Centrosaurus

Description

Centrosaurus was a medium-sized ceratopsid that was significantly smaller than its famous relative Triceratops. Adult Centrosaurus reached 5.5 m in length and weighed 1.4 tonnes, whereas fully-grown Triceratops exceeded 8 m in length and weighed over 10 tonnes. Walking on all fours, it had a rotund body, stocky legs, a muscular tail of moderate length. Its large head was perched at the end of a short neck. Its feet had four toes and its hands five fingers, each digit equipped with a hoof.

Centrosaurus belongs to the subfamily Centrosaurinae. The Centrosaurinae possessed a short, shield-like frill that extended at the back of the head. While paleontologists suspect the frill was entirely covered with skin, two large, symmetrical openings perforated the bony structure. A series of small horns lined the frill’s outer edge, and two larger horns projected forwards from the midline. Unlike the Chasmosaurinae (the other ceratopsid subfamily, and the one to which Triceratops belongs), centrosaurines had pronounced nasal ornamentation. They grew large nasal horns or bosses, and smaller horns above the eyes. By comparison, Chasmosaurinae had large brow horns and short nasal horns.

Did you know?

In the species name Centrosaurus apertus, “centrosaurus” derives from Greek and means “prickly lizard,” referring to the hook-shaped horns at the back of the dinosaur’s frill. “Apertus” derives from the word “aperture,” and refers to the frill’s two large openings.

Like all ceratopsians, Centrosaurus had a parrot-like beak. It possessed 28-31 closely-packed teeth, each of which sat directly on top of 3-5 fully-formed teeth ready to erupt when the top tooth was worn. When the upper teeth came in contact with the lower teeth, they rubbed against each other in a scissor-like fashion to slice vegetation.

Several specimens of Centrosaurus preserve patches of fossilized skin. The skin on the belly and sides of the animal consisted of small polygonal scales. In the lower region of the thigh, the skin consisted of a mixture of small polygonal and larger round scales. A row of long, raised scales ran along the top of the tail. Paleontologists believe that these animals had “cheeks,” either fleshy or muscular, to help keep food in the mouth while chewing. The colour of the skin is unknown.

Range and Habitat

Fossil remains attributable to Centrosaurus are found in the lower part of the Dinosaur Park Formation of southern Alberta and Saskatchewan, and in the upper Oldman Formation of southeastern Alberta. The fossils date to the end of the Cretaceous period, between 76.5 and 75.3 million years ago. Yet unpublished fossil discoveries suggest that Centrosaurus may also have inhabited northern Montana.

Centrosaurus lived on the western coastal plain of the Western Interior Seaway, an inland sea that connected the modern-day Arctic Ocean to the Gulf of Mexico, dividing North America in two. It appears to have inhabited both humid and arid habitats.

Reproduction and Development

Although presumed to have laid eggs, paleontologists have not discovered eggs or eggshell fragments for Centrosaurus or any ceratopsid. However a recent study discovered that most dinosaurs, including the primitive horned dinosaur Protoceratops, laid soft-shelled eggs, like many lizards and turtles. So it is possible that Centrosaurus also laid soft-shelled eggs, which are less likely to fossilize than hard-shelled eggs.

Paleontologists have discovered fossil remains representing nearly all growth stages of Centrosaurus, from very young to fully-grown individuals. These fossils allow paleontologists to study the changes in cranial ornamentation (horns and frill) that Centrosaurus underwent during its lifespan. In young individuals, the nasal horn began as a narrow, triangular structure that curved backwards. As the animal grew, the horn became larger and it gradually changed orientation, first becoming straight and eventually curving forward in mature individuals. The horns above the eyes started as small bone growths in young individuals, grew into tall pyramids as the animals matured, and transformed into either tall, sharp horns or low, rough bumps in adult individuals. The length of the frill and size of the openings in the frill increased through the lifespan of the animal. The horns ornamenting the periphery of the frill were absent in young individuals and grew with age.

Diet

Centrosaurus was herbivorous. Its sharp parrot beak allowed it to crop food from plants and trees. Ceratopsids could not chew by moving their lower jaws from side-to-side like mammals do. Instead, the lower jaws moved up-and-down and possibly slightly front-to-back, allowing the upper teeth to shear against the lower teeth to slice plant matter. Although paleontologists have not discovered stomach contents for ceratopsids, their jaw adaptations and quadrupedal stance indicate they had a strong bite and fed on low-growing, tough plants, such as woody browse.

Behaviour

Centrosaurus, like many ceratopsids, was a gregarious animal. It lived in herds composed of individuals that ranged in size from the very young to adults. Catastrophic events, like vast coastal floods produced by storms or hurricanes, killed large herds of Centrosaurus. The carcasses were buried to form bonebeds. These bonebeds range from hundreds of meters to several kilometers in extent (like the Hilda megabonebed) and preserve the remains of hundreds to thousands of individuals. Paleontologists have discovered over 20 Centrosaurus bonebeds in Alberta and Saskatchewan, which indicates that those catastrophic floods were frequent and widespread. One Centrosaurus individual discovered in a bonebed had an advanced form of bone cancer (osteosarcoma) on its right lower leg, which would have undoubtedly induced a severe limp. The fact that this individual lived for a long time and did not succumb to predators earlier suggests that it was protected by the herd.

For a long time, paleontologists believed that the horns and frills of ceratopsids were used for defense against predators like tyrannosaurs. However the lightly-built nature of the frill in many ceratopsids, the great amount of variation in frill ornamentation within species, and the fact that many horns point inwards rather than outwards suggest these features were used for other purposes. The absence of frill injuries in Centrosaurus suggests that this species did not use its frill for protection, but rather as a display structure to attract mates or scare competitors. Rather than head-butting, Centrosaurus may have used its nasal horn to hit the flanks of its adversaries.