The taxonomic system for the hierarchical (ranked) classification of living organisms (and initially also of rocks, but that failed) is very simple. Organisms are assigned into different groups based on their characteristics, and these groups are hierarchical. The figure below shows the seven main types of groups. Kingdom is “higher” than phylum, while class is “lower”, and so on.
For some, but not all groups there are subdivisions of these group types, such as subclass (subdivision within a class), infraorder (subdivision within a suborder; i.e. infra- is “below” sub-), and grouping of groups – e.g. superorder (group of orders).
Since the system is hierarchical, organisms belonging to the same class also belong to the same phylum and kingdom. For instance, all animals belonging to the class Reptilia (reptiles, then) also belong to the phylum Chordata (animals with a notochord, or backbone) and the kingdom Animalia (animals).
Now, the order Primates (primates), although belonging to the class Mammalia instead of Reptilia, also belongs to the phylum Chordata and kingdom Animalia. This might complicate things, but it is simply due to that the two classes Reptilia and Mammalia both belong to the same phylum and (therefore) kingdom. Notice that the name of the class is written with a capital first letter when you refer to the actual group. If you instead write “carnivorans” (belonging to the mammalian order Carnivora; not equal to “carnivore”, which refers to a feeding strategy – not a taxonomic group), you are really referring to the members of the group, and you do not use capital letters. This rule is useful for distinguishing between, for example, Primates and primates.
While on the subject of formal rules, the genus and species are special. First, both are always written in italics. Always. Second, the genus name is written with capital first letter, but the species never has a capital. Third, you may refer to the genus alone, e.g. Tyrannosaurus, but never ever write only the species name. Never. This is because there may be several different species with the same name, (for instance, they may be named after the same discoverer) but they never belong to the same genus (if they do, they are simply not allowed to have the same species name). In this way, we get an endless variety of specific names for an endless variety of species. Finally, you may shorten the genus name to only the first letter (capital) followed by a dot and the species name (if you do not include the species name, you may not shorten the genus name – it would be silly to write something like “T. had remarkably short arms”). For example, we take the genus Tyrannosaurus (species name is excluded, since I refer to the genus), which has one species: Tyrannosaurus rex – although some researchers argue that Tarbosaurus bataar really belongs to Tyrannosaurus; in that case, we would also have Tyrannosaurus bataar (the species name is the same, but is assigned to a different genus). Notice that I should not shorten the genus name here, since it may be unclear what I mean by T. bataar.
Names can be discarded or invalidated, usually by showing that two very similar species actually are the same, in which case the name given first is the one that remains valid. Rejected names are written within quotation marks, and never italicised. A classical example is that "Brontosaurus exelsus" and Apatosaurus ajax were shown to be the same species (and therefore also belonged to one and the same genus); Apatosaurus, being the first to have been described and named, was kept (both genus and species name).
Another notable convention is that families tend to end with -idae, superfamilies with -oidea and subfamilies with -inae (their members would then be -ids, -oids, and -ines, respectively). For example, we have the Hadrosauroidea (superfamily), Hadrosauridae (family) and Hadrosaurinae (subfamily).
Nowadays, the taxonomic system has been overshadowed by phylogenetic systematics, or cladistics. Cladistics is favoured because it systematically investigates evolutionary relationships, rather than just putting the organisms into different groups; cladistics tries to work out how they evolved, and how closely related different organisms are. Taxonomy, on the other hand, merely groups similar-looking organisms together in order to make some sense of the overwhelming chaos of life we have out there.
The groupings can differ quite a lot between taxonomy and cladistics, since cladistics has a predilection for groups that include all the descendants within an evolutionary line (i.e. monophyletic clades). Therefore, cladistics disregards the group Reptilia, as it does not include birds (class Aves) and mammals, both which have their origins in the reptilian evolutionary line. Instead, cladistics just group them all together as Amniota.
