FIGURE 1.

Phylogeny of selected major groups of amniotes. Selected synapomorphies are indicated by gray bars. Two basal extinct lineages Parareptilia and Archaeothyris (representing Ophiacodontidae) are included to indicate the retention of shared ancestral characters (large gray box). Sauropsida includes all extant reptiles and their extinct relatives. Extant anapsids (turtles) are omitted due to their uncertain placement within Sauropsida. Synapsida and Sauropsida are clades of equal taxonomic rank, with a last common ancestor congruent with the ancestor of all crown group amniotes (Amniota). Archosauria is a clade nested within Sauropsida. As such, it has a lower taxonomic rank than either Sauropsida or Synapsida, even if shared derived characters of this group distinguish them from other amniotes. Representative taxon images were downloaded from PhyloPic (http://phylopic.org/). All images are uncopyrighted except parareptilia (http://phylopic.org/image/00b96cf3-1802-4bda-a6cc-76aea0f6f05e/) which is owned by Nobu Tamura (vectorized by T. Michael Keesey), under the following creative commons license (https://creativecommons.org/licenses/by-sa/3.0/legalcode).