The angular process of Monodelphis domestica (Didelphidae, Marsupialia) and its relation to the middle ear: an ontogenetic and evolutionary morphologic study

In most marsupials, the angular process is inflected medially. By using an ontogenetic series of Monodelphis domestica, the development of this characteristic structure has been described. In contrast with the eutherian mammals, in marsupials there is retained a close connection between the dentale and the tympanicum and goniale; it is well known that these 2 elements of the middle ear are derived from the angulare and prearticulare of the reptilian lower jaw. At the neonatal stage, the dentale and tympanicum are both relatively vertically orientated; during the following 2 weeks, they take an increasingly oblique position, which is primarily caused by the rapid growth of the braincase. Only after the eruption of the first teeth, the ascending ramus of the dentale takes a more and more vertical position, whereas the angular process remains with its tip near the medioventral floor of the tympanic bulla. The bulla shows at this place a rectangular fenestra which is covered by a membrane of loose connective tissue; the tip of the angular process, which is always free of muscular insertions, maintains contacts with this fenestra throughout life. During juvenile and adult life stages, the process becomes somewhat removed from the fenestra for obvious reasons, but at a gape of about 40 to 50 degrees it inevitably must touch the "inferior tympanic membrane" and possibly also the tympanic ring. It is speculated that the relationship between the angular process and the tympanic bulla represents a specific form-function complex for sound transmission, which may be a modified retention from archaic mammalian conditions. Further details of the ontogenetic development of the tympanic region have been described which may be of some relevance for the evolutionary morphology of mammals: The tympanic process of the petrosal, which fixes the posterior end of the tympanic ring, is formed by 'Zuwachsknochen' (additional bone) but not by cartilage. The styloid process remains cartilaginous throughout life: its free tip ends in the lateral wall of the tympanic cavity and it is closely connected with the collum mallei and the posterior end of the tympanicum; it guides the chorda tympani and may therefore be homologous with the cartilage of Spence. The cartilage of Paauw is interpreted in terms of functional morphology. A model of evolutionary transformation of the dentale-tympanicum complex in mesozoic mammals in outlined on the basis of the ontogenetic findings in Monodelphis and other didelphid and dasyurid marsupials.