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Diversity

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)

Geographic Range

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)

Habitat

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)

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