Figure 3.

Evolutionary and ontogenetic development of the mammalian middle ear ossicles. (A) Comparison of the skull of a neonatal marsupial (Monodelphis domestica; above) and a basal amniote (below). The exocranium (brown) of the basal amniote is adopted from Captorhinus, and it is combined with a hypothetical ‘reptilian’ endocranium of a fetal stage (blue = neural endocranium, purple = viscerocranium, yellow = hyobranchial skeleton). The hyostylic suspension of the jaws in the basal amniote already shows the future arrangement of the ear ossicles: articular and quadrate (which will become malleus and incus, respectively) forming the primary jaw joint, and hyomandibula (which will become the stapes) inserted into the fenestra ovalis of the otic capsule. Neonatal marsupials retain the primary jaw articulation and the elements of the secondary joint (dentary and squamosal more densely stained) lie still far apart; both are gradually approaching each other in phylogeny and ontogeny, and finally develop an ‘Anlagerungsgelenk’ (‘appositional joint’); the red circle in the basal amniote indicates the future position of the tympanic membrane at the angle of the lower jaw, the yellow circle that of the future tympanic membrane behind the quadrate in squamates (adopted from Maier & Werneburg, 2014). (B) Evolutionary series of fossil synapsids (Dimetrodon, Thrinaxodon, Morganucodon) and postnatal stage of Monodelphis showing the gradual transformation of the angle of the lower jaw into sound‐transmitting middle ear structures (modified from Allin, 1975; and Maier, 1990: medial view on the left, lateral view on the right). (C) Postnatal ontogeny of Monodelphis demonstrating the translocation of the ectotympanic (red colour) from the lower jaw to the skull base (modified from Maier, 1990; adult stage in posterior view, early postnatal stages as cross‐sections). (D) Close‐up of the jaw and ear region in a 7‐day‐old Monodelphis, resembling that of Triassic cynodonts (lateral view; modified from Maier, 1990). (E) Posterior view of the ear region of Morganucodon from the Triassic‐Jurassic boundary, showing that the quadrate and articular elements of the primary jaw joint are functionally replaced by the squamosal and dentary of the secondary articulation (which is more laterally positioned). The primary elements are transformed into incus and malleus, but they are still attached to the lower jaw (modified from Kermack et al. 1981). The hatched arrows indicate that the middle ear structures of the adult marsupial are reached by two pathways: the phylogenetic and the ontogenetic.