Triceratops horridus (Marsh 1889)

Currently, there are two species commonly recognized in the Triceratops genus, Triceratops horridus and Triceratops prorsus. Many other Triceratops species have been proposed, but they are now generally recognized as different growth stages of one of the other two species (1). There is strong evidence for the differentiation of the remaining two species. The two different Triceratops are found in different levels at the Hell Creek Formation, demonstrating that they did not live at the same time (2).

With its horny frill and prominent brow horns, Triceratops horridus is one of the most recognizable dinosaurs. It lived in the Late Cretaceous (68 to 65 million years ago), up until the mass extinction that killed all non-avian dinosaurs. At 8 m in length and weighing approximately 9000 kg,T. horridus was the largest of the frilled ceratopsians and dwarfed all land animals alive today (1). Fortunately for paleontologists who wish to study this spectacular dinosaur, T. horridus is commonly found in fossil deposits. Paleontologists previously have thought that there were large numbers of Triceratops species as well. Of the many Triceratops species described, today only two are generally recognized: Triceratops horridus and Triceratops prorsus. The other “species” are now understood to be different growth stages or genders of T. horridus or T. prorsus (2).

Triceratops horridus lived approximately 68 to 65 million years ago in the Late Cretaceous. It has been found at a variety of locations throughout the United States, such as Wyoming, Montana, Colorado, and the Dakotas (1). By this time, the plants in the landscape were similar to the ones we have today. It lived among the final non-avian dinosaurs, up until the mass extinction event that marked the end of the Cretaceous (2).

Abundant T. horridus fossils have been found at the Hell Creek Formation. The environment when T. horridus was alive was very different from what is seen in that location today. The environment represented by the Hell Creek Formation was a subtropical forested floodplain. Plants such as gingko, cypress, ferns, and angiosperms were common here. There was a wide variety of other dinosaur species represented at this site as well, such as Thescelosaurus, Edmontosaurus, and Tyrannosaurus rex (3). Based on bite marks found on Triceratops bones, paleontologists have concluded that T. rex likely preyed upon T. horridus (4).

The first Triceratops specimen was discovered near Denver, Colorado in 1887. This fossil consisted of only the brow horns of Triceratops attached to a small portion of the skull. With only this small amount of material, Othniel Charles Marsh mistakenly identified this fossil as a large bison. He named the species Bison alticornis (1). A more complete skull was collected a year later from the Lance Formation, though this skull was originally described in the Ceratops genus (2). Marsh later amended his description and reclassified the skull as Triceratops (3).

Like all of the other horned and frilled ceratopsian dinosaurs, Triceratops horridus was an herbivore. They were not able to lift their head very high, so they fed at low levels. To feed on higher material, they may have used their bulk or horns to knock down trees and other tall plants (1, 2).

The teeth of T. horridus were packed into dense batteries, containing up to 800 teeth. Unlike humans who only have two sets of teeth throughout their life, T. horridus replaced their teeth constantly and without limit. Only a small number of their teeth were in use at any one time, the rest held in reserve as replacements for when a tooth wore out (1). T. horridus needed constant replacement teeth due to the wearing nature of its diet. It fed on tough and fibrous plants, such as palms or cycads (3, 4).

Although there is evidence for herding behavior in some ceratopsian dinosaurs, the horned and frilled relatives of Triceratops, there is little evidence for herding in Triceratops itself. Herding is strongly implicated in Centrosaurus, which has been found in large numbers in bone beds (1). Triceratops, while abundant, is rarely found with more than one individual at a single location. The first instance of multiple Triceratops found at one site was discovered in 2005 at the Hell Creek Formation. Three juvenile Triceratops were found grouped together. Based on depositional setting, these dinosaurs most likely died together and were rapidly buried, such as during a sudden flood event (2).

Though this is only a single instance and future finds may contradict this hypothesis, the discovery of three juvenile Triceratops together suggests that these animals aggregated in groups when young. It is possible that juvenile Triceratops had different social behavior than adults of their species (2).

The brow horns and tall frill of Triceratops horridus are easily the most recognizable features of this dinosaur. Since its discovery, paleontologists have wondered what the purpose of these features may have been. Several hypotheses stand out among these. The first is that the horns and frill were used for combat and defense (1).

Triceratops locked in dramatic combat with Tyrannosaurus rex is a popular image, an idea first postulated by Sternberg in 1917 (2). Though originating from a time riddled with fanciful, but ultimately incorrect, theories about dinosaurs, there is actual some evidence that this may have occurred. Triceratops specimens have been found with partially healed tyrannosaur tooth marks on the frill and horn (3). Given that these animals were able to heal from their injuries, it can be inferred that they survived these dangerous encounters. However, Triceratops bones found with Tyrannosaurus bite marks on the hips imply that Tyrannosaurus did have successful predation events (4).

Other Triceratops frills show evidence that they may have fought among one another as well. Some specimens have been found with healed wounds in their skull or frill which appear to been inflicted by horns of other Triceratops. The skull of Triceratops is built to absorb shock from such blows. At the base of the prominent brow horns is a large open-air sinus. These sorts of sinuses can be found in modern animals that use their heads for ramming, such as goats and sheep (5).

Regardless of whether Triceratops used its horns and frill in combat, it is highly likely that the frill functioned in display as well. In Ceratopsidae, the group containing the other horned and frilled dinosaurs, there is a wide variety of different cranial adornments. There is no clear functional reason for the vast array of differently shaped frills, so it is likely that they played a role in courtship instead. This is similar to the courtship patterns of many modern animals, such as the horned wildebeast, which uses its horns to impress a potential mate (6).

The frill of Triceratops is unusual among ceratopsids in that it does not have any fenestra, or holes. It was thick and strongly built with an extensive network of blood vessels. Some paleontologists think that the frill had such a large blood supply to make it more effective at heating or cooling Triceratops. It could have turned its frill to face the sun to warm itself, or directed blood to the frill to cool itself. Modern elephants use their ears in the same way (5).