Symplesiomorphy
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A symplesiomorphy or symplesiomorphic character is in cladistics a trait which is shared (a symmorphy) between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa considered are more closely related to each other than to the more distant taxa, as all share the more primitive trait. A close phylogenetic relationship, that the taxa form a certain clade to the exclusion of certain other taxa, can only be shown by the discovery of synapomorphies: shared traits that have originated with the last common ancestor of the taxa considered, or at least in the branch, not including the taxa to be excluded, leading to it.
The concept of the symplesiomorphy shows the danger of grouping species together on the basis of general morphologic similarity, without distinguishing between resemblances caused by either primitive or derived traits, as was common before cladistics became popular in the 1980s.
A famous example is pharyngeal gill breathing in bony and cartilaginous fishes. The former are more closely related to Tetrapoda (terrestrial vertebrates, which evolved out of a clade of bony fishes) that breathe via their skin or lungs, rather than to the sharks, rays, et al.. Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrates.
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