Centrosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ceratopsidae

Genus: Centrosaurus

Species: Centrosaurus apertus

Introduction

Fossil records indicate that Centrosaurus, a single-horned ceratopsid, was a prolific dinosaur that dominated western Laurasia in the latter part of the Late Cretaceous period (99–66 million years ago). Extensive fossil deposits have been discovered in North America and as far east as Asia, demonstrating that ceratopsids migrated between the two continents when they were still joined by a land bridge.

The largest dinosaur fossil bone bed ever discovered in the world to date, near Hilda, Alberta, Canada, contains the fossilized remains of hundreds of Centrosaurus specimens, ranging from juveniles to adults. The site spans an area of 2.3 square kilometers (1.4 sq mi). Scientists believe that the mass death was caused by drowning, proposing that a tropical storm caused flash flooding that swept over the plain where Centrosaurus herds were browsing. Because of their size, the great creatures could not tread water for long and drowned. When the flood waters receded, the bodies of the felled creatures concentrated in specific areas.

The finding reveals a great deal about Centrosaurus's development and lifestyle, including an insight into the purpose of its elaborate frill and prominent horn, and how the animal developed from youth to adulthood. Scientists have been able to infer that Centrosaurus was a social animal that probably practiced complex social behaviors.

Classification

Scientists classify dinosaurs in different ways. The Linnaean system is a hierarchical classification system that narrows species into order, genera, and species. In the late twentieth and early twenty-first century, paleontologists moved towards cladistic or phylogenetic classification systems to classify dinosaurs according to their common ancestry based on shared physical characteristics.

Fundamentally, dinosaurs are classified into one of two orders: Ornithischia (bird-hipped dinosaurs) or Saurischia (lizard-hipped dinosaurs).According to Linnaean taxonomy, the herbivorous (plant-eating) Centrosaurus is in the order Ornithischia. It then falls under the Ceratopsidae family of dinosaurs, which are characterized by beaks and elaborate horns and frills, and further falls within the Centrosaurinae subfamily, a group of horned dinosaurs.

Centrosaurus is a member of the larger Ceratopsia infraorder, of which Triceratops is the most well-known genus. It was a hulking beast, weighing up to 8 tons (17,600 pounds). Smaller and more primitive ceratopsians have been discovered in Asia, some of them weighing as little as 25 kilograms (55 pounds). The ceratopsians were characterized by horned faces, elaborate neck frills and pointed, parrot-like beaks. The number, size, and position of horns, as well as the size and shape of the frill, differ between genera. Ceratopsians were herbivorous animals that walked on four legs.

Cladistic analysis is based on more detailed physical characteristics; for example, the shape of the skull and features of bone and skeletal arrangement. Centrosaurus is a member of the Centrosaurinae clade or “group” that consists of Centrosaurus, Styracosaurus, Coronosaurus, Rubeosaurus, and Spinops. The Centrosaurinae clade is characterized by a long nasal horn, short brow horns, and a comparatively short frill decorated by elaborate spikes. The bone beds in Alberta revealed that all species in this group looked identical until they reached sexual maturity, at which point they developed differences in frill ornamentation. The differences are so subtle between Centrosaurus and Styracosaurus that some scientists have questioned whether they are, in fact, variations of the same genus.

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 ornithischians, such as Centrosaurus, and theropods, such as Allosaurus, and proposed they be reclassified together into a new clade, Ornithoscelida.

Anatomy

Centrosaurus is distinguished by a single large horn on its nose, short brow horns over the eyes, two hooks that arch over the frill, and bones that decorate the frill's scalloped rim. C. apertus is distinguished by two hooked horns that adorn the front of the frill.

The animal's two-ton frame was supported on four strong legs. The unusual structure of ceratopsid legs, however, makes it difficult for scientists to accurately model how these animals walked. Some believe that the front legs were a little splayed, producing a somewhat sprawling posture at the front of the animal and an upright posture at the rear. Others believe that Centrosaurus walked with a gait and posture reminiscent of today's rhinoceroses. It is estimated that the walking pace for these animals was approximately 4 kilometers (2.5 miles) per hour, with a maximum running speed of approximately 30 kilometers (18 miles) per hour.

The growth pattern of ceratopsid dinosaurs is well documented. Scientists know, therefore, that hatchlings were born without horns and frills. These features only developed as the animal approached sexual maturity. Fossil records confirm that juvenile centrosaurine dinosaurs (a group consisting of five genera, including Centrosaurus) looked identical in their youth. The genera are largely indistinguishable until the horns and frills begin to grow and develop specific traits. Centrosaurus also exhibits sexual dimorphism (a difference in form between males and females). Scientists have been unable to definitively distinguish the sex of fossilized remains, but males may have sported larger and more elaborate frills than females. These combined factors complicate the identification of centrosaurine fossils. Evidence also suggests the horn changed shape over the individual's lifespan.

Intelligence

Despite that Centrosaurus had a large head relative to the size of its body, its brain was smaller than expected of a similarly-sized reptile. The EQ score for ceratopsians ranges from 0.7 to 0.9 (Hopson). This compares with 0.2 for sauropods, 1.0 for crocodiles, up to 2.0 for theropods and 5.8 for dromaeosaurids. Centrosaurus's limited intellectual prowess surprises paleontologists considering that it was very likely a social animal that practiced complex social behaviors.

Reproduction and Population

Fossilized remains of mass graves containing multiple generations of Centrosaurus dinosaurs provide an excellent indication of how these animals developed from youth to adulthood. Scientists know that Centrosaurus did not develop its horns and frill until it approached sexual maturity and its full body size. This implies that the frill and other facial ornamentation served a reproductive purpose. Scientists theorize that Centrosaurus might have used its horn and frill in ritual combat against members of its own species to challenge sexual rivals and to attract a mate. This theory is supported by evidence of puncture wounds found on the fossilized skulls of numerous specimens. Similar behavior is observable today in such horned mammals as antelope. According to research from the Hell Creek Formation in 2010, the Triceratops frill was made of spongy tissue loaded with blood vessels, which would render defensive use of the frill potentially quite painful and dangerous to the ceratopsian. This calls into question the defensive use of the frills on other similar creatures like Centrosaurus. Scientists John Scannella and Jack Horner of the Museum of the Rockies theorize that it is more likely that the frill was an indication of maturity.

It is not known if ceratopsid dinosaurs brooded (provided parental care of their eggs) or if they cared for their young after hatching. Herding behavior implies, however, that the young were adopted into adult groups. This supposition is confirmed by the wide range of ages represented in the Alberta bone beds. Nonetheless, paleontologists are somewhat perplexed by the absence of remains of very young Centrosaurus animals in the bone beds. They speculate that smaller carcasses may have been washed away or taken by carnivorous scavengers.

Diet

The shape of Centrosaurus's teeth and jaw indicate that it was herbivorous. Its pointed beak was designed for cropping vegetation and its powerful jaw was designed for grinding leaves and branches. Centrosaurus was also equipped with a dental battery (rows of teeth), which ensured that worn teeth were constantly replaced.

Given its bulking size and rigid posture, it is likely that Centrosaurus grazed on mainly low-lying plants, such as ferns, cycads, and small conifers. As such a large animal, Centrosaurus had a high energy requirement necessitating that it spend most of the day feeding. Centrosaurus's high food demand also suggests that it roamed over a wide geography. Fossil sites and evidence of mass herding behavior also suggest that Centrosaurus preferred open terrain, such as wide plains and marshland.

Behavior

The accumulation of multiple Centrosaurus remains at fossil sites indicates that these animals lived in herds for at least part of the year. Ceratopsid bone beds rarely contain mixed species, implying that Centrosaurus lived primarily with its own species. This has important implications for the purpose of these dinosaurs’ elaborate horns and frills. Aside from providing protection against predators, scientists believe that variations in facial ornamentation helped ceratopsid dinosaurs to recognize their own species. Scientists investigating signs of scarring on Centrosaurus skulls and frills caused by puncture wounds have concluded that most injuries are consistent with battling members of their own species.

The ritual purpose of Centrosaurus's horns and frills suggest that these animals practiced relatively complex social behaviors. Centrosaurus very likely participated in mating displays, ceremonial combat, territorial defense, and social ordering. This assumption is further bolstered by evidence indicative of sexual dimorphism (a difference in form between sexes) within the genus. Scientists deduce that females of the species may have been impressed by larger, showier males. Such behavior is evident today in modern horned animals such as antelope, elk, and sheep.

Habitat and Other Life Forms

The climate of the Late Cretaceous period was warm and humid, contributing to lush forests and fertile plains. In addition to gymnosperms (non-flowering, seed-bearing plants), angiosperms (flowering plants) were rapidly evolving into many different species. Scientists agree that herbivorous animals, such as Centrosaurus, helped to spur the rapid rise of angiosperms. Along with insects, herbivorous dinosaurs assisted by spreading pollen between flora.

Centrosaurus lived in the continent we know today as North America. In the Late Cretaceous period, North America was divided into east and west by a shallow sea. Centrosaurus lived on the western side of the sea, with fossilized remains having been unearthed in Alberta, Canada, and in Wyoming and Montana in the United States.

Centrosaurus shared its environment with a diverse number of dinosaur and primitive mammal species. This included herbivores, such as the heavily armored Panoplosaurus; a number of duck-billed dinosaurs, including Corythosaurus and Gryposaurus; as well as multiple genera of ceratopsids. Carnivorous non-avian theropods were also prolific during the Late Cretaceous period in western Laurasia. Centrosaurus may have used its horn and thick frill to defend itself from dromaeosaurs, troodontids, or tyrannosaurids, such as Daspletosaurus and Gorgosaurus. Numerous fossilized teeth from the large Albertosaurus have been found among the Centrosaurus bone beds in Alberta, indicating that scavenging theropods picked through the remains before they were buried in sediment.

Research

Centrosaurus should not be confused with the stegosaur Kentrosaurus that lived in Africa during the Late Jurassic period. Both names derive from the Greek word kentron, meaning “spike” or “sharp point.” In the case of Centrosaurus, “sharp point” refers to spikes on the animal's frill, not its horn. When Lawrence M. Lambe discovered and named the species from a fossil unearthed in Canada in 1904, the find did not include a nasal horn. Centrosaurus is also sometimes confused with Monoclonius, which was discovered in 1872 by Edward Drinker Cope in Montana, United States. The animal's name, meaning “single sprout,” describes the root of one of its teeth; again, not its horn. Its identification and naming was based on only a few fragments of skeleton, including some hip bones, vertebrae, and a few teeth. Today, many paleontologists believe that Monoclonius is a juvenile example of Centrosaurus or another centrosaurinae genus.

The largest dinosaur bone bed ever discovered was near Hilda, Alberta, Canada, during the 1990s. What scientists found was a high concentration of Centrosaurus apertus. The bone bed itself covers 2.3 square kilometers, and it has helped scientists to formulate a new theory on the reason for so many bone beds in the Alberta badlands (arid terrain eroded by wind and rain). They speculate that instead of herds drowning while crossing flooded rivers, they drowned in tropical floods on the then-coastal flat land, with no higher ground for retreat. As the heavy dinosaurs could not tread water for long periods of time, they drowned; and they were then carried away and deposited in large numbers in various spots.

The Alberta badlands hold a diverse assortment of dinosaur fossils. Excavations there have unearthed more than forty-five different genera, including primitive aquatic animals, amphibians, reptiles, and mammals. The area is of such paleontological significance that the Dinosaur Provincial Park in the valley of the Red Deer River was designated a World Heritage Site in 1979. Dinosaur Provincial Park has provided hundreds of specimens to museums around the world.

In the twenty-first century, some paleontologists claimed to have found preserved dinosaur proteins, blood, or bone cells, which generated controversy within the field as such organic materials typically decay within thousands to a few million years. Researchers Evan Saitta and his colleagues, who conducted aseptic analysis of Centrosaurus bones in the late 2010s, argued their findings disproved such claims. Instead of collagen, they discovered large concentrations of relatively rare modern bacteria living inside the Centrosaurus bones. Others thought microbes may vary among fossils and, in some cases, might even to help preserve ancient tissues. Regardless, the finding opened up a new line of scientific inquiry into the interplay between living microbiomes and host fossils, and encouraged additional caution in excavation.

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