Ornithischia, better known simply as the "ornithosaur", is one of the three common orders of dinosaurs (along with Ornithischia and Ornithopteryx). The other two are the Ceratopsidae and the Stegosaurus. Ornithischia was the first order of dinosaurs to survive the Cretaceous geological period, which lasted from the end of the Triassic period to the beginning of the Jurassic period in the Early Jurassic. This is also the place where many modern birds got their name.
What kind of Ornithischia did Ornithischia look like? Well, it's hard to say. Ornithischia doesn't have a complete skeleton, so we can't say for sure what kind of plumage Ornithischia had. However, there are several clues that paleontologists have found concerning the diet of Ornithischia.
The majority of ornithischians belong to the Ornithischia major clades. These include Ceratopsidae, Ankylosaurians and Cavenians. Within Ornithischia, there's a great deal of variety, which probably explains why there are so many different kinds of dinosaurs and why it's so difficult to determine what Ornithischia actually look like.
The bones of Ornithischia are relatively large and varied in shape, with the exception of the small "pelvis Ornithischia". The overall size of Ornithischia dinosaurs is compared to modern-day birds and dinosaurs, such as Triceratops. The difference is that the pelvic bone of Ornithischia has a much shorter fibula than the pelvic bones of modern birds and T-Rexes. However, the Ornithischian was only known to have a tail, so it's not clear whether it had a prehensile tail or not.
Since Ornithischia has a very diverse anatomy and a number of subspecies, it's not surprising that we still don't know much about Ornithischia well. Only recently have researchers been able to reconstruct most of the features of Ornithischia well. It has been proposed that Ornithischia came into existence during the Cretaceous geological period, which ended with the start of the Age of the dinosaurs some 65 million year ago. The same theory is applied to Ornithischia-suspectans, another sub-species of Ornithischia. These subspecies are also supposed to arrive at the same time as ornithischians.
The most complete Ornithischia fossils come from the Cerrozoic acidated rocks of South Africa. The rocks contain fossils of small dinosaurs, like ornithischians and triceratops, along with several kinds of small fish - including mollusk - and even the very first dinosaurs to walk the earth. All this was found in Ordovirgins Formation. Ordovirgins Formation isn't really a well known area of palaeontology, and the exact formation of the Ordovirgins is still a subject of great debate. It's believed that the dinosaurs in this formation came about after the same kind of explosion that caused the mass extinction of dinosaurs in the later Cretaceous geological period, but palaeontologists disagree.
Two main subspecies of Ornithischia are represented by fossils from India and China. The smallest subspecies of Ornithischia are represented by the very small hornbills called Haemorrhoids that lived alongside ornithischians and triceratops. The most complete Haemorrhoids fossils belong to the Ornithischia ceratophora, which are a type of egg laying birds. This includes numerous species of ostriches and meadow birds. The largest subspecies of Ornithischia are represented by the gigantic hornbills called Caudipteryx which roamed the deserts of central China and northern Mongolia.
Ornithischia meleagris, a type of small bird, is sometimes called the megalosaurus. A small sub-species, Ornithischia imbricata, also known as the horsebird or false rock pigeon is represented by a fossil record which has jutted out over 100 million years. Ornithischia grafaceata, a subspecies of the robust ostricole, is represented by the very large Caudipteryx. Most ornithischians and triceratops have been classified into two main categories, meaning that ornithischia can either be categorized into a large order or into several smaller groups.
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