One of the most active areas of dinosaur research in the last two decades has been a thorough reconsideration of their relationships using a new approach called cladistics, which is also known as phylogenetic systematics. Unlike the Linnean system, which puts organisms into hierarchical categories, cladistics attempts to determine the many speciation events that resulted in the separation by branching of all organisms, living and extinct. In simpler terms, cladistics is a method of analyzing the evolutionary relationships between groups to construct their family tree.
Cladistics groups organisms on the basis of shared derived characters (synapomorphies) and uses a philosophical concept called parsimony, which holds that the simplest branching pattern (the one with the fewest steps) is most likely close to the true one. Scientists using cladistics do not place organisms into nested categories like the Linnean system, because they assume that each branch occurs by the same simple process of speciation. However, the various Linnean categories are still widely used for placement of groups into categories, as is done in this book.