By the classification followed here, the living members of the class Mammalia is divided into two subclasses: Prototheria (egg-laying mammals) and Theria. Within the Theria, the infraclasses Metatheria (sometimes treated as the infraclass Marsupialia) and Eutheria are equivalent to the familiar marsupials and placentals. You may wish to review the higher-level taxonomy for the marsupials and placentals at this time.
Traditionally, the marsupials have been treated as a single order, the Marsupialia. However, it has long been noted that placing all marsupials into a single order obscures the differences and relationships within the infraclass. The marsupials have now been subdivided into several orders. (McKenna and Bell recognize the Marsupialia as a cohort.) We'll look first at characters that, for the most part, unite the members of the infraclass. We'll then look at the relatively primitive order Didelphimorphia and follow up with a look at some of the more notable adaptations of the other orders, most of which are limited to the Australian region. Although marsupials were widespread in the past, the Australian and Neotropical biogeographic regions contain almost all marsupials, with only a few moving northward in the New World.
The major feature of marsupials is the relatively short gestation period, resulting in the birth of relatively undeveloped young. No marsupials have litters in excess of 1% of the body weight of the mother, whereas placental mammals produce litters that may be as much as 50% of the maternal weight. The character that laymen tend to think of as most diagnostic (the presence of a pouch, or marsupium) is absent in some taxa (and, as noted, present in echidnas). Likewise, the use of the term "placental" suggests the lack of an allantoic placenta (a particular type of placenta found in all therians) in marsupials, but some do have such, though of simple nature and functioning for only a short period of time.
Characteristically, auditory bullae (often absent) are formed largely from the alisphenoid. The braincase commonly is small and narrow. The brain tends to have small cerebral hemispheres and convolutions are simple. There usually are large openings (vacuities) in the hard palate. The angular process of the lower jaw in inflected in all but a few taxa. Epipubic bones are present in most. You've been exposed to the primitive tooth formula in the sections on dentition, but to reinforce this, the primitive tooth formula for marsupials is 5/4, 1/1, 3/3, 4/4 = 50, with only the last premolar being preceded by a lacteal (milk) tooth. The upper molars typically have a stylar shelf on the labial side, often bearing cusp-like structures easily mistaken for the paracone and metacone. The lower molars generally have the hypoconulid closer to the entoconid than to the hypoconid ("twinned"), rather than pretty much equidistant as in the placentals.
Many Australian marsupials have very specialized hind feet. The first digit often is set apart from the other digits (much as the thumb in humans or even more extreme) or, in a number of cases, lost. Somewhat more unusual among mammals in general is a strong tendency for a highly developed 4th digit together with digits 2 and 3 being bound together for much of their length, thus acting functionally much like a single digit more or less balancing digit 5. At one extreme (in the kangaroo Macropus), digits 2/3 and 5 are almost vestigial, with the weight being bourn on digit 4.
Although the vast majority of members of one subdivision of marsupials (magnorder Australidelphia of McKenna and Bell) occur in the Australian biogeographic region, some are known as fossils in Antarctica and the New World, and the genus Dromiciops (one species) from south-central Chile is assigned to this taxon, though long considered a didelphid.
In Australia and environs, the marsupials have radiated into a large number of ecological niches that elsewhere are filled by placental mammals. Thus there are carnivorous, grazing, arboreal, and fossorial types (among others). The fossil record shows that South America marsupials likewise underwent a considerable radiation. Among these were some borhyaenids that showed considerable convergence in skull characteristics to wolves and others to sabertooth cats. Other marsupials became very rodent-like, with some converging strongly with kangaroo rats and jerboas, including the saltatorial adaptations and enlarged auditory bullae.
Living New World marsupials (McKenna and Bell's magnorder Ameridelphia) belong to two orders: Paucituberculata and Didelphimorphia. The former includes the family Caenolestidae, represented in western South America by some three genera and four species, also vaguely shrew-like in appearance and probably in actions. The majority of New World marsupials are members of the family Didelphidae, the opossums. There are about 65 species known, of which one species, Didelphis virginiana, reaches the United States (other species reach north into Mexico).
A number of authors have compared marsupials and placentals. The comparison below is largely based on Kielan-Jaworowska, Bown, and Lilligraven, 1979, as presented by Vaughn, 1986.
| Marsupialia | Placentalia |
|---|---|
| Relatively small braincase | Relatively large braincase |
| Posterior palatal vacuities usually present | Posterior palatal vacuities seldom present |
| Usually a poorly developed auditory bulla derived from the alisphenoid | Auditory bulla usually well developed and of various origins |
| Angle of lower jaw usually inflected medially | Angle of lower jaw usually not inflected medially |
| Dental formula derived from 5/4, 1/1, 3/3, 4/4 | Dental formula derived from 3/3, 1/1, 4/4, 3/3 |
| Only third premolar preceded by deciduous tooth | Diphyodont dentition, with replacement of antemolars |
| Epipubic bones in both sexes of most forms | No epipubic bones |
| None possesses os baculum or os clitoridis | Os baculum or os clitoridis common |
| Cerebral hemispheres usually small | Cerebral hemispheres tend to be large |
| Young carried externally on nipples and often carried in a pouch | Young in nest or active; rarely carried on the nipples and never in a pouch |
| No true embryonic trophoblast | Embryonic trophoblast present, as distinguished from inner cell mass |
| Erosion of uterine wall by extraembryonic membranes of developing young uncommon | Erosion common |
| Choriovitelline placenta in most forms; chorioallantoic placenta in only a few genera | Chorioallantoic placenta usual instead of choriovitelline placenta |
| Young altricial | Often young precocial |
Last Update: 21 Jan 2008
Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso