Cladistics works as a classification system by showing how organisms with certain inherited traits have common ancestors, which makes any organism with those characters a member of a clade. Consequently, all animals that have a notochord, pharyngeal gill slits, and a dorsal nerve cord belong to Chordata. Classification of animals with these shared traits places humans in the same clade as sharks and Dinosauria. Similarly, the formation of bones in vertebrates is an ancestral trait that characterizes the clade Vertebrata. However, subsequent evolutionary innovations of vertebrates since their formation resulted in new clades through geologic time, which paleontologists try to define on the basis of character data. For example, Chordata eventually gave rise to the character of four limbs, which partially defines Tetrapoda. Tetrapoda had a clade develop, Amniota, with the evolution of egg-laying ability (Chapters 6 and 8), which then had other reptile-like clades form that eventually had clades develop from each of them. All of this diversification of amniote clades resulted ultimately in the origin of Dinosauria as a clade, most likely by the Middle Triassic or earliest part of the Late Triassic (Chapter 6). Mammalia also originated as a clade from a common amniote ancestor shared by dinosaurs and mammals.
As is typical in scientific endeavors, total agreement is still lacking about exactly what traits define Dinosauria as a clade. The consensus reached thus far is that Dinosauria is defined primarily on the basis of synapomorphies related to locomotion, such as these previously mentioned anatomical traits:
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