Fig 6.

A cladogram depicting the phylogenetic relationships of the major tetrapod vertebrate lineages. A triangle labels the putatively closed choanae in crocodilians and mammalians. Due to their distant phylogenetic relationship, the most parsimonious conclusion leads to two independent derivations of closed choanae, once in mammals and once in crocodilians. Additionally, the open choana formation is the likely ancestral condition in amniotes, since basal (non-amniote) lineages such as lungfishes and amphibians also develop open choanae.