28 July 2010

Marsupial Family Tree Established

Genetic evidence has provided a definitive phylogenetic tree for the seven orders of marsupials. It shows that all four Australian marsupial orders have their origins in a single South American marsupial ancestor.

The orders, with representative members are:

South American marsupial orders

Didelphimorphia, Virginia opossum;
Paucituberculata, shrew opossum;
Microbiotheria, monito del monte;

Australian marsupial orders

Notoryctemorphia, marsupial mole;
Dasyuromorphia, Tasmanian devil;
Peramelemorphia, bilby;
Diprotodontia, kangaroo.

Microbiotheria and the Australian marsupial orders form the cohort Australidelphia, which was based on the belief that the five orders shared a common ancestor (which they do). But, the new genetic research indicates that all Australian marsupial orders share a common ancestor not shared by Microbiotheria, and proposes a new name for the four “true” Australasian orders (Euaustralidelphia).

A common ancestor of Paucituberculata and all five marsupial orders of the cohort Australidelphia broke off from the order Didelphimorphia, which includes the Virginia opossum, around 130 million years ago.

Three of the Australian marsupial orders, Notoryctemorphia, Dasyuromorphia, and Peramelemorphia, share a common ancestors not shared by the Australian marsupials of the order Diprotodontia, which includes the kangaroo.

While marsupials in Australia are all descendants of the common ancestors of South American marsupials (and the reverse is not true), the Australian marsupials are more diverse:

There are about 250 species of marsupials, and they are found in a variety of habitats. About two-thirds of them live in Australia, Tasmania, or New Guinea, where they have evolved into a wide variety of forms, including plant-eaters such as kangaroos, koalas, and wombats, and also animals such as bandicoots and quolls, which have sharp teeth and feed largely on insects and other invertebrates.

The remainder of the world's marsupials live in the Americas. They include about 70 different kinds of opossum, one of which—the Virginia opossum—is the only marsupial found in North America.

There were once marsupial families distinct to Antarctica, but as that continent grew colder many millions of years ago, they went extinct.

The three species of montremes such as the platypus are only found in the places where Australian marsupials are found, but genetic evidence appears to favor the theory that their closest common ancestor is shared with placental mammals rather than with marsupial mammals, something called the Therian hypothesis, as opposed to the Marsupionata hypothesis that montremes share a closest common ancestor with marsupials, although the data are not definitive.

No comments: