Ceratopsians and Pachycephalosaurs

Ceratopsians and Pachycephalosaurs

Around 144 million years ago, began the emergence of the Ornithischian dinosaurs during the cretaceous period and diversified into North America and Asia.

Ornithischians were classified as having a hip structure similar to that of birds, although they are not the descendants of birds. Marginocephalians, meaning "fringed heads" are a group of Ornithischians that have a distinctive skull structure, consisting of a slight shelf or bony frill on the back of the skull, a unique palate, and a short hip structure. These herbivores include two major groups: the Ceratopsians and the Pachcephalosaurians.

These plant-eaters include the Ceratopsians, horned dinosaurs such as the Triceratops, Styrachosaurus, Pentaceratops, and the Protoceratops. Pachycephalosaurians, thick-skulled dinosaurs like such as the Stegoceras and the Pachycephalosaurus. (1)

Ceratopsians and Pachycephalosaurs are closely related in their characteristics. Ceratopsians processed a saddle-shaped boney frill that extended from the skull to the neck and typically had horns over the nose and eyes. The most popular was the triceratops, which could reach over 26 feet and weigh in excess of twelve metric tons. Their frills served as two major functions. It protected the vulnerable neck from being harmed. The second major function that the frill provided was due to the fact that the frill contained a network of blood vessels on its underside, which were used as a means to get rid of excess heat. The Pachycephalosaurs were considered to be bipedal. They were also found to have thick skulls, flattened bodies, and tail that were covered in an array of body rods. Pachycephalosaurs were thought to have been more than fifteen feet long and processed a skull that was surrounded by a rounded dome of solid bone. It was thought that they used their heads in combat or mating contests, but that was disproved fairly recently, which I will discuss later in the paper. Both Ceratopsians and Pachycephalosaurs were "bird-hipped" and both of these suborders contained a backwards pubic bone. Both were Marginocephilia, or "fringed heads", which is one of three clads under the Orinthiscia order. They were also herbivore dinosaurs that inherited their fringe at the back of the skull from earlier ancestors.(2) Their classification is as follows. The Pachycephalosaurs were of the kingdom animalia; the phylum chordate, the class Archosauria, the subclass Orintodira, and the order Orinthiscia. The Ceratopsians were of the kingdom Animalia, the phylum Chordate, the class Archosauria, the order Rinthiscia, and the suborder Ceratopia.

The Ceratopsians are broken down into three different families: the Ceratopsidae, the Psittacosauridae, and the Protoceratopsidae. The most primitive group is the Psittacosauridae. There is only one member in this family and that is the Psittacosaurus, found in the lower cretaceous on the continent of Asia. It is considered to be a smaller form of the Ceratopsian, measuring around two meters in length. Psittacosaurus contain possess only an overhanging shelf, as apposed to the larger and more advanced Ceratopsians that possess and actual frill. There is a major difference found in the teeth of the Psittacosaurus from the earlier and more primitive Ornithischians in that "the enamel is thicker on one side of each tooth." The thickened side of the upper teeth can be found on the outside, whereas the thickened side of the lower teeth can be found on the inside of the tooth. This thickening provides the dinosaur with a "self-sharpening" edge that assists with the ability to chew threw vegetation in a more efficient way. "Psittacosaurus was probably bipedal since the fore limb is about one-half the length of the hind limb. However, the hand is broad and could have supported a portion of the body weight. Ossified tendons lay along the back from the thorax to the middle of the tail. The tibia is longer than the femur, suggesting that Psittacosaurus was fairly agile."(3)

The next two families are classified into the group called Neoceratopsia, consisting of the Protceratopsidae and the Ceratopsidae. The Neoceratopsia set themselves apart from the Psittacosaurus in the following ways: "The head and frill are much larger in the neoceratopsians, the beak of Neoceratopsians is more compressed and was probably used to pluck rather than slice plants, Neoceratopsians have horns on the nasal bones and above the orbits, the first three cervical vertebrae in Neoceratopsians are fused, and the shaft of the ischium is curved forward in Neoceratopsians."(3)

The smallest of the Neoceratopsians is the Protoceratopsids, measuring around two to five meters long. A number of Protoceratops skeletons from various growth stages have been recovered in Mongolia, making them the main source of knowledge about the Protoceratopsids. Many studies on these skeletons have revealed various significant shape changes that took place during the growth of these dinosaurs. Males and females were found to have skulls that were shaped differently. Nests with eggs have been found along with Protoceratops fossils, giving concrete evidence as to how dinosaurs reproduced, which I will discuss later. They have upward arching nasals, although some had only a small horn found on the nasal. The teeth were arranged in a fashion that was very suitable for the chewing of the teeth. When the jaw was closed they met at an angle which facilitated this. "The rest of the skeleton of Protoceratopsids is similar to the larger Ceratopsids, but less robust. One major exception is the existence of ossified clavicles in Protoceratops, but not in the Ceratopsids."(3)

The final family is the Ceratopsidae, which are found in the Upper Cretaceous but are only found in North America, not Asia. These dinosaurs possessed noticeably larger skulls measured around four to eight meters long. They possessed frills that were a lot greater in size than the previous two families, which explains the reason for the uniquely larger head. These frills were found to have horns or spikes. "The Ceratopsidae is divided into two subfamilies: the Centrosaurinae and the Chasmosaurinae. The two subfamilies can be distinguished by the size and shape of their frills. Centrosaurines have a relatively shorter frill, greater ornamentation along the edge of the frill in the form of spikes and hooks, and a nasal horn larger than the orbital horns. Centrosaurines also tend to be slightly smaller animals. Examples of Centrosaurines are Styracosaurus with long spikes on the frill, Centrosaurus with forward curved hooks on the frill, and Pachyrhinosaurus with a huge and blunt process in place of the nasal horn. Chasmosaurines have