Classification of any group of animals, including the dinosaurs, relies on the character or 'shape' of certain parts (or collection of parts) of the animal. With extinct animals such as dinosaurs, of course, no soft parts are preserved as fossils, so we must rely on the preserved bones of the animals to construct a classification and to deduce their evolutionary history. Different dinosaurs are not all completely different, but have certain characters in common. By looking at what dinosaurs share characters, it is seen that dinosaurs fall into certain groups. The first person to recognise that all dinosaurs are a distinct group was the Reverend Richard Owen, in 1841. Owen also coined the term 'Dinosauria', meaning 'terrible lizard'.

A large number of dinosaurs had been discovered and described by 1887, when Professor Harry Seeley noted that the dinosaurs could be divided into two distinct groups based on the differences in the structure of the hip bones. The dinosaurs are therefore divided into two groups, the 'reptile-hipped' and 'bird-hipped' dinosaurs. This significant evolutionary change arose soon after the evolution of the dinosaurs.


The reptile-hipped dinosaurs have the more primitive hip structure, as seen in the archosaurs. In this type of hip, one of the three main bones (the pubis) points forward. Included in the reptile-hipped dinosaurs are the well-known carnivores such as Velociraptor, and the large herbivores such as Diplodocus. The saurischian dinosaurs include the largest land animals ever to have lived, such as Seismosaurus, which is estimate to have weighed up to 100 tonnes. The arrangement of bones in the saurischian hip is seen below.


The bird hipped (or ornithischian) dinosaurs, as their name reflects, have a very similar hip structure to living birds. All of these dinosaurs were herbivores, and the main groups include forms such as Hypsilophodon and the 'duck-billed' hadrosaurs such as Maiasaura (an ornithopod), the horned dinosaurs such as Triceratops (a ceratopsian) and other large herbovores such as Stegosaurus and the club-tailed Ankylosaurus. All major groups of dinosaurs can be grouped together to form an evolutionary 'tree' that illustrates their evolutionary relationships. The diagram below shows the arrangement of bones in the ornithischian hip, in which the pubis points backward parallel with the ischium.