Infraclass Metatheria
marsupial mammals

Metatherian mammals, also known as marsupials, comprise around 272 species. They are an ancient group, very diverse in body form, and they occupy an enormous range of ecological niches. Today, most marsupials are found in Central and South America (around 70 species) and Australasia (around 200 species). Radiations took place on both of these continents during the Cenozoic, at a time when there were few placental competitors. Present marsupial faunas are very diverse, with some startling parallels with placental mammals (e.g., marsupials with similar morphologies and life histories as moles, anteaters, shrews, primates, carnivores, and many others). Some marsupial life histories and morphologies are seemingly without placental mammal parallels, for example, kangaroos. Past marsupial faunas were even more incredible. In Australia, for example, were rhinoceros-sized marsupial herbivores, kangaroos nearly 10 feet tall, and carnivorous lion-like forms with shearing teeth and retractile claws. In South America, where parallel radiations of large placental herbivores may have denied these herbivorous niches to marsupials, marsupials filled many carnivore niches (including a sabretooth marsupial "cat") and many rodent-like forms. It seems clear on both continents that invasion by placental mammals is correlated with a decline in number and diversity of marsupials. However, it is unclear whether placental mammals caused the disappearance of marsupials through competition or the apparent pattern of replacement is the result of random historical events. (Moeller, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000; Wilson and Reeder, 1993)

Today, most marsupials are found in Central and South America (around 70 species) and Australasia (around 200 species). One species is found in temperate North America (Didelphis virginianus). Metatheres diverged from the lineage leading to eutherian (placental) mammals by the Middle Cretaceous. Early diversification of metatheres is thought to have taken place in North America although, by the middle Miocene, the lineage became extinct on that continent, only reappearing around the time that North and South America regained contact in the Pliocene. The earliest marsupials are believed to have resembled North American opossums and other members of the family Didelphidae. A few fossil marsupials are known from Europe, Africa, and Asia, but this group was never well established on those continents. (Moeller, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

From deserts and dry scrubland in Australia to tropical rain forests in South America, there are at least a few, and often many, species of Metatheria present. These animals occupy an enormous variety of terrestrial habitats throughout these two continents. The single species found in temperate North America (Didelphis virginia) naturally inhabits moist woodlands, but is common in towns and small cities. Metatheres have evolved to fill many niches in many habitats. Many species are fully terrestrial, many are arboreal, and at least one species, yapoks, is semi-aquatic. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Until recently, metatherians were classified in a single order, Marsupialia, with around a dozen families. Improving information on phylogeny, and an effort to make classification better represent the evolutionary history of the group, has led to several schemes that divide metatheres into a number of orders. The one currently followed in the Animal Diversity Web is that proposed by Aplin and Archer (1987) and followed by Wilson and Reeder (1993) in their authoritative summary of the classification of mammals. It divides marsupials into two "cohorts." The cohort Ameridelphia contains two neotropical groups, the families Didelphidae and Caenolestidae, in addition to a number of extinct groups. The cohort Australidelphia contains 16 extant families (including the recently extinct family Thylacinidae), several extinct families, and Microbiotheriidae, a family found only in the Andes of Argentina and Chile. These 18 families are organized into 7 orders. This classification relies heavily on new information from studies of molecular biology, but is increasingly supported by studies of morphology and the fossil record. This is a substantial improvement over the older Marsupialia classification, but we suspect that the last word about the higher taxonomy of marsupials remains to be written. (Aplin and Archer, 1987; Thenius, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Marsupials differ from placental mammals in a number of important and obvious ways. The palate of marsupials is usually "fenestrated," that is, it contains large gaps or spaces in its bony surface. The angular process of the dentary is inflected (bent) medially in almost all marsupials. The braincase is small and narrow. It houses a relatively small and simple brain compared to that of similar-sized placental mammals. The jugal is large, extending posteriorally so that it contacts, and forms part of, the glenoid fossa. The lacrimal canal is slightly anterior to the orbit so that it opens on the surface of the face rather than inside the orbital space. The bullae are sometimes not ossified. When they are, they are formed largely by extensions from the alisphenoid.

Tooth form varies considerably among species of marsupials, but an easy and reliable character for recognizing members of the group is that the number of incisors in the upper jaw is different from the number in the lower (except in one family, the Vombatidae). The number is equal in most (but not all) placental mammals. Also, the maximum number of incisors (seen in several families) is 5/4, in contrast to 3/3 in placentals. The number of premolars and molars also differs between the groups (3/3 4/4 in marsupials, 4/4 3/3 in placental mammals), and the pattern of tooth replacement (milk teeth by adult teeth) differs, but these traits are difficult to use to recognize specimens. Postcranial skeletons of marsupials differ from those of placental mammals in that modern marsupials have epipubic bones in the body wall, projecting anteriorally from the pelvis. Epipubic bones are vestigial in recently extinct thylacines and were absent in at least one extinct group. The presence of epipubic bones is shared with monotremes. (Moeller, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Distinguishing among most of the orders and families of modern marsupials is not difficult. Two frequently-used characteristics are the conformation of the feet, and the number and position of the lower incisors. The second and third toes of syndactylous species are mostly enclosed in a sheath of skin and appear fused, except for the claws. Members of the orders Peramelemorphia and Diprotodontia are syndactylous. Others have separate toes, sometimes referred to as polydactylous. Members of the Diprotodontia and Paucituberculata have a pair of enlarged, forward-projecting (procumbent) lower incisors, a condition called diprotodonty. Other groups are polyprotodont, with numerous small and unspecialized lower incisors. Marsupial moles (Notoryctemorphia) are an unusual group, probably because of their extreme specialization to a fossorial mode of life. Their incisor morphology is not clearly diprotodont or polyprotodont and their feet are neither polydactylous nor syndactylous. (Moeller, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

While some species of metatheres live only 1 to a few years, some species, such as coarse-haired wombats, have lived up to 26 years in captivity. ("Marsupials", 1990)

Metatheria has undergone two major adaptive radiations (in Australia and South America) and exhibit a vast array of behavior as a consequence of evolving to fill a variety of ecological niches on these two continents. Thus, generalization of their behavior is difficult. Marsupials may be arboreal, terriculous, or fossorial and at least one species is semi-aquatic (yapoks or water opossums). Their locomotion may include walking, gliding, hopping, running and swimming. Marsupials have not evolved powered flight. Activity patterns also run the full spectrum of possibilities: marsupials may be diurnal, nocturnal or crespuscular. Seasonal behavior may include hibernation in some species, while others remain active throughout the year. Some species are social, while others are solitary. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

As with mammals in general, vision, olfaction, hearing and touch are all important to varying degrees among species of Metatheria. Communication can take many forms as well. Some marsupials communicate with acoustic signals, particularly during mating or territorial encounters. Many species have conspicuous color patterns that may convey information about sex or species identity. Pheromones may also used in communication of reproductive receptivity. (Nowak, 1991)

Ominivory, insectivory, carnivory, and herbivory are all common food habits among Metatheria. Some groups that exhibit omnivory are American oppossums (Didelphidae), Australian possums (Phalangeridae) and bandicoots (Peramelidae). Insectivory and/or carnivory can be observed in several groups as well (e.g., Dasyuridae). Many marsupials, such as koalas (Phascolarctidae), wombats (Vombatidae), and kangaroos (Macropodidae), are strictly herbivorous. (Nowak, 1991)

Except for a very few species (e.g., the extinct thylacine wolf), marsupials are rarely top carnivores and thus are subject to predation by a host of mammalian, reptilian and avian predators wherever they occur.

With their great diversity of food habits, behavior and habitat use, metatheres can substantially impact their communities and ecosystems in a variety of ways. For example, metatheres may help pollinate plants, distribute seeds, or control pest populations. Most species are prey for other species and thus are an important component of many food webs. Species that dig burrows (e.g. wombats and marsupial moles) create habitat for other organisms and/or help aerate soil. Parasites of marsupials are certainly as diverse as their hosts.

Metatheres generally do not have large detrimental effects on human economics. Some herbivorous species may be minor crop pests. Wombats are extirpated in some areas because their burrows cause injury to livestock, or because European rabbits, which have become significant pests in Australia, use wombat burrows as dens. (Nowak, 1991)

Humans benefit from metatheres in a variety of ways. Many are eaten as food or their body parts are used as some sort of resource (e.g., leather is made from kangaroo hides and koalas and brushtail possums were once taken for their fur). Some species that eat mice or insects can help control agricultural pests. Metatheres are valuable for the ecotourism industry--drawing many tourists to Australia. Some species are even kept as pets (e.g. Petaurus outside of Australia). (Nowak, 1991)

Many marsupials are threatened or endangered. The International Union for the Conservation of Nature and Natural Resources (IUCN) currently lists over 200 species (i.e., more than 2/3 of all marsupials) as being of some level of concern. Habitat destruction, overexploitation, and competition with exotic species and livestock have greatly reduced many populations. A number of species have gone extinct within the past two centuries as a direct result of human activity (e.g., Thylacinus, Macropus spp.). (IUCN, 2004; Nowak, 1991)