The oldest reptiles having mammal-like features, the synapsids, occur in rocks of Pennsylvanian age formed about 305 mya. However, the first mammals do not appear in the fossil record until Late Triassic time, about 210 mya. Hopson (1994) noted, "Of all the great transitions between major structural grades within vertebrates, the transition from basal amniotes [egg-laying tetrapods except amphibians] to basal mammals is represented by the most complete and continuous fossil record Structural evolution of particular functional systems has been well investigated, notably the feeding mechanism... and middle ear, and these studies have demonstrated the gradual nature of these major adaptive modifications."
A widely used definition of mammals is based on the articulation or joining of the lower and upper jaws. In mammals, each half of the lower jaw is a single bone called the dentary; whereas in reptiles, each half of the lower jaw is made up of three bones. The dentary of mammals is joined with the squamosal bone of the skull. This condition evolved between Pennsylvanian and Late Triassic times. Evolution of this jaw articulation can be traced from primitive synapsids (pelycosaurs), to advanced synapsids (therapsids), to cynodonts, to mammals. In mammals, two of the extra lower jaw bones of synapsid reptiles (the quadrate and articular bones) became two of the middle-ear bones, the incus (anvil) and malleus (hammer). Thus, mammals acquired a hearing function as part of the small chain of bones that transmit air vibrations from the ear drum to the inner ear.
Mammals stapes incus malleus tympanic bone new tympanic membrane
development of angular bone to form tympanic bone
Diagrammatic skulls showing the changes in the jaw articulation and the ear region in the evolution from reptile to mammal. In reptiles, the lower jaw is made up of several bones on each side and there is only one ear bone, the stapes, on each side. In mammals, the lower jaw is made up of only one bone on each side and the other jaw bones have taken on new functions in the middle ear. The reptilian articular bone becomes the malleus bone of the middle ear of mammals and the quadrate bone of the reptilian jaw becomes the incus bone of the middle ear of mammals. The angular bone is lost. Modified from
Savage and Long (1986). Jm
Reptiles stapes tympanic membrane quadrate articular angular bone ower jaw
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