Metatheria

Angular process of dentary inflected; effectively monophyodont (most primary teeth are resorbed without erupting); maximum dental formula 5/4, 1/1, 3/3, 4/4; upper molars with broad cingulum and stylar cusps; hypo- and entoconulids closely spaced; large vacuities in palate; nasals with triangular lateral process at articulation with frontals; auditory bulla, if present, formed by alisphenoid; brain relatively small, with cerebral hemispheres joined by enlarged anterior commisure; anastamosing capillary network over brain absent; epipubic ("marsupial" bones present; chorioallantoic membrane transient or absent; vestigial, transient egg shell may be present; no corpus luteum (young ejected at end of estrus cycle); usually marsupium ("pouch") for immature young; cursorial forms and large carnivores relatively rare.

South American, apparently more basal, marsupials; based on molecular data; rostrum long; sagittal crest present; dental formula 5/5, 1/1, 3/3, 4/4; spermatozoa paired in epididimus.

Small to medium-sized semi-arboreal, mostly omnivores with some exceptions. Long rostrum; narrow braincase and prominent saggital crest; primitive dental formula 5/4, 1/1, 3/3, 4/4; incisors small; canines large; molars tricuspid; jugal contacts glenoid fossa; plantigrade feet; hind feet have opposable digit 1 with nail, not claw; tail prehensile in living species; stomach simple, with small cecum; marsupium simple fold or absent; usually opportunistic omnivores; range still expanding in NAm.; very plesiomorphic and little changed since K; solitary; females larger.

Small, terrestrial rodent-like. Elongate, rounded skull (supraoccipital without distinct lambdoid crest results in globular braincase); pointed rostrum; small eyes; dental formula: 3-4/2-4, 1/1, 3/3, 4/4; $ 1-2 lower incisors large & procumbent; 1st premolar with 1 cusp; 1st molar enlarged and blade-like; most molars have hypocone; $ flap of skin on upper lip; large olfactory bulbs; cerebral fissures absent (primitive brain); $ atlas with mobile cervical rib; tail long, with hair, not prehensile; humerus large & heavy; 3-chambered stomach with short intestine; no marsupium; insectivores & omnivores; unique, 4-limbed jumping style.

Australian radiation of metatherians. Based largely on molecular data, tarsal specializations (?).

Very small (25g), mouse-like form. One living species (Dromiciops) in montane bamboo forests of Chile. Dental formula 5/4, 1/1, 3/3, 4/4; incisors relatively large and spatulate; lower incisors somewhat procumbent; cheek teeth with low, rounded cusps; premaxilla elongated; nasals expanded posteriorly; paroccipital processes absent; auditory bullae greatly enlarged, with ($) entotympanic bone which forms posterior 2/3ds of bulla (in fact, entire posterior portion of skull appears "inflated"); marsupium present, with anteroventral opening; tail long & "moderately prehensile"; may store fat at base of tail; insectivorous (grubs) and/or herbivorous; scansorial; may hibernate; builds nests; no social group beyond mother and young, breeding pairs in season.

Dental formula 4/3,1/1, 2-3/2-3, 4/4; incisors small or blade-like; carnassials?; some (Thylacine) with digitigrade posture and cursorial habit, strikingly parallel to placental wolves; no syndactyly; marsupium absent or poorly developed.

2 living species (10-20 cm) with no fossil record. Dental formula: 3-4/3, 1/1, 2/3, 4/4 (varies?); all but last premolar unicuspid; with paracone & metacone fused; molars zalambodont, with talonids absent; eyes reduced, vestigial & without lens; cornified plate on rostrum; auditory pinnae absent; 5 posterior cervical vertebrae fused; short (1-3 cm) leathery tail with distinct rings; manus 2-4 coordinate as spade, 1 & 5 reduced; pes 1 has nail, 2-4 long claw, 5 vestigial; silky fur (as placental moles-- presumably for same function: no nap to permit quicker reversals in tight spaces); highly specialized for fossorial habit in desert soils; may "swim" through soil without permanent burrow; females may have deep nests (?); frequently feeds above ground; marsupium opens posteriorly, divided into 2; insectivore; solitary.

Dental formula 4-5/3, 1/1, 3/3, 4/4; incisors small; dilambdodont molars; long, narrow rostrum; skull table flattened; typically very long "ears" (pinnae); elongated feet; syndactyly of pes 2&3 (grooming adaptation?); pes 4 largest; halux vestigial or absent; digitigrade; some forms with extreme cursorial adpatations; also some with chorioallantoic membrane; insectivorous.

Herbivorous & omniverous forms plus Thylacoleo. Jaw shortened; diprotodont (not surprisingly) = large pair of procumbent incisors on lower jaw; 3 pairs of upper incisors; no lower canines; heavy tail common; elongated hind limbs; frequent syndactyly (second and third digits on the feet fused up to the base of the claws, with the claws not being fused); hallux absent; males do not show evidence of mammary glands; jumping locomotion very common; mostly herbivorous, with some insectivores (plus Thylacoleo).