Mammalian jaw

A jaw in which the dentary forms the lower jaw and articulates directly with the squamosal bone. This joint distinguishes mammals from other living vertebrates.

Dentary–squamosal joint: The jaw joint between the dentary bone of the lower jaw and the squamosal region of the skull. This articulation defines the mammalian jaw.
Mammaliaformes: A group of synapsids encompassing mammals and their closest extinct relatives. The dentary–squamosal joint first appears among early mammaliaforms, before crown mammals.
Skull: The bony or cartilaginous framework that encloses and protects the brain and supports the face. The squamosal component of the mammalian jaw joint belongs to this framework.
Reptile jaw: A jaw system in reptiles, typically using a quadrate–articular joint and multiple bones in the lower jaw. Its usual joint contrasts with the dentary–squamosal articulation of mammals.
Dentary: The principal bone of the lower jaw in mammals and many other vertebrates. In mammals, it alone forms the lower jaw and meets the skull at the jaw joint.
Morganucodon: An Early Jurassic mammaliaform known from fossils showing a transitional jaw and middle-ear anatomy. Its fossils preserve both the new jaw joint and remnants of the ancestral one.
Lower jaw: The movable skeletal structure that supports the lower teeth and participates in biting and chewing. In mammals, this structure is formed by the dentary bone.
Bird jaw: A lightweight, beak-bearing jaw system in birds, with the quadrate participating in the jaw joint. Birds retain the ancestral quadrate-based articulation rather than the mammalian joint.
Squamosal bone: A skull bone that contributes to the temporal region and, in mammals, the jaw joint. Its articulation with the dentary is the defining joint described here.
Hadrocodium: A small Early Jurassic mammaliaform known from a skull with a fully detached middle ear. It illustrates a later stage in the separation of jaw and ear structures.

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