Fig. 2.

The amniote jaw skeleton has undergone evolutionary diversification in form and function. a Development of upper and lower cartilaginous elements of the mandibular arch in a generalized ancestral amniote. The cartilaginous upper jaw of an ancestral amniote consists of the palatoquadrate cartilage, which is a single element. The lower jaw contains Meckel’s cartilage with the articular cartilage at the most proximal end. b During the radiation of amniotes into the major clades of reptiles (including snakes, lizards, turtles, crocodiles, and birds) and mammals, the ancestral amniote condition became modified. c In reptiles and birds, all that persists of the palatoquadrate is the epipterygoid and quadrate cartilages that develop along condensations of NCM (dashed lines). The quadrate is the upper portion of the jaw joint and the articular cartilage is the lower portion. d In place of the palatoquadrate, the functional upper jaw of reptiles and birds is composed of dermal bones including the premaxilla (pm), maxilla (mx), and palatine (pa). The lower jaw is also made up of several dermal bones that surround Meckel’s cartilage including the dentary, surangular (su), angular, and splenial (sp). The amniote jaw skeleton is derived entirely from neural crest mesenchyme (NCM; light blue) whereas some elements in the skull roof are derived from mesoderm (orange). e In mammals, the epipterygoid contributes to part of the alisphenoid bone and the quadrate becomes the incus (in), which is an ossicle in the middle ear. In mammals, the articular cartilage becomes the malleus (ma) in the middle ear. f Mammals evolve an entirely new jaw joint between the dentary and squamosal (sq) bones, as the incus (in) and malleus (ma) become incorporated into the middle ear. The lower jaw is reduced to a single bone, the dentary (i.e., mandible)

(Modified and adapted from [19, 37, 73, 102, 138, 140, 396])