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Ceratopsia

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Ceratopsia (Greek: "horned faces") is a group of herbivorous, beaked dinosaurs which thrived in North America and Asia during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. Early members such as Psittacosaurus were bipedal. Later members, including ceratopsids like Centrosaurus and Triceratops, became very large and developed elaborate facial horns and neck frills. While the frills might possibly have been a form of armour, protecting the vulnerable neck from rear predator attack, they were probably too fragile for defense and may have been used for display or thermoregulation.

Triceratops is by far the most well-known ceratopsian to the general public. It is traditional for ceratopsian genus names to end in "-ceratops", although this is not always the case. One of the first named genera was Ceratops itself, which lent its name to the group, although it is considered a nomen dubium today as it has no distinguishing characteristics that are not also found in other ceratopsians.

It has been noted by several nomenclaturists that the name Ceratopsia is actually incorrect linguistically, and that it should be "Ceratopia." However, this spelling, while technically correct, has been used only rarely in the scientific literature and the vast majority of paleontologists continue to use Ceratopsia. As the ICZN does not govern taxa above the level of superfamily, this is unlikely to change.

Contents

Anatomy

Ceratopsians are easily recognized by features of the skull. On the tip of the upper jaw is a unique bone, the rostral bone, which is not found in any other group of dinosaurs. Along with the predentary bone, which tips the lower jaw in all ornithischians, the rostral forms a superficially parrot-like beak. Also, the jugal bones below the eye are very tall and flare out sideways, making the skull appear somewhat triangular when viewed from above. This triangular appearance is accentuated in later ceratopsians by the rearwards extension of the parietal and squamosal bones of the skull roof to form the neck frill.

Taxonomy

Ceratopsia is usually considered an infraorder within the suborder Marginocephalia, which also includes pachycephalosaurids.

The most basal known ceratopsians are Yinlong from the Late Jurassic of western China, along with Chaoyangsaurus and the family Psittacosauridae from the Early Cretaceous Period. The rostral bone and flared jugals are already present in all of these forms, meaning that the very early evolution of Ceratopsia still remains to be discovered.

Neoceratopsia includes all ceratopsians more derived than psittacosaurids. Another subset of neoceratopsians is called Coronosauria, which currently includes all ceratopsians more derived than Auroraceratops. Coronosaurians show the development of the neck frill and the fusion of the first several neck vertebrae to support the increasingly heavy head. Within Coronosauria, three groups are generally recognized, although the membership of these groups varies somewhat from study to study, and some animals may not fit in any of them. One group can be called Protoceratopsidae and includes Protoceratops and its closest relatives, all Asian. Another group, Leptoceratopsidae, includes mostly North American animals that are more closely related to Leptoceratops. The Ceratopsoidea includes animals like Zuniceratops which are more closely related to the family Ceratopsidae. This last family includes Triceratops and all the large North American ceratopsians and is further divided into the subfamilies Centrosaurinae and Ceratopsinae (previously known as Chasmosaurinae).

Classification

Following is a list of ceratopsian genera by classification and location:

INFRAORDER CERATOPSIA

There are several other fragmentary Asian forms which may or may not be valid: Asiaceratops, Kulceratops, Microceratops, and Turanoceratops. Also Serendipaceratops, significant due to its location in Australia, and Notoceratops; remains (now missing) from Argentina.

Phylogeny

Paleontologists today agree on the overall structure of the ceratopsian family tree, although there are differences on individual taxa. There have been several cladistic studies performed on basal ceratopsians since 2000. None of them have used every taxon listed above and many of the differences between the studies are still unresolved.

Xu & Makovicky

Xu Xing of the Chinese Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, along with Peter Makovicky, formerly of the American Museum of Natural History (AMNH) in New York City, and others, published a cladistic analysis in the 2002 description of Liaoceratops. This analysis is very similar to one published by Makovicky in 2001. Makovicky, who currently works at the Field Museum of Natural History in Chicago, also included this analysis in his 2002 PhD thesis. Xu and other colleagues added Yinlong to this analysis in 2006.

CERATOPSIA
\--Yinlong
\--Chaoyangsaurus
\--PSITTACOSAURIDAE
NEOCERATOPSIA
\--Liaoceratops
\--Archaeoceratops
CORONOSAURIA
\----LEPTOCERATOPSIDAE
\   \--Asiaceratops
\   \--Montanoceratops
\   \--Udanoceratops
\   +--Leptoceratops
\
\  +--Graciliceratops
\/__PROTOCERATOPSIDAE
\  +--Bagaceratops
\ +--Protoceratops
CERATOPSIDAE
In this analysis, Asiaceratops has a variable position, coming out as the most basal leptoceratopsid when Liaoceratops is included, or as the most basal neoceratopsian when Liaoceratops is not included. The authors also conclude that Turanoceratops is a nomen dubium and not a basal ceratopsid as previously proposed.

You & Dodson

You Hailu of the Chinese Academy of Geological Sciences, also in Beijing, was a co-author of the previous analysis, but in 2003, he published his own analysis along with Peter Dodson from the University of Pennsylvania. The two presented this analysis again in 2004. In 2005, You and three others, including Dodson, published on Auroraceratops and inserted this new dinosaur into their phylogeny.

CERATOPSIA
\--PSITTACOSAURIDAE
NEOCERATOPSIA
\--Chaoyangsaurus
\--Liaoceratops
\--Archaeoceratops
\--Auroraceratops
CORONOSAURIA
\----PROTOCERATOPSIDAE
\   +--Bagaceratops
\  +--Protoceratops
\----LEPTOCERATOPSIDAE
\   +--Leptoceratops
\  +--Montanoceratops
CERATOPSIDAE
Asiaceratops is found to be a nomen dubium in this analysis, as is Turanoceratops.

Chinnery

Brenda Chinnery, formerly of the Museum of the Rockies in Bozeman, Montana, published a description of Prenoceratops in 2005 and included a new phylogeny.

CERATOPSIA
\--PSITTACOSAURIDAE
NEOCERATOPSIA
\--+--Chaoyangsaurus
\ +--Asiaceratops
\--Liaoceratops
\--Archaeoceratops
CORONOSAURIA
\----LEPTOCERATOPSIDAE
\   \--Montanoceratops
\   \--Prenoceratops
\   \--Udanoceratops
\   +--Leptoceratops
\--Bagaceratops
\----PROTOCERATOPSIDAE
\   +--Graciliceratops
\  +--Protoceratops
\--Zuniceratops
CERATOPSIDAE

Paleobiology

Biogeography

Ceratopsia appears to be Asian in origin, as all of the earliest members are from Asia. Fragmentary remains, including teeth, which appear to be neoceratopsian, are found in North America from the Albian stage (112 to 100 million years ago), indicating that the group had dispersed across the Bering Strait by the middle of the Cretaceous. Almost all leptoceratopsids are North American, aside from Udanoceratops, which may represent a separate dispersal event back into Asia. Ceratopsids and their immediate ancestors, such as Zuniceratops, are not found in Asia or any other continent, and appear to be endemic to western North America (You & Dodson 2004; Chinnery 2005).

Individual variation

Ceratopsian skulls are the most commonly preserved elements of the skeleton, and several species are known only from skulls. There is a great deal of variation between and even within ceratopsian species. Complete growth series are known for several basal ceratopsians, including Psittacosaurus and Protoceratops, where scientists have found more or less complete remains of individuals of every age, from hatchling through very old adults (Erickson & Tumanova, 2000; Brown & Schlaikjer, 1940). There is also significant sexual dimorphism in Protoceratops and in ceratopsids as well (Lehman 1990; You & Dodson 2004; Dodson et al. 2004).

Ecological role

Protoceratops is the most common dinosaur in the Mongolian sediments where it is found, and Triceratops fossils are far and away the most common dinosaur remains found in the latest Cretaceous rocks in the western United States, up to 70% of the fauna in some areas, indicating that some ceratopsians were the dominant herbivores in their environments.

Many species of ceratopsians appear to have been gregarious, living in herds. Bonebed deposits in North America indicate that some species may have lived in herds of thousands of animals.

References

 

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