Phylogeny of living Amniota

Major lineages are distinguished by the number and placement of temporal openings. The ancestral condition is probably the absence of such openings in the anapsid ("without arches") skull, as seen in turtles (Anapsida). One derived conditions 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 skull, with both high and low openings. In all Diapsida except Sphenodon, the diapsid structure has been further modified such that the two openings are not distinct. The change in number of temporal openings from 1 2 3 is called a transformation series.  [The reconstruction shown above is one of several theories: temporal openings in synapsid and diapsid skulls may not be homologous, and the absence of arches in the anapsid skull may be a secondary condition.]

Text material © 2005 by Steven M. Carr