The evolution of highly specialized, increasingly robust and varied horns and frills in ceratopsians begs the question of their utility and function. No one doubts that horns and frills were used for protection. The ceratopsian frill provided protection for the neck, normally one of the most vulnerable parts of the body to be targeted by predators. Horns on the nose and brow were undoubtedly dangerous weapons when thrust into action by an angry ceratopsian. Why, however, was there such a great variety in the morphology of frills and horns? Why did some ceratopsids have brow horns and others nasal horns? Why were horns often curved up or even down? What was the purpose of the elaborate scallops, knobs, and spikes that adorned the frills of many of these dinosaurs? Paleontologists discuss several secondary reasons for the development of such horns and frills in the ceratopsians.
Frills and jaws. Frills may have provided additional surface area for the attachment of jaw muscles. Some paleontologists have thought that such muscles were long and stretched onto the wide, flat area of the top surface of the frill. This would have required a set of jaw muscles nearly 5 feet (1.5 meters) long in a dinosaur such as Torosaurus. Because increasing the length of a muscle does not make it stronger, however, and other equally large plant-eating dinosaurs did not find it necessary to evolve such long jaw muscles, it is more likely that the jaw muscles were attached to the base of the frill and no further.
than the chasmosaurines. Their body length ranged from about 13 to 23 feet (4 to 7 m) long. With some exceptions, centrosaurine brow horns were usually greatly reduced but their frills were elaborately adorned with spikes, bony knobs, and distinctive outlines. Representative centrosaurines included Achelousaurus, Centrosaurus (Late Cretaceous, Alberta), Einiosaurus (Late Cretaceous, Montana),
Visual display. The frills of the horned dinosaurs varied widely from individual to individual within the same species. The study of the various stages of growth of ceratopsians has also shown that the frill grew large and prominent when the animals reached sexual maturity. This fact alone suggests that frills, and horns as well, were not meant purely for defensive purposes because an animal lived for most of its youth without them. Frills also may have figured prominently in attracting members of the opposite sex. Perhaps the largest and most decorative frills were viewed as belonging to the most desirable mates. Large and prominent frills also may have been colored or arrayed with various studs, spikes, and bony plates. When the animal was viewed face to face, the frill probably made the dinosaur look much bigger. In addition to attracting a mate, this visual display could have been used to intimidate a predator or even another rival horned dinosaur. Lowering the head and shaking the frill from side to side might have served as a challenge or a warning to stay away.
Social combat. The male members of several kinds of modern horned animals often use their weaponry in head-to-head competition with rival males. These animals are not trying to kill each other but rather are testing each other, either to establish or maintain dominance in their herd's social hierarchy. It is possible that horned dinosaurs also behaved in this way. They may have locked nasal horns and swung their heads from side to side or butted each other to gain dominance. There is even some evidence of skull wounds because of the accidental stabbing of one male horned dinosaur by another.
Pachyrhinosaurus, and Styracosaurus. Centrosaurines also sported some of the most unusual headgear of all dinosaurs. The forward-pointing nasal horn of Einiosaurus ("buffalo lizard") resembled a giant can opener. In addition to a prominent nasal horn, Styracosaurus ("spiked lizard") had six sizeable spikes flaring outward from its neck frill.
The chasmosaurines were typically larger than the centrosaurines, spanning a range from 17 to 30 feet (5 to 9 m). The largest may have weighed close to 8.5 tons (7.7 tonnes). All had long, prominent horns over the eyes and a smaller horn on the nose. The longest of these brow horns was that of Torosaurus at about 32 inches (80 cm). In life, the horn would have been longer when covered with its kera-tinous sheath. Representative members of the chasmosaurines were Eotriceratops (Late Cretaceous, Alberta), Triceratops, Torosaurus, Chasmosaurus, and Pentaceratops (Late Cretaceous, New Mexico).
Although trackway evidence is not widespread for ceratopsians, there is abundant evidence that horned dinosaurs traveled in large groups or herds. Extensive bone beds, often with a jumble of 100 or more skeletons of juveniles and adults, have been found for Anch-iceratops (Late Cretaceous, Alberta), Chasmosaurus, Centrosaurus, Pachyrhinosaurus, and Styracosaurus. This suggests that ceratopsids gathered in large herds and possibly migrated.
An extensive bone bed of Centrosaurus was discovered in 1977 in Dinosaur Provincial Park in Alberta, Canada. The bone bed includes the remains of 300 to 400 individuals ranging from young to old. Those individuals were probably once part of an even greater herd that had tried to cross a flood-swollen river. Many of them drowned in the crossing. Their bodies were washed downstream and accumulated along the river bed, where they eventually became fossilized. The presence of shed theropod teeth at the site suggests that scavenging predatory dinosaurs picked over the bodies before they were buried.
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