Reptiliomorpha

Reptiliomorpha is a clade of tetrapods that includes reptiles and their closest fossil relatives. The name literally means "reptile-form." This group diverged from the lineage leading to amphibians early in the evolution of tetrapods. While the name suggests a close relationship to reptiles, reptiliomorphs encompass a broader range of extinct forms as well as modern reptiles.

Early reptiliomorphs, such as Westlothiana, appeared during the Carboniferous period. These animals possessed a mix of traits, some resembling amphibians and others more typical of reptiles. Key evolutionary innovations that define the reptiliomorph lineage include changes in the skull, limbs, and skeleton that were adaptations for a more terrestrial lifestyle.

The reptiliomorpha clade is often contrasted with the anthracosauria, another major group of early tetrapods that are more closely related to amphibians. The development of a more amniotic egg, which allowed reproduction away from water, is a defining characteristic of the Amniota, a subgroup within reptiliomorpha. Modern reptiles, birds, and mammals are all amniotes, highlighting the significance of this evolutionary step. Fossil discoveries continue to refine our understanding of the precise relationships and evolutionary history within Reptiliomorpha.