Biologists classify all types of living things using a standard system (kingdom, phylum, class, order, family, genus and species). Just like all organisms, dinosaurs are classified using this system.
Each individual type of dinosaur can be placed at a particular position in this system, however, we can also see that various types of dinosaurs have things in common and can be grouped together.
In modern biological classification systems, scientists use a method known as "Cladistics" to try group organisms together: the more closely related they are in evolutionary terms (that is to say the more recent their shared common ancestor), the more closely they should be grouped together. Thus, two animals which are in the same genus are more closely related that two animals which are only in the same family, and two animals which are in the same family are more closely related than two which are only in the same order (and so on).
Of course, trying to classify living things, or fossils is not always easy. Scientists have to try find common features shared between different animals, and use these to group them together into larger and small groups.
Today, we know that dinosaurs can be divided into two large groups (orders), and are then able to subdivide each of these orders further into separate lineages. The first person to recognize that dinosaurs could be divided into these two orders was Harry Seeley. In 1888, he published book proposing the division of dinosaurs into these two major groups, based on the shape of the animal's hip and pelvic bones - he called these groups Ornithischia ("bird-hipped") and Saurischia ("lizard-hipped").
Saurischian pelvic bones:
Over a hundred years later, we continue to use Seeley's classification system, but have also now recognized smaller subgroups and sub-subgroups within Ornithischia and Saurischia.
Here is a chart showing how the different groups and subgroups of dinosaurs are related, and therefore classified:
(to see how dinosaurs are related to other animals, please see our dinosaur family tree)
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