Chasmosaurus, meaning “opening lizard”, is a genus of ceratopsian dinosaur from the Upper Cretaceous Period of North America. Its name derives from the Greek chasma: “opening” and sauros: “lizard”. The word chasma refers to the large openings in its frill. It was discovered in 1898 by Lawrence Lambe of the Geological Survey of Canada. He only recovered part of the neck frill and not knowing much about it named it Monoclonius belli. When more fossils were discovered that were more complete, the dinosaur was renamed under the genus Chasmosaurus in 1914, again by Lawrence Lambe. Since that date, more and more Chasmosaurus remains have been unearthed.

There are a number of species of Chasmosaurus known. These include C. belli, C. canadensis, C. brevirostris, C. russelli, and C. irvinensis. Another specimen, C. mariscalensis, discovered in 1989 in Texas, has been renamed Agujaceratops.

Chasmosaurus was between 16 and 20 feet in length at adulthood and weighed about 4 tons. It was an averaged sized ceratopsian dinosaur. It was an herbivore. Chasmosaurus had a long, large frill. Its face and jaws were long as well. Long-frilled dinosaurs typically have longer faces and jaws than short-frilled animals. Long frills were a relatively late development in evolution, since even the Chasmosaurus dates from 76 to 70 million years ago. The frill of the Chasmosaurus has been described as “heart-shaped”, since its bone structure consists of two large loops from a central bone. The frill may have been brightly colored, to draw attention to its size or maybe as a mating display.

The frill was so large and flimsy however, and most likely did not provide any defense from predation. In the event of a herd of chasmosaurs being attacked by a predator, the males may have formed a ring and, with frills facing outwards, would have presented a formidable challenge for the predator. There were three main facial horns on the skull of Chasmosaurus, one on the nose and two on the brow. In different findings, specimens have been discovered with either short brow horns, or long brow horns. With some debate, it is believed that short horns are from female specimens, while long horns are found on males.

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