Phylogeny of living Amniota

    Major lineages of the Amniota are distinguished by the number and placement of temporal openings in the skull. The ancestral condition is probably the absence of such openings in the anapsid ("without arches") skull, as seen in modern turtles (Anapsida). One derived condition is a synapsid skull with a single, low opening, as seen in Mammals, the only living members of the lineage Synapsida. A further modification found in the ancestor of all other living amniotes is the diapsid ("two-arch") skull, with both high and low openings. In all living Diapsida except the Tuatara (Sphenodon), the diapsid structure has been further modified such that the two openings are not distinct. In Crocodilia, this is connected with strengthening of the skull for greater bite pressure, and in Aves for lightning of the skull as a single fused structure.

    The change in number of temporal openings from 0 1 2 is called a transformation series.  The reconstruction shown above is the classical explanation: temporal openings in synapsid and diapsid skulls may not be homologous, and the absence of arches in the anapsid skull seems to be derived from the diapsid condition.