In the past, systematists may have named paraphyletic or polyphyletic groups due to overall similarity of features among a group of organisms and/or lack of understanding of their relationships. In other words, birds are descendants of a particular clade of dinosaurs-meaning that the Dinosauria clade is not extinct at all!Ĭontinued Use of Paraphyletic and Polyphyletic Group Names One subclade of Archosauria is the Dinosauria in turn, a subgroup of the Dinosauria is Aves, the birds. rex, and the bird are all assigned to the monophyletic Archosauria clade. Each nested monophyletic group represents a different level of classification. Notice how each differently colored polygon represents a monophyletic group (clade): a common ancestor and all of its descendants. The phylogenetic tree below shows the relationships of seven types of vertebrates, one of which is the extinct dinosaur Tyrannosaurus rex. For instance, Class Mammalia is a monophyletic group of animals that share the characteristics of having hair and mammary glands. Rather, systematists generally try to define groups by critical features (synapomorphies) that unite them. While each node on a phylogenetic tree can be used as the anchoring point for defining a monophyletic group, systematists do not necessarily seek to name every possible group. Thus, any given taxon can belong to many monophyletic groups, some more inclusive, and some more exclusive, depending upon the ancestral node that defines each group. As discussed in Section 2.1, monophyletic groups (clades) form nested sets on a tree.
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