The angular is a large bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 in the lower jaw (mandible) of amphibians and reptiles (birds inclused), which is connected to all other lower jaw bones: the dentary (which is the entire lower jaw in mammals), the splenial

Splenial

The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....

, the suprangular

Suprangular

The suprangular or surangular is a jaw bone found in most land vertebrates, except mammals. It's usually in the back of the jaw, on the upper edge, and is connected to all other jaw bones: dentary, angular, splenial and articular. It is often a muscle attachment site....

, and the articular

Articular

The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...

. It is homologous to the tympanic bone bone in mammals, due to the incorporation of several jaw bones into the mammalian middle ear

Evolution of mammalian auditory ossicles

The evolution of mammalian auditory ossicles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution.In reptiles, the eardrum is...

early in mammal evolution.