nondiapsid

Nondiapsid is a descriptive term used in vertebrate paleontology and comparative anatomy to refer to amniotes that do not possess the diapsid skull condition, which features two temporal openings in the skull behind the eye sockets. In other words, nondiapsids are amniotes lacking the two fenestrae characteristic of diapsids. The term encompasses two broad groups: anapsids, which have no temporal openings at all, and synapsids, which have a single temporal opening located lower on the skull.

Skull morphology is central to the distinction. The diapsid condition, common among most reptiles and birds, involves two openings that reduce skull weight and provide attachment space for jaw muscles. Anapsids lack these openings, a configuration seen in some early amniotes. Synapsids possess one opening, which alters muscle attachment and skull mechanics compared with diapsids. The nondiapsid label thus describes skull plans rather than a single cohesive lineage.

Evolutionary context and classification have evolved with new data. Most living amniotes are diapsids (including lepidosaurs and archosaurs and their descendants such as lizards, snakes, turtles, crocodilians, and birds, though turtle skull anatomy has long been debated). Nondiapsids today are represented mainly by extinct lineages: synapsids, which gave rise to mammals, and various anapsid groups such as parareptiles. In modern cladistics, anapsids are often regarded as a paraphyletic group rather than a true clade, and the term nondiapsid is used more as a descriptive shorthand than as a strict taxonomic category.

See also: diapsid, anapsid, synapsid, sauropsida, mammal.