Triceratops

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ceratopsidae

Genus: Triceratops

Species: Triceratops horridus

Introduction

With its distinctive head ornamentation consisting of three horns and frill of bone at the rear of the skull, Triceratops is one of the most-recognized Late Cretaceous dinosaur genera. Often compared to modern-day rhinoceros in both appearance and behavior, it was one of the last dinosaurs to evolve and one of the last to become extinct.

Having roamed the earth for many millions of years, dinosaurs were the most successful animal species to have ever existed in terms of longevity. The oldest ceratopsian dinosaurs, an herbivorous group, the members of which feature bony frills at their skull bases as well as unique beak structures, first appeared some 160 million years ago (Ma). The ceratopsian dinosaurs diversified in North America and Asia during the Late Cretaceous Epoch, a little less than 100 Ma. However, the horned and neck-frilled species found in the Ceratopsidae family, including Triceratops, only existed during the Late Cretaceous in North America.

Incomplete fossil records mean it is not possible to identify and describe all species that have evolved and become extinct over the course of the earth's incredibly long 4.5 billion-year history. Current scientific opinion states, in fact, that more than 99 percent of all species that have ever lived on earth are now extinct. Though extinction may be the ultimate end for all life, mass extinction episodes are relatively rare. Evidence from paleontological studies indicates that the K/T extinction event, which was responsible for the final extinction of the dinosaurs, occurred between the Cretaceous and the Tertiary (Paleogene) periods some 66 Ma. The cause of this event is not agreed on, although most theories involve meteoric impact, volcanism, sea regression, and/or climate change. It is certain that the concurrent or separate occurrence of any of these would have had a dramatic effect on the world's ocean currents, rainfall, vegetation, and global temperatures, all of which could have pushed declining species into extinction.

Classification

Paleontologists employ two main classification methods for naming and grouping dinosaur species: the Linnaean system and cladistics. The Linnaean system, developed by Carl Linnaeus in the mid 1700s, attempts to classify dinosaurs by placing them into hierarchical groupings based on their similarities to each other. It follows a generalized pattern to classify organisms that consists of kingdom, phylum, class, order, family, genus, and species.

Cladistics is the second classification system used and is favored by many paleontologists. First introduced in the 1950s, cladistics aims to highlight relationships between organisms in regards to kinship and ancestry. In this way, cladistics classifies organisms into “clades” based on the premise that organisms with common characteristics share common ancestry.

In 1887, British paleontologist Harry Seeley determined that dinosaurs could be divided into one of two orders, Ornithischia (or bird-hipped dinosaurs) and Saurischia (or lizard-hipped dinosaurs). All Cerapoda dinosaurs, including Triceratops horridus, belong to the Ornithischia order, which includes species that existed as far back as the Late Triassic period.

Due to the relative rarity of fossilization, a significant percentage of the world's extinct species are undiscovered and undescribed. Classification may be obvious for some species, but the incomplete fossil record can lead to imperfect classification and ambiguity for others. The classification of Triceratops horridus has been debated among paleontologists for many years. Triceratops is the most recognized of the Ceratopsidae family; there has been contention over the subfamily to which it belonged—the Centrosaurinae, the Ceratopsinae, or the Chasmosaurinae—based on the length of its neck frill or the length of its horns. Current morphological research places Triceratops within the Chasmosaurinae subfamily.

There is also debate regarding the number of species of Triceratops. Following the discovery of the first Triceratops fossil in 1889, at least sixteen species were proposed and described. Improved technology and modern research on found specimens indicates, however, that there are most likely only a handful of species (Triceratops horridus, Triceratops prorsus, Triceratops sulcatus, Ojoceratops fowleri, and Tatankaceratops sacrisonorum). It is thought that the mistaken identification of numerous species was actually based on the same species during different times in its life cycle. Another possibility is that due to the few specimens available for some purported species, the differences observed are actually due to individual variation or abnormalities. A 2014 analysis of Hell Creek specimens based on strata and skull features indicated that the earlier T. horridus likely evolved into the species T. prorsus over a couple million years.

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

Anatomy

One of the heaviest quadrupedal dinosaurs, Triceratops horridus possessed relatively short but strong legs, with its forelimbs and back-limbs ending in hoof-like claws. At 3 meters (10 ft) long, its massive head was almost one-third the length of its entire bulky body.

Featuring three horns, a bony frill 1.8 meter (6 foot) wide, a parrot-like toothless beak, powerful jaw, and hundreds of cheek teeth that were replenished, Triceratops was a distinctive herbivorous dinosaur. Its three horns included one short horn located above its nostrils and 1 meter (3 foot)-long horns situated above each of its eyes. The two large horns grew and developed over the course of the dinosaur's lifetime, starting as small stubs before curving during the juvenile stage and re-curving in the other direction in the adult stage. Triceratops’ neck frill was also very distinctive and lacked the presence of fenestrae (holes in the bone) that are common in other ceratopsids. The lack of fenestrae could have been a reflection, though, of the animal's maturity, as 2010 finds in the Hell Creek Formation have persuaded some to abolish the genus Torosaurus and reassign it as a growth stage of Triceratops.

Intelligence

One method used to measure animal intelligence is the encephalization quotient (EQ). Although all dinosaur species rank on the lower end of the intelligence scale compared with animal species today, with an EQ rank between 0.7 and 0.9, Triceratops horridus falls toward the lower to intermediate end of dinosaur intelligence. (Comparatively, humans have an EQ of 7.44 and dolphins measure a 5.31.) Endocasts of Triceratops demonstrate no distinctions in its brain regions, which differ from its sister group of pachycephalosaurs and may explain why it has a lower EQ.

Triceratops’ intelligence also falls below that of most carnivore species, again supporting the theory that most herbivore dinosaurs were less intelligent than their carnivorous cousins.

Reproduction and Population

A great deal of behavioral information must be inferred from preserved dinosaur remains, which makes it difficult to accurately measure the past population numbers of prehistoric animals. Given the relative quantity of fossil records, however, it appears Triceratops horridus was one of the largest in size and most abundant herbivorous species to exist in the North American region during the Late Cretaceous.

As with most dinosaurs, Tricertops was thought to have been an oviparous, or egg laying, species. Precise determinants of sex are usually destroyed during the fossilization process, making it difficult to measure sexual dimorphism. Current paleontological theory suggests that female dinosaurs may have possessed a wider, more robust pelvic anatomy to allow for the passage of eggs. Unlike birds, however, it is unlikely the very large dinosaur species practiced contact incubation due to the likelihood of crushing the eggs. Although there is evidence to suggest post-hatching parental care possibly occurred in some dinosaur species, it is still an issue of debate among paleontologists in regards to Triceratops, with fossil evidence providing a patchy picture.

Diet

All species of Triceratops were herbivorous primary consumers, meaning they received all of their energy through eating vegetation. Paleontologists believe this species most likely consumed cycads and other low-lying vegetation that could be cropped by their beak-like mouths easily. Its specialized jaw and beak-like mouth suited a grasping and plucking feeding action, rather than a biting and grazing action, which is further supported by their “cheek-teeth.” These batteries of hundreds of grinding teeth were adapted for feeding on vast quantities of rough, fibrous vegetation, thought to include cycads, palms, and ferns.

Although the dinosaur was very large, Triceratops was part of the food chain. Fossil evidence from tooth marks has shown that carnivores such as Tyrannosaurus rex actively hunted Triceratops species.

Behavior

Prehistoric animal behavior is difficult to determine and researchers must infer much from fossilized remains or from behavior seen in animals today. Current paleontological evidence from bone beds in North America suggests that ceratopsian dinosaurs, possibly including Triceratops, traveled in herds. Such behavior is representative of many large herbivorous species today, and it is thought some species of large grazing dinosaurs utilized the same behavior as a form of defense against predators.

Perhaps the most contentious debate among paleontologists in regards to Triceratops, however, is the function of its cranial adornment. Traditional views stated that the frill and horns were used in combat and defense against the saurischian predators that resided in the same areas, including Tyrannosaurus rex. Some bitemark evidence has suggested that Tyrannosaurus preyed upon or scavenged Triceratops. Most evidence has suggested that the frill may have functioned to disperse body heat or aided species recognition and was most likely related to courtship rituals and reproductive competition than active defense.

Habitat and Other Life Forms

Many new plant and animal species emerged and evolved during the Cretaceous period. Although it ended with a mass extinction event, the Cretaceous was a particularly successful time for dinosaurs.

The Triceratops species lived during the Late Cretaceous Epoch, between approximately 70 and 66 million years ago. The Late Cretaceous was also the time of carnivore species such as Tyrannosaurus and Albertosaurus, and herbivore species such as the huge armored Ankylosaurus and the helmet-crested duck-billed Corythosaurus.

Although the Late Cretaceous experienced a general cooling trend, its climate has been described as a “greenhouse climate” with little change in the equator-pole thermal gradient and no ice in the polar regions. Such a climate meant that dinosaur species were dispersed widely between the warm-temperate and far north cool-temperate areas. Many areas in the Northern Hemisphere inhabited by Triceratops still exhibited fern-dominated gymnosperm (seed-bearing plants) vegetation. Rapid radiation and diversification of angiosperms (flowering plants) was occurring as well. Triceratops appeared to have inhabited areas of forest that included stands (areas featuring specific tree species) of bald cypress, metasequoia, sequoia, sycamore, and magnolia.

Research

Given the sturdy nature of Triceratops skulls, quite a number have been preserved in the fossil record. The first Triceratops horridus remains were discovered in western North America in 1887 and were initially thought to belong to an extinct form of unusually large bison. In 1889, Othniel Charles Marsh corrected his original assumption and classified the remains as a species of Triceratops.

Despite Triceratops being one of the commonly found dinosaur fossils in the famed Hell Creek Formation in Montana, it is only recently that a Triceratops bone bed has been located. In 2005, the first Triceratops bone bed was discovered and was believed to have contained only three juvenile individuals. Thereafter, researchers more thoroughly investigated the species Torosaurus, with its three horns (at a different angle than Triceratops) and thinner and two-holed frill, and have suggested that Torosaurus was indeed a growth stage of Triceratops and not a separate genus. These scientists, John Scannella and Jack Horner at the Museum of the Rockies in Montana, suggest that the skull and frill of Triceratops went through a great many changes over the course of its life and that the tissue of the frill was actually very spongy and full of blood vessels. The nature of the frill tissue leads these scientists to believe that the use of the frill for defense would have been very painful and, instead, the frill signaled the creature's maturity. Researchers Nicholas Longrich and Daniel Field challenged that view, saying a lack of transitional fossils suggested Torosaurus and Triceratops were distinct species, which Scannella disputed.

In September 2010, it was announced that two additional species close to Triceratops were discovered in Utah's Grand Staircase-Escalante National Monument, Utahceratops gettyi and Kosmoceratops richardsoni. Both Utahceratops and Kosmoceratops were thought to belong to Chasmosaurinae as well. Such finds pose a challenge to paleontologists, who are seeing related but completely different species from those found in northern rock formations of Montana and Canada. A number of other ceratopsians have also been unearthed in such varied locations as China and Arizona. Among them were Auroraceratops, Nasutoceratops, Crittendenceratops, Titanoceratops and Wendiceratops.

Additional Triceratops specimens were located throughout the 2010s. In 2011 a scientific paper described a ceratopsid from the Hell Creek Formation dating to 65 Ma, making it the most recent dinosaur fossil found to date and the closest to the K-T extinction boundary. A couple years later three Triceratops skeletons of varying ages were found together in Wyoming, including one about three-quarters complete, the most intact to date. Then, in 2019 a ceratopsid was discovered in Colorado and a partial skull was found in North Dakota.

Bibliography

Baron, Matthew G., et al. “A New Hypothesis of Dinosaur Relationships and Early Dinosaur Evolution.” Nature, vol. 543, no. 7646, Mar. 2017, pp. 501–6, doi:10.1038/nature21700. Accessed 25 Mar. 2020. ‌

Department of Palebiology, Smithsonian National Museum of Natural History. Smithsonian Institution, www.paleobiology.si.edu. Accessed 20 Mar. 2020.

Dinobase. University of Bristol, dinobase.gly.bris.ac.uk. Accessed Mar. 2009.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. 3rd ed., Cambridge UP, 2016.

Grabianowski, Ed. “Everything You Need to Know about the Scientific Controversy That Could Change Triceratops Forever.” Gizmodo, 14 May 2012, io9.gizmodo.com/everything-you-need-to-know-about-the-scientific-contro-5909645. Accessed 27 Mar. 2020. ‌

Hopson, James A. “Relative Brain Size and Behavior in Archosaurian Reptiles.” Annual Review of Ecology and Systematics, vol. 8, 1977, pp. 429–48.

Jerison, H. J. “Brain Evolution and Dinosaur Brains.” American Naturalist, vol. 103, , no. 934, 1969, pp. 575–88.

Lucas, Spencer G. Dinosaurs: The Textbook. 6th ed., Columbia UP, 2016.

Martin, Anthony J. Introduction to the Study of Dinosaurs. 2nd ed. Blackwell, 2006.

Natural History Museum, www.nhm.ac.uk. Accessed 20 Mar. 2020.

Nichols, Douglas J., and Kirk R. Johnson. Plants and the K-T Boundary. Cambridge UP, 2008. Cambridge Paleobiology Series.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Updated and revised ed., U of California P, 2000.

Poinar, George, and Roberta Poinar. What Bugged the Dinosaurs? Insects, Disease, and Death in the Cretaceous. Princeton UP, 2007.

UCMP. University of California Museum of Paleontology, 2020, www.ucmp.berkeley.edu. Accessed 20 Mar. 2020.

Weishampel, David B., et al, editors. The Dinosauria. 3rd. ed., U of California P, 2007.

Willis, K. J., and J. C. McElwain. The Evolution of Plants. 2nd ed., Oxford UP, 2014.