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

Angular

The angular is a large bone in the lower jaw of amphibians and reptiles , which is connected to all other lower jaw bones: the dentary , the splenial, the suprangular, and the articular...

. It forms the jaw joint by articulating with the quadrate bone

Quadrate bone

The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...

 of the skull.

In mammals, the articular bone has migrated to the middle ear

Middle ear

The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...

 to become the malleus

Malleus

The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...

, while the quadrate bone becomes the incus

Incus

The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....

. Paleontologists regard this modification as the defining characteristic of mammalian fossils.