The Ornithischia order of dinosaurs is a classification of herbivorous creatures that thrived throughout the Mesozoic era beginning over 200 million years ago. The ornithischia family of dinosaurs includes many well known dinosaurs such as the Stegosaur, and the Triceratops, and it is divided into two suborders called the Cerapoda and the Thyreophora clades, as well as five major groups known as Infraorders. Whether they were horned, frilled, or duck-billed, among them these dinosaurs shared two very distinct attributes that were essential to their longevity. These creatures had the benefit of an additional predentary bone in their lower jaw that allowed them to move their beaks and improved their efficiency as grazing animals; additionally, these dinosaurs had a pelvic and hip structure closely related to that of a modern day bird.
The dinosaurs of the Ornithischia order weren't exactly as light as a feather however, they were actually the armored battlewagons of the animal world during the Triassic, Jurassic, and even into the Cretaceous period. Endowed with armored plates, spikes on their tales, or massive horns atop their boney heads, the Ornithischia dinosaurs used these evolutionary developments to defend themselves against ferocious attacks by predators and they migrated according to environmental factors and the availability of food. Not all of them were endowed with lethal tools of physical force, some relied upon a more benign manner of defense. The frilled dinosaurs that belonged to the ornithischia order used these apendages to ward off potential adversaries.
The remains of these dinosaurs have been found all over the world, with the exception of Antarctica. The Ornithischia order contains five small Infraorder's and it is clear that their order was diverse enough for some of them to survive even after the breakup of the supercontinent Pangea, well into the Cretaceous period. While the Greek meaning of the term Ornithischia is literally "of a bird", the dinosaurs of this order are not the predecessor to birds as we know them today. Birds in fact evolved from theropods, which during the time of the Ornithischia order were some of the predominant predators of the day.
Some dinosaurs of the Ornithischia order such as the Triceratops, had large horns mounted on defensive plates that protected the creature during battle; and the Stegosaurus had large spikes mounted at the end of its stiffened tail that could be used to fend off an attacker. It is clear that their unique physical attributes were more defensive in nature, especially among the frilled Ornithischia species. Regardless of the lethality that some of them did possess however, they weren't above using a show of force to avoid a battle, even if their hostility was only a guise. Due to the ferocity of the predators that hunted them, their armor was a necessary evolutionary step that ensured that these creatures could survive from the onset of the Triassic period, into the early part of the Cretaceous period.
In total the Ornithischia order of dinosaurs includes five subgroups of classification called Infraorders, with the fifth group only having been added recently. The most distinctive external feature of these creatures is their beaks, large horns, and in some cases the presence of external skeletal armor. Despite their ferocious look and lethal tools, these dinosaurs were herbivorous and their uniquely designed beaks were used to snip off large pieces of dense plant life. The Ornithischia order was aided by a predentary bone in their jaw that allowed for the growth and movement of their beaks. This evolutionary development made them extremely efficient in terms of grazing and it is no doubt that given their large size, their unique hip morphology must have aided them in traveling the vast distances required to satisfy their dietary requirements.
The five groups of the Ornithischia order are:
Ornithopod- The ornithopods were some of the most abundant dinosaurs of the ornithischia order and included many duckbilled dinosaurs. They evolved over time from being bipedal creatures, to some having straight backs that gave them greater speed while running on all fours. The Iguanodon and Shantungo are examples of the bipedal ornithopod, while the Hadrosaurus is an example of the straight-back ornithopod.
Pachycephalosaurs- The most recent addition to the list of subgroups, the pachycephalosaurs were separated from the rest of the order and given their own group because of new fossil findings. Many of the recent fossil findings of these creatures consist of only their large boney head; the dinosaurs Stegoceras and Stenopelix belong to this group.
Stegosaurs- With armor plates running down their backs, and a tail that was embedded with sharp spikes, the stegosaurs were certainly a force to be reckoned with. While their purpose hasn't yet been proven conclusively, the armor plates on the stegosaurs back were used either for shows of dominance, or as a tool of thermoregulation.
Ankylosaurs- The ankylosaurs were the last of the Ornithischia order to die out, and some have been traced back as recently as the late-cretaceous period.
Ceratopians- These were the horned and frilled dinosaurs, including the triceratops, and they are also known for having extremely boney heads.
Fossil remains of dinosaurs belonging to the Ornithischia order have been found on all continents except for Antarctica. With slight evolutionary changes taking place after the continental split, many of these creatures endured until the Cretaceous-Tertiary event that led to the mass extinction of all dinosaurs on earth. The sheer size and longevity of their order makes them one of the most abundant types of dinosaur fossils that have been found, with new discoveries constantly broadening our understanding of the ornithischia order.