What is a "dinosaur"?
The word "dinosaur" comes from the Greek "terrible lizard" (δεινος + σαυρος), but the translation "fearfully great lizard" (thanks katycat) perhaps conveys better the awe felt by the first paleontologists. However, many paleontologists still consider it lacking in scientific rigour. It is usually used to encompass two different orders of animal - Saurischia and Ornithischia - which are as closely related to each other as they are to the extinct pterosaurs and modern crocodiles. Therefore these four orders are grouped under the heading Archosauria - "ruling reptiles".
The defining feature of what we call a dinosaur - that is, a Saurischian or Ornithischian - are the position of the legs. In modern reptiles, and indeed the other reptiles of the Mesozoic era, the legs come from the side of the body and the body of the animal is "slung" between them. However, dinosaurs had their legs positioned directly beneath them, like those of a modern mammal. This allowed dinosaurs to be a great deal heavier than the lizards that had come before, since legs in this position are much better weight-bearing structures.
The Two Orders of Dinosaur
What, then, is the difference between the saurischians and the ornithischians?
Saurischian means "lizard-hipped", since these dinosaurs had their hips arranged like those of a modern lizard. The pelvic girdle had a pair of flat bones at the top, attached to the spine; these are the ilium bones. Crucially though, from the hip sockets protuded a pair of "ischium bones" pointing down and back, and a pair of "pubis bones" pointed down and forwards. Early saurischians were carniverous and bipedal, and used the forelimbs to grasp prey. When sleeping, they lay on their front and the strong pubis bone lay along the ground and provided support.
The ornithischian ("bird-hipped") dinosaurs had a different arrangement of pelvic bones. The ilium and ischium bones were unchanged but the pubis bones pointed backwards, parallel to the ischia. Therefore below the pelvis was a large space and ornithischians evolved to fit the huge gut, necessary for plant-eating, into this space. Thus ornithscians, who were all herbivorous, could remain on two legs.
The other key difference between the orders occured in the skull. The skull of ornithischians had an extra bone absent in saurischians, and known as the predentary. This bone held the horny beak that allows the easiest seperation of the two groups.
The lizard-like hips are considered more primitive and it seems likely that the bird-like hip evolved from them. However, in a classic case of evolutionary convergence, bird-like hips seem to have evolved independently several times. For example, a group of dinosaurs discovered in the 1980s were clearly carnivorous saurischians and yet had a distintly bird-like hip arrangement. Likewise, during the Jurassic period, another group of bird-hipped animals evolved - birds! Incidentally, birds did not actually evolve from the bird-hipped dinosaurs but rather from the sauriscians! (Thanks again katycat)
Within these two orders existed a number of suborders to further categorise the dinosaurs.
Recall that saurischians had originally been herbivorous. As they began to increase in size, it was no longer possible to balance with such a large gut in front of the pelvis. Therefore, the largest saurischians took up a four-footed existence. These are known as the Sauropods ("lizard footed") and include well known dinosaurs such as Diplodocus. Those saurischians that remained bipedal inevitably became carnivorous and are known as Theropods ("mammal footed") - a suborder that includes many of our favourite dinosaurs such as T. Rex and the velociraptor.
The pelvic girdle of ornischians allowed many of the species to remain two-footed and these are grouped under the suborder Ornithopoda ("bird-footed") and Pachycephalosauria ("bone-headed"). However, many others became four-footed and developed heavy armour to compensate for the inevitable increased vulnerability. These are split into three suborders; Stegosauria ("roofed lizards") such as Stegosaurus, Ankylosauria ("welded lizards") such as Ankylosaurus and Ceratopsia ("horned heads") such as the marvellous Triceratops.
All dinosaurs belong to the
From there, they are split into the two orders and further into the seven sub-orders:
- Order: Saurischia
- Order: Ornithischia
Each dinosaur is then furthur classified into families (of which there are too many to list here), and then is given a name according to the usual taxonomic system of binomial classification. For example, Diplodocus is a member of the Sauropods within Sauropodomorpha, and is technically known as Diplodocus longus.
Phew! That should be all you need to classify any dinosaur. Please do /msg me with any corrections or additions as you see fit.