Plesiomorphy and symplesiomorphy

[a] Apomorphic and synapomorphic characteristics convey much information about evolutionary clades and can be used to define taxa.

A backbone is a plesiomorphic trait shared by birds and mammals, and does not help in placing an animal in one or the other of these two clades.

Other clades, e.g. snakes, lizards, turtles, fish, frogs, all have backbones and none are either birds nor mammals.

Being a hexapod is plesiomorphic trait shared by ants and beetles, and does not help in placing an animal in one or the other of these two clades.

All of these terms are by definition relative, in that a trait can be a plesiomorphy in one context and an apomorphy in another, e.g. having a backbone is plesiomorphic between birds and mammals, but is apomorphic between them and insects.

Phylogenies showing the terminology used to describe different patterns of ancestral and derived trait states. [ 1 ]
Imaginary cladogram. [ 2 ] The yellow mask is a plesiomorphy for each living masked species, because it is ancestral. [ 2 ] It is also a symplesiomorphy for them. But for the four living species as a whole, it is an apomorphy because it is not ancestral for all of them. The yellow tail is a plesiomorphy and symplesiomorphy for all living species.