Taxonomy is the science of classifying organisms into groups by structure, origin, common ancestor, etc. One goal is to recognize the inherent order in nature. Taxonomic classifications are used to organize dinosaur species and understand how animals are related to each other as time progresses. Two taxonomic systems are used, modern cladistics and the older Linnaean system.

1. Cladistics is a method of classifying organisms by common ancestry, based on the branching of the evolutionary family tree. Organisms that share common ancestors (and therefore have similar features) are grouped into taxonomic groups called clades. Cladistics can also be used to predict properties of yet-to-be discovered organisms. Phylogenetic systematics is a method of classification based on the evolutionary history of organisms, dividing organisms into meaningful groups and subgroups. It was developed by Willi Hennig, an entomologist, in 1950, but was not well accepted until decades later.

Cladistics is based on three principles:

• Groups of organisms are descended from a common ancestor.
• At each node (divergence of a population), there are two branching lines of descendants.
• Evolution results in modifications of characteristics over time.

A cladogram is a branching diagram that depicts species divergence from common ancestors. They show the distribution and origins of shared characteristics. Cladograms are testable hypotheses of phylogenetic relationships. Dinosauria On-line has a huge cladogram of the dinosaur species, and "The Dinosaur Pages" has an index of dinosaur clades.

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2. The Swedish scientist Carolus Linnaeus (1707-1778) developed the two-name (Genus and Species) system for identifying and classifying living things. The Linnaean System is based on a simple hierarchical structure in which organisms are sorted using the nomenclature of Kingdom, Phylum, Class, Order, Family, Genus and Species. Organisms belonging to the same kingdom are not necessarily very similar; organisms belonging to the same species are very similar and can produce offspring.

Click here to see an example of a Linnaean classification of the dinosaurs from "The Dinosaur Reference Center."

Taxonomic Classification

TAXON*	STEGOSAURUS	HUMAN
Kingdom Phylum Class Order Family Genus Species	Animalia Chordata Archosauria Ornithischia Stegosauridae Stegosaurus stenops	Animalia Chordata Mammalia Primates Hominidae Homo sapiens

*Extra taxon levels can be used when needed (for example, suborders).

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