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How do we know which kinds of dinosaurs were most closely related?

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For us to understand the genealogy, the relationships, among dinosaurs we use the same techniques and tools as other biologists use to understand how different groups of organisms are related. The primary one that we use is a technique called cladistics, and what cladistics is, is that we just look at number of attributes in the animal and then we try to find the shortest path through those attributes. When I say shortest path I mean the least evolutionary steps. I mean, for instance if we had, you know, three animals in the room with us now, and we had, say we had a human, say we had a mouse, and say we had a lizard. Well one of the attributes that links the mouse and the human together is hair, so if you would say that the mouse and the lizard were most closely related, you would have to have two evolutionary steps, the independent origin of hair. But by saying that hair only originated once, it's evidence for a preferential relationship between humans and mouse. So that's a very simple example, but we do that with hundreds of characters and hundreds of animals sometimes and then we have computer algorithms, which actually go through the matrix and find the shortest path through it, and it results in a family tree of these animals.