Cerapoda

Introduction

Cerapoda species include the horned, frilled, crested, and domed herbivorous dinosaurs. They were particularly prevalent during the Late Jurassic and Cretaceous and were widely dispersed throughout the Northern Hemisphere.

The phylogenetic definition of Cerapoda is a group that consists of Parasaurolophus walkeri and Triceratops horridus and their most recent common ancestor and all descendents. Cerapoda include three main subgroups, Ornithopoda, Ceratopsia, and Pachycephalosauria (the latter two of which form the Marginocephalia).

Fast Facts

Pronunciation: The term Cerapoda (SER-a-POHD-a) was first introduced by Paul Sereno in 1986 and means “horn-feet”.

Time Period: Middle Jurassic (176 million years ago) to Late Cretaceous (66 million years ago)

Size: 60 cm to 16.5 m (3–55 ft) in length

Weight: 0.5 kg to 23 ton (1–50,700 lb)

Diet: Herbivorous

Location: World-wide for Ornithopoda species, but apparently restricted to the Northern Hemisphere for Marginocephalia species.

Lifespan: Up to 30 years of age

Homologous Traits

All dinosaurs are defined as either bird-hipped ornithischians or lizard-hipped saurischians. Cerapods are a subgroup of Ornithischia, and therefore share all those traits that define dinosaurs, which include upright posture, modified fourth and fifth digits on the hands, three-toed feet, specialized crests on the humerus and tibia bones, and a femur with a ball-like head at one end, as well as all those traits that define the Ornithischia such as a backwards pointing pubis and ischium, an unpaired predentary bone at the lower jaw, a toothless snout, a narrow palpebral “eyelid” bone, leaf-shaped cheek teeth, at least five sacral vertebrae, and hardened tendons above the pelvis area to stiffen the backbone. Although not possessed by all ornithischian dinosaurs, Cerapoda are perhaps most famous for the ornate physical features of their skulls, including frills, horns, armor, and dermal plates, such as the tubular skull structure of Parasaurolophus or the three cranial horns seen on Triceratops.

Cerapods are characterized by the spaces (diastema) between their premaxillary and maxillary teeth, a thick and uneven layer of enamel on the inside of their lower cheek-teeth, and possessing no more than five premaxillary teeth. Cerapods are divided into two main subgroups, the Ornithopoda and Marginocephalia. Ornithopods lacked armor and had no hole in the bone of the outer lower jaw. They are noted for their elongated pubis bone and numerous herbivorous adaptations, such as strong and closely packed teeth with extensive wear and depressed jaw joints. Members of Marginocephalia are defined by relatively subtle pubis adaptations, as well as by the shelf or ridge of bone around the back of their skull, which varied greatly in size depending on the species. With the possible exception of a few fragmentary fossils, all cerapod marginocephalians are classified into the bone-headed Pachycephalosauria or the horned Ceratopsia clades.

The pachycephalosaurs are best noted for their extraordinary helmet-like skulls, which could be almost 25 centimeters (10 in) thick. Pachycephalosaurs also possessed rows of dermal ossifications around the back of the head, strengthened vertebrae, and relatively primitive dentition and jaw morphology; they were small in size. The ceratopsians are predominately characterized by the presence of a unique rostral bone located on the upper jaw, which formed the dorsal part of the beak. This bone is found in no other animal on earth and is therefore highly diagnostic. Ceratopsians are also defined by their flared and pointed cheek-bones (jugals) and very strong jaws. Many earlier ceratopsians did not possess horns or neck frills, but these structures are well recognized on later ceratopsians such as the famous Triceratops.

Evolutionary Divergences

Cerapoda differ from their sister-clade Thyreophora in that they did not possess body armor. It is believed that Cerapoda first appeared during the Middle Jurassic period, having evolved from more primitive forms. The evolutionary split between the Thyreophora and Cerapoda (Neornithishcia) occurred about 200 million years ago during the Early Jurassic.

The Cerapoda clade consists of Marginocephalia together with Ornithopoda. The evolutionary division between these two groups occurred about 165 million years ago. Early basal forms of ornithopods likely descended from a common Asian ancestor that existed prior to the Cretaceous period. The split between heterodontosaurids and euornithopods groups of Ornithopoda occurred relatively quickly. By the Late Cretaceous, species had dispersed from Asia to North America, Europe, and finally into South America.The two Cerapoda groups classified under Marginocephalia are the bone-headed Pachycephalosauria and the beaked Ceratopsia–superficially different in appearance but evolutionarily and phylogenetically closely connected. The most basal of the Ceratopsia appeared during the Jurassic Period in northern China or Mongolia, with the more advanced species later dispersing into North America. A small bipedal Asian species known as Psittacosaurus is thought to represent the primitive ceratopsian group, although, interestingly, all later more derived ceratopsians developed quadrupedal lomotion. Basal Pachycephalosauria were also bipedal and first appeared in the fossil record during the Middle Cretaceous in Central Asia. Unlike the ceratopsians, the later pachycephalosaurs remained bipedal and they dispersed into North America and Europe during the Cretaceous, with many species evolving ever larger head domes.

Creatures in This Group

Ornithischia are one of the two orders of Dinosauria, predominately classified by the arrangement of their hips. Basal ornithischian dinosaurs split to form a single Jurassic species known as Lesothosaurus and the Genasauria clade. Genasauria is currently divided into the Thyreophora (“shield bearer”) and Cerapoda (“horn-foot”), which is further divided into the Ornithopoda (“bird-foot) and Marginocephalia (“fringe heads”). The common ancestral link between the thyreophorans, marginocephalians (pachycephalosaurs and ceratopsians), and ornithopods means that species from these divisions are grouped together in the node-based Genasauria clade.

The term Cerapoda is often used interchangeable with Neornithischia, which is also defined as containing the Ornithopoda, Ceratopsia, and Pachycephalosauria. Strictly speaking, however, these two terms are not synonymous with each other, with neornithischians defined as all genasaurians more closely related to Parasaurolophus walkeri than to Ankylosaurus magniventris or Stegosaurus stenops. Specifically, Cerapoda is a subclade of the stem-based Neornithischia clade.

Cerapoda dinosaurs include Ornithopoda and Marginocephalia, which is, in turn, comprised of Ceratopsia and Pachycephalosauria. Ornithopoda is a stem-based clade consisting of the small Middle Jurassic Heterodontosauridae and the Middle Jurassic to Late Cretaceous Euornithopoda branches and including species noted for their crests, duck-bills, and beaks. Marginocephalia dinosaurs are noted for the bony shelf at the back of the skull and include the parrot-beaked ceratopsians, such as the Late Cretaceous Triceratops from western North America, and the bony-headed pachycephalosaurians, such as the Late Cretaceous Pachycephalosaurus from North America.

Ecology

Cerapods lived in many different habitats and roamed the terrestrial landscape from the Middle Jurassic to the end of the Cretaceous some 65 million years ago. Asian pachycephalosaurs were found in arid desert-like environments, while many North American marginocephalians were found in wide temperate coastal plains, with pachycephalosaurs even found within mountainous regions. Whatever the region, Cerapods were adapted to an herbivorous diet, which they consumed by shearing foliage from low-lying vegetation then grinding and/or chewing it. The chewing behavior seen in ornithopods was particularly sophisticated and quite unique among reptiles. Paleontologists believe that the adoption of chewing behavior significantly influenced the successful radiation of these species during the Cretaceous period when flowering plant species expanded and diversified. Herbivores develop various defensive mechanisms to protect themselves or avoid predation from the carnivorous theropods. Cerapods evolved a number of different techniques as they did not possess the famous body armor seen in the thyreophorans species, nor were they as fast as many of the carnivores. Many paleontologists believe, for example, that ornithopods relied on safety in numbers, with many species living in large non-segregated herds, as well as having better running stability and maneuverability than the faster theropods. The other group of cerapods (the marginocephalians) also adopted a number of defensive mechanisms and structures. The hard dome-headed pachycephalosaurians are thought to have used their thick skulls as battering rams, with the hard bone helmets and shock absorbency adaptations used to “punch” the more vulnerable underside of predators, while the ceratopsians likely used their famous horns for defense.

However, many paleontologists believe that the domes of pachycephalosaurs and horns of ceratopsians were not only used as defensive structures. There is growing evidence to suggest that both these anatomical features were also involved in intraspecific behavior such as sexual display, social dominance, mate selection, and ritualized combat. It is thought that many cerapods were gregarious and lived in large non-segregated herds and, therefore, such behaviors were likely. This assumption is strongly supported by the growing unearthing of multiple bone beds of cerapods, some of which have exceeded 100 individual dinosaurs. The gregarious nature of these species is also supported by the ornate frills and horns and nasal crests, which are structural adaptations of social living.

Paleontology News

Although Ceratopsia was first introduced by Othniel Charles Marsh in 1890, the Pachycephalosauria suborder was not named until 1974 by Teresa Maryańska and Halszka Osmólska. Furthermore, the Cerapoda clade and Marginocephalia, which is comprised of Ceratopsia and Pachycephalosauria, was not formed until 1986 by Paul Sereno. Many consider the work of Sereno to be among the most influential in terms of ornithischian classification, with his research dominating our understanding of Cerapoda.

According to later literature, however, Sereno discarded the taxon “Cerapoda” in favor of “Neornithischia,” a taxon term introduced by Cooper in 1985, which Sereno defined as a stem-based clade. The term “Cerapoda” is still used by many paleontologists, but its questionability does once again highlight the difficulty many researchers have in correctly classifying dinosaurs and accurately defining the phylogeny of these species.

Bibliography

Books

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. New York: Cambridge University Press, 2009.

• A book aimed at non-specialists and those with little understanding or background in dinosaurs, it focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

—. Evolution and Extinction of the Dinosaurs. New York: Cambridge University Press, 2005.

• This is a very well-illustrated text, covering complex scientific processes and dinosaur species in an informative and comprehensible way.

Lucas, Spencer George. Dinosaurs: The Textbook. New York: McGraw-Hill, 2005.

• This book gives students a great précis regarding important aspects of dinosaur discovery, behavior, biology, and classification.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

• Introduction to the Study of Dinosaurs is exactly as the title claims, a comprehensive and up-to-date introduction to dinosaurs, providing scientific based chapters on the major dinosaur clades, as well as anatomical, physiological, and behavioral information.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond hill, on: Firefly Books, 2009.

• This text provides useful information on more than 500 dinosaurs, such as discovery and location and characteristics like anatomy and diet.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkeley: University of California Press, 2007.

• This book is an in-depth text providing resources and scientific papers on dinosaurs, especially looking at the saurichia and ornithisia orders in regards to their evolution, distribution, and ecology.

Journals

Brusatte, Stephen L., Sterling J. Nesbitt,, Randall B. Irmis, Richard J. Butler, Michael J. Benton, and Mark A. Norell. “The Origin and Early Radiation of Dinosaurs.” Earth-Science Reviews 101.1–2 (2010): 68–100.

Langer, Max C., Martin D. Ezcurra, Jonathas S. Bittencourt, and Fernando E. Novas. “The Origin and Early Evolution of Dinosaurs.” Biological Review 85 (2010): 55-110.

Nesbitt, Sterling J. “The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades.” Bulletin of the American Museum of Natural History 352 (2011): 1-292.

Additonal Works Used

Butler, Richard J., Paul Upchurch, and David B. Norman. “The Phylogeny of the Ornithischian Dinosaurs.” Journal of Systematic Palaeontology 6 1(2008): 1-40.

“Ornithischia” and “Cerapoda.” Wikipedia. Web. June 2011. <http://en.wikipedia.org>.

Sereno, P. C. “Basal Archosaurs: Phylogenetic Relationships and Functional Implications.” Journal of Vertebrate Paleontology 11.S4 (1991): 1-53.