Table 2 Features listed by de Beer [1] as characteristic of marsupials. Several of the features do not concern the chondrocranium directly, but are listed here for the sake of completeness. The titles of each of three sections are taken from de Beer’s [1] formulation
“Reptilian” features of marsupials that are shared with monotremes and are absent in placentals ([1]:465): “the basal plate is broad” “the cochlear capsules are small” “the canalicular capsules lie directly above the cochlear, and the medial walls of the auditory capsules thus form the main part of the lateral walls of the cranial cavity in the posterior region” “the parietal plates are low” “the occipital arches are vertical” “the tegmen tympani is very small and there is no lateral prefacial commisure” “the incus lies dorsally to the malleus” “the stapes is columelliform” “the orbital cartilage and the sphenoethmoid and orbitoparietal commisures form a wide band” “the carotid foramina pierce the trabecular plate; i.e. no alicochlear commissures” “the internal carotid arteries enter the cranial cavity directly” “no mesethmoid bone is developed (feature shared by certain placentals)” “no ethmoidal cells or sinuses are excavated (except in Phascolarctos)” “the premaxillae bear a dentinal egg-tooth…” |
“The characteristic features of the developing marsupial skull apart from those shared with monotremes (see above) and with placentals (see below)…are not clear cut” ([1]:466): “The presence of two pairs of hypoglossal foramina (feature shared with some rodents and Sirenia)” “The presence of a foramen rotundum between the ala temporalis and processus ascendens (feature not present in Didelphis, Dasyurus; shared with some Carnivora and Primates)” “The loss of the pila metoptica” “The diagonal position of the plane of the foramen olfactorium” “The small size of the frontals” “The large size of the lachrymals” “The great length of the jugals” “The poor development of the palatine process of the maxillae” “The extension of the alisphenoid to form a bulla surrounding the tympanic cavity” “The inflection of the posterior angle of the dentary” “The outer bar to Jacobson’s capsule (shared by Dasypus, Tupaia, Macroscelides, Chrysochloris, Orycteropus)” “The papillary cartilage (shared with Tupaia, Macroscelides, Miniopterus and some Rodentia)” |
“… features shared by marsupials and placentals…” ([1]:466): “The subdivision of the foramen olfactorium advehens to form the cribiform plate” “The substitution of the fenestra rotunda and aqueductus cochleae for the foramen perilymphaticum” “The presence of a crista semicircularis in the nasal capsule” “The loss of the pila antotica” “The loss of the septomaxilla (except in the Xenarthra)” “The fusion of the reptilian pterygoid (secondary pterygoid cartilage) with the lateral wing of the parasphenoid” “The fusion of the cartilaginous elements of the 2nd and 3rd branchial arches to form thyroid cartilages” |