Because Linnean Taxonomy is no longer adequate, a new science of taxonomy called Phylogenetic Taxonomy is in use today. Phylogenetic Taxonomy groups animals based on ancestry, or phylogeny, with membership determined only by advanced characteristics. The node based Dinosauria is defined as the most recent common ancestor of Ornithischia and Saurischia, and all its descendants. Clades are diagnosed by advanced characteristics that set them apart from their ancestors. Clades are not family trees as the true ancestry of every animal would have to be known.
Clades are hypotheses of ancestry, and are governed by the rules that govern all of science.
A closely related science to Phylogenetic Taxonomy is called Cladistics. Cladistics is the science of discovering and representing the most parsimonious distribution of advanced characteristics among taxa. Cladistics produces branching diagrams called cladograms. Cladistics is used to test phylogenetic hypotheses, as the distribution of advanced characteristics should mirror the taxa's phylogeny, and the resulting cladograms are often used to represent the hypothesized phylogeny. Phylogenetic Taxonomy and Cladistics are therefore often used synonymously, although technically they are two different disciplines.