Only related species can be put together in a given taxa. (Walking dinosaur costume)As related taxonomic groups evolve from one another the degree, or grade, of morphological difference between them is used to further separate them. But unlike the true separation of species, the boundaries of genera, families, etc., are arbitrary ones, set by humans. For instance, the big-cat genus Panthera excludes cougars and house cats. (Realistic dinosaur costume)There is no a priori reason for doing thisthey are close relatives. But most catologists think there are enough differences to put house cats in another genus, Fehs. It is much as meterologists arbitrarily set the boundaries between differing cloud types-there is no precise moment when a little cumulus cloud becomes a great cumulonimbus. Stellar astronomers, who can choose precise surface temperatures to classi$r stars, have an easier task. But higher taxa are by no means entirely artificial. Just as the cumulonimbus thunderhead is different from the flufff cumulus cloud, the family Tlrannosauridae really is different from the Allosauridae. In the case of genera, the rule of thumb is to separate when there is one significant functional difference present. As morphological and functional differences acqunulate, then families, orders, and so on are split off.
A good system contains as much information as it can while remaining simple enough to grasp in a single good look. (Animatronic dinosaur) Only the evolutionary high points, the major nodes of functional and phylogenetic divergence, should be formally recognized. In attempting to classi$r extinct animals, it is tempting for paleontologists to designate as many phylogenetic clades as possible, but this results in bewildering arrays of mega, hypo, and micro orders and families. If this tendency is followed to its logical conclusion, every two genera would be in a separate systematic group. To avoid such unmanageable proliferation, only the normal subrankings listed above should be used. These are all that are needed. Besides, the uncertainty inherent in the phylogenetics of long-extinct groups also works against unduly complex rankings.