Chasmosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ceratopsidae

Genus: Chasmosaurus

Species: Chasmosaurus belli

Introduction

Like other members of their Ceratopsidae family, Chasmosaurus bears among the largest skulls of the dinosaur world. Known for their great horns and elaborate neck frills, ceratopsids such as Chasmosaurus emerged in the Late Cretaceous period (99 to 66 million years ago) and were extraordinarily successful in their environment, especially in North America. Their numbers flourished, and Triceratops, a close relative of Chasmosaurus, has become one of the most widely recognized dinosaurs in the world.

Chasmosaurus and its relatives are remarkable not only for their distinctive appearance, but also for the wealth of their fossil finds. Dozens of skulls and numerous partial and complete skeletons, ranging from juvenile to adult, have been unearthed throughout North America.

Chasmosaurus belli is of particular note as it was the first fossil find of the Chasmosaurus genus and has been the most common Chasmosaurus species found. At least two other species—Chasmosaurus irvinensisChasmosaurus russelli—were identified as well.

Classification

The classification system commonly used for dinosaurs and other animals is the hierarchical Linnaean system, developed by Carl Linnaeus in 1735. This system comprises different tiers of classification (class, order, genus, and species) based on shared characteristics within each grouping. Species, the lowest tier, is based on the most specific shared traits. Later, the biologist Ernst Haeckel added the phylum tier, which is the highest tier that links animals.

According to the Linnaean system, the four known species of Chasmosaurus have been classified under the order Ornithischia. Dinosaurs in this order were known for their bird-like hip structure, with the pelvic bone pointed down and back towards the tail. They also tended to be herbivores and to have hoof-like claws on their feet. Within the ornithischian order, Chasmosaurus is further classified as a member of the Ceratopsidae family. The ceratopsians comprise herbivorous quadrupeds with beaks, notable head or neck frills, and horns, generally on the nasal area of the skull if not elsewhere. They had large skulls and torsos, with short, sturdy legs and tails. The Chasmosaurus genus is particularly noted for its unusually large neck frill with two wide openings in the skeletal frame and its short nasal horn. Two of the species (C. belli and C. russelli) also had two large brow horns. (In fact, Chasmosaurus irvinensis is distinct because it lacked the two brow horns.)

Scientists may also use another classification system, called phylogenetic or cladistic classification. This system classifies animals in family trees, called cladograms, grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. Within cladistics, Chasmosaurus falls under the ceratopsian branch, or clade, of the dinosaur family tree. The ceratopsian clade breaks down into further branches, including that of Neoceratopsia and Coronosauria. The Ceratopsidae, of which Chasmosaurus is a member, fall along the coronosaurine branch. This smallest sub-family includes close relatives of Chasmosaurus, such as Triceratops.

As with any dynamic study in the field of science, cladistic analyses serve as hypotheses and can change over time as new specimens are discovered and as the science advances. In 2017 Matthew Baron and his colleagues posited a common ancestor for the ornithischians, such as Chasmosaurus, and theropods, such as Allosaurus, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Chasmosaurus is most noted for its enormous skull and accompanying horns and frill. For one of the species, C. russelli, only skull fossils exist for study. Complete skeletons exist for C. belli, and one nearly complete skeleton, absent its tail, has been found for C. irvinensis. Each skull reveals the characteristic large size of ceratopsians, complete with deep-set facial structure and broad snout ending in a short parrot-like beak. The snout itself contains large nasal cavities with a short nose horn. C. belli, as well as C. russelli, also possess long horns mounted over the eye sockets, while C. irvinensis lacks the horns but has ridges, known as "rugosities," and pits in their place. These features indicate that the species may once have had such horns, which curve upwards in the other three species.

Atop each Chasmosaurus skull extended a large, magnificent neck frill. This frill stretched back over the dinosaur's shoulders and was largely hollow in the adult Chasmosaurus. Two fenestrae, or large openings, lightened the skeletal frame, making it likely that the frill was more for ornamentation or intimidation than for fighting or defense. The edge of the frill was rimmed by protruding bits of triangular bone.

The skeleton of Chasmosaurus was broad and heavy, with a thick torso and four powerful, stout legs with five clawed toes on each foot. Sturdy ornithischian hips gave way to a short but thick tail.

Intelligence

Slower-moving herbivores tend to fall lower on the EQ scale than their faster, carnivorous, predatory counterparts. Ceratopsians, such as Chasmosaurus, are believed to have ranked somewhere in the middle of dinosaur intelligence, between 0.6 and 0.9, smarter than most other herbivores, but just below the plant-eating hadrosaurs.

Reproduction and Population

Fossil evidence suggests that ceratopsian dinosaurs, including Chasmosaurus, were among the most populous species within their respective regions during the Late Cretaceous. Scientists speculate that they were surpassed demographically only by their fellow herbivores, the hadrosaurs. One study proposes that ceratopsians made up between a quarter to more than half of the population in their communities during the final stages of the Late Cretaceous.

Chasmosaurus and other ceratopsians featured distinctive head frills and horns that may have served some defensive function but just as likely served as ornamentation to attract mates. Fossil evidence reveals variation among skin patterns and colors on the prominent frills, which might have made them specially designed to signal and compete for mates. They might also have used their horns and bony frills to conduct physical contests such as wrestling horns and head-butting—similar to those of modern buck deer in order to show off for females. Varying heights in the brow and nasal horns also suggest that male and female members were differentiated in part by these features.

Existing evidence indicates that most dinosaurs, including Chasmosaurus and other ceratopsians, were oviparous. This means that they reproduced by laying eggs. In fact, the fossil remains of ceratopsians have contributed a great deal to the study of dinosaur reproduction and nesting. Fossil sites have revealed that ceratopsians like Chasmosaurus laid their eggs in communal nesting grounds. Females from one ceratopsian species, Zuniceratops, are believed to have made nests from dirt and vegetation in which to embed their eggs. Chasmosaurus may have guarded their nests and cared for hatchlings collectively.

Diet

Chasmosaurus, and its many relatives among the ceratopsian family, were undoubtedly herbivores. Like the hadrosaurs, another successful plant-eating species of the Late Cretaceous, they likely spent much of their time searching for and consuming vegetation. The Late Cretaceous, particularly in North America, offered an abundance of diverse plant life, including the long-standing gymnosperms, such as conifers and cycads, and the newly evolved angiosperms, or flowering plants. Scientists speculate that Chasmosaurus most likely fed on low-lying and high-fiber gymnosperms and ferns; however, they continue to debate whether gymnosperms or ferns likely made up the bulk of the ceratopsian diet. Nevertheless, the horns and beaks of Chasmosaurus make it plausible that they also pulled down higher vegetation.

Behavior

Fossil evidence for ceratopsian dinosaurs indicates that they were largely social animals. The collective fossil beds of Chasmosaurus and many of its relations, for example, indicate that they were herd animals who traveled with others of their kind. However, the fossil beds of some of their relatives indicate that other ceratopsian dinosaurs may have congregated in looser communities. Most scientists seem to agree that Chasmosaurus and other ceratopsians did live in large herds, with some groups numbering into the hundreds, but that they were not consistently migratory. Instead, they likely occupied a certain home range across which they moved. The highly differentiated frills of Chasmosaurus, the structure of which was more ornamental than functional, also indicates a gregarious nature similar to that of other ceratopsians. Scientists believe the skin patterns of these frills were unique and colorful, and that Chasmosaurus used such decorative physiology to appeal to mates among numerous competitors.

As with other social animals, the herd probably provided a degree of protection against predators. The size and horns of Chasmosaurus equipped it for defense, and despite its heavy structure, research reveals that they could move quickly when needed—such as to escape the pursuit of predators. In general, however, they likely had a slow quadrupedal gait designed for browsing.

Habitat and Other Life Forms

Chasmosaurus and most of its ceratopsian relatives thrived in the western regions of the United States and Canada during the Late Cretaceous, a time of increasing plant and animal diversification. The climate and landscape of this time period differed markedly from that of the same region today. The climate was far warmer, likely sub-tropical, and the land was more likely covered in thick forests and swamps. Within this habitat, the plant-eating Chasmosaurus thrived.

The ceratopsians themselves (along with many other species) did not emerge until the Late Cretaceous, and the presence of ancient ceratopsian fossils in Mongolia suggests that they migrated from one continent to the other after the split of the great landmass Laurasia, which once comprised Asia and North America.

Even as animals diversified, new plant life emerged across the disparate continents. Though scientists believe that Chasmosaurus most likely fed on ferns and gymnosperms, new flowering angiosperms had evolved and may have contributed to the diet of the ceratopsians.

Among the dinosaurs that thrived during this period were also predators of Chasmosaurus. Specifically, many large carnosaurs, including the tyrannosaurids, stalked the land (Chasmosaurus mariscalensis and Tyrannosaurusrex shared the same time span). It is likely that such predators either hunted herds of Chasmosaurus or followed them in order to scavenge fallen members of the group.

Research

In 1914, Canadian paleontologist Lawrence Lambe named the first fossil of a new dinosaur genus, Chasmosaurus. Lambe first discovered the fossil, a fantastic neck frill, in 1902 in the Dinosaur Park Formation in Alberta, Canada. Subsequent finds of an additional 12 skulls convinced him and his colleagues that they had encountered a new species. Named for the notable ridges and ravines carved into its characteristic neck frill, this first Chasmosaurus became known as the species Chasmosaurus belli. The fossil bore a close resemblance to Triceratops, a more commonly known ceratopsid that had been unearthed in the Lance Formation of Wyoming in 1889. However, Chasmosaurus was found to predate his more famous cousin, dating back as far as 75 million years.

Following the discovery of Chasmosaurus belli, a number of additional Chasmosaurus species have been located, including Chasmosaurus russelli, in the same rock formation in Alberta by Charles Sternberg, in 1940; Chasmosaurus mariscalensis, in more distant Aguja Formation of Texas by Thomas Lehman, in 1989 (later reidentified as a new kind of ceratopsid named Agujaceratops); and Chasmosaurus irvinensis, in the Dinosaur Park Formation by Robert Holmes, in 2001. Other ceratopsid relations include contemporary dinosaurs such as Centrosaurus and Styracosaurus, also found in Alberta by Lambe in 1904 and 1913, respectively. A possible ancestor, Protoceratops, was discovered in Mongolia in 1923.

A 2016 PLoS ONE study posited that based on its frill shape and ornamentation, C. irvinensis really belonged to a distinct, but related, genus called Vagaceratops. It also acknowledged other proposed Chasmosaurus species, including C. canadensis, for which the study authors argued there was insufficient data.

In addition to the Dinosaur Park Formation and nearby Horseshoe Canyon Formation in Alberta, Canada, numerous ceratopsid finds have been made throughout the western United States. Among the richest finds are the Lance Formation of Wyoming, but Montana, Texas, New Mexico, and North and South Dakota have also yielded important finds.

A near-complete immature ceratopsid skeleton believed to belong to C. belli was found in Red Deer River, Dinosaur Park, in 2010 and described in 2016. The 1.5 meter (4.9 foot) skeleton lacked forearms, shoulders, and tip of the tail but included skin impressions on the ribs and the scleral rings in the eye sockets. The three-year-old's spine and limb bones were not completely joined yet, and its proportions differed from those of an adult, with shorter snout and larger eyes. Most notably, however, its shorter frill was fused shut, narrowed at the back, arched, and ridged in the middle—a very different shape from that of the adult. Paleontologists believed the rare find would aid in understanding the ceratopsid life course, growth rates, physiology, size, and population structure.

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