nondiapsids

Nondiapsids is a historical term used to describe amniotes that are not part of the clade Diapsida, which is defined by two temporal skull openings on each side. In traditional classifications, nondiapsids include two main lineages: Anapsida, which lack temporal openings entirely, and Synapsida, which have a single opening on each side of the skull. The use of the term is largely historical or non-cladistic, and modern analyses often complicate or reject a clean separation, especially regarding turtles, which are sometimes discussed as anapsid-like but are now often placed within Diapsida in many analyses.

Anapsida comprises lineages that retained no temporal openings in the skull. These groups are known mainly from the early amniote record and are largely extinct. Synapsida, by contrast, is the lineage that gave rise to mammals. Early synapsids, such as the pelycosaurs, were prominent in the Permian, and later therapsids gave rise to the mammals in the Mesozoic and Cenozoic eras.

In terms of evolutionary significance, the concept of nondiapsids highlights early amniote diversification before the diapsid skull architecture became widespread among reptiles and birds. Skull morphology has historically been a key character in amniote phylogeny. However, the term is less used in contemporary cladistics because it can be paraphyletic and because some lineages—such as turtles—are variably placed within Diapsida depending on the data and method used.