In systematics, describing a group of organisms that contains all the descendants of a particular single common ancestor. In cladistics such a grouping is called a clade and is the only type of group regarded as valid when constructing classification schemes. Hence the monophyletic grouping Theria contains the marsupial and placental mammals, together with their extinct Mesozoic relatives, all of which share an immediate common ancestor not shared by the more distantly related egg-laying mammals (comprising the Prototheria). Similarly, birds and crocodiles are the living representatives of a monophyletic group, Archosauria, and are more closely related to each other than to other living reptilian descendants. Consequently the grouping ‘reptiles’, used in many modern classification systems, is not monophyletic but paraphyletic, since it excludes the birds (and mammals). Compare polyphyletic.