Why are birds and crocodiles grouped closely in cladistic classification?

In cladistic classification, birds and crocodiles are grouped closely because they are both members of the subclass Archosauria. Unlike phenetics, which classifies organisms based on overall physical similarity, cladistics defines groups solely based on their most recent common ancestor and shared derived characteristics, known as synapomorphies.

As archosaurs, both groups descend from a single ancestral lineage that diverged into two primary branches: the Pseudosuchia (which includes crocodiles) and the Ornithosuchia (which includes birds and their dinosaurian ancestors). Because birds and crocodiles share this common archosaurian ancestry, cladistics places them in a closer evolutionary relationship to each other than either group has to other reptiles, such as lizards. This classification reflects their shared evolutionary history rather than their current outward appearance or ecological roles.12