Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass Synapsida included taxa that exhibit a single pair of temporal fenestrae (synapsid condition). Subclass Diapsida included taxa that feature two pairs of temporal fenestrae (diapsid condition) and those that are descended from ancestors that had them. The absence of temporal fenestrae is known as the anapsid condition. Mesosaurids, pareiasaurids, and captorhinids are examples of fossil reptiles that lack temporal fenestrae, whereas turtles are the only living reptiles that lack them; these reptiles were placed together in the subclass Anapsida. Certain Paleozoic reptiles (for example, millerettids) that exhibited synapsid-like temporal fenestrae but clearly did not belong in Synapsida (or Diapsida) were placed in Anapsida.