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i to a phylum is a division, otherwise the categories are the same. Under this classification scheme, dinosaurs are categorized as below, with the more exclusive grades descending to the right:

Phylum Chordata

Subphylum Vertebrata Class Reptilia

Subclass Diapsida

Infraclass Archosauria Superorder Dinosauria Order Saurischia Order Ornithischia

For humans, the categories would be: Phylum Chordata, Subphylum Vertebrata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, and with Homo sapiens as the species.

The modern and more commonly used classification method applied to dinosaurs, began in 1984, is the phylogenetic classification. This classification is also known as cladistics because it is based on placing organisms into units called clades, which are supposed to represent their evolutionary history or phytogeny. Thus, clades are groups of organisms composed of an ancestor and all of its descendants. They are defined on the basis of synapomorphies, which are shared, evolutionarily derived anatomical characteristics, also known as characters. For example, all mammals have synapomorphies of hair and mammary glands, which they share with ancestral mammals. Cladistic classifications are basically explanations of evolutionary relationships between organisms and are best summarized in a diagram called a cladogram (Fig. 1.3).

A cladistic classification for dinosaurs based on characters, where one clade branches to another to show descent to the lower right, is:

Cladistics produces a bush with many branches, rather than a ladder with many rungs.

Cladistics produces a bush with many branches, rather than a ladder with many rungs.

Chordata Tetrapoda Amniota Reptilia Diapsida

Archosauriformes Archosauria Ornithodira Dinosauria Saurischia Ornithischia

This may look like a "line of descent," but is not because:

1 it does not include the many branches that emanate from each clade; and

2 it does not show the timing for the evolution of a new clade (Chapters 5 and 6).

In other words, a clade did not have to become extinct in order for the next clade to evolve. Because verbal descriptions of phylogenetically-based classifications can become confusing, cladograms are more commonly used to explain them instead.

Sauropoda o

Ceratosauria

Avialae

(chapter 15)

Sauropoda o

Ceratosauria

Sauropodomorpha

(chapter 10)

Pachycephalosauria Ceratopsia o

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