the middle Cretaceous. Willow, sycamore, and poplar trees appeared, along with magnolias, figs, and other herbaceous (herb) plants. Along with this new fauna, insect life rapidly diversified. Because of its small stature, Stegoceras would have relied on low-lying vegetation as the major source of its diet.

The dinosaur population thrived during the Late Cretaceous period in what is now North America. Triceratops, Tyrannosaurus, and Parasaurolophus were a few of the most well known dinosaurs. Scientists believe that in the Late Cretaceous period, many dinosaurs continued to develop and diversify. For example, hadrosaurs diversified into about 24 kinds, and the ceratopsians divided into more than 10 types, the most famous being Triceratops.

Additionally, the Late Cretaceous period was characterized by the emergence and evolution of several important animals. Placental mammals (which include such modern mammals as primates, whales, and rodents) had just evolved. Marsupials (which include the North American opossum as well as the kangaroos and koalas found in present-day Australia) also first appeared during the Cretaceous.

Along with the fierce tyrannosaurids (including the famed Tyrannosaurus rex), smaller predators such as Chirostenotes, and Troodon may have preyed upon the small plant-eating Stegoceras. Unlike the quick-footed ornithomimids that could outrun predators, Stegoceras, with its stout hind limbs, would have made much easier prey.

Research

Canadian paleontologist Lawrence Lambe discovered the first Stegoceras in the Belly River region of Alberta, Canada (now called Dinosaur Park Formations) in 1902. When first discovered, only fragmentary fossils were unearthed. Because the newly found teeth fragments resembled those of Troodon, a meat-eating predator of North America during the Late Cretaceous period, what we now call Stegoceras was considered Troodon, as well. Subsequent finds and further examination of an additional Stegoceras domed skull confirmed that it was distinct from Troodon.

With roughly ten types of pachycephalosaurs described, the fossil evidence from which to learn more is extremely limited. Most pachycephalosaurs are known mainly from their preserved skulls or thickened skull roofs. Skeletons are extremely rare or absent, with the exception of Stegoceras. Advances in high-resolution computed tomography (HRCT) have allowed scientists to examine the few valuable remains without destroying them.

Paleontologists utilizing HRCT have advanced several notable theories about Stegoceras and other pachycephalosaurs. For example, some scientists argue that many of the variations in pachycephalosaur skull thickness and size is evidence of ontogenetic (growth of a single organism) development and is not phylogenetic (evolutionary) development. Simply, these scientists argue that many pachycephalosaur fossils are the remains of dinosaurs at different ages, rather than being distinct species.

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