Metatheria

Lycopsis longirostris

Metatheria is a taxonomic group of mammals that includes all marsupials and their extinct relatives. Metatherians are distinguished from Eutheria, the placental mammals, by various reproductive and physiological traits. The most notable characteristic of Metatheria is the presence of a pouch in many species, where the immature offspring complete their development after a relatively short gestational period.

Characteristics

Metatherians share several unique features that differentiate them from other mammals. These include differences in the structure of their teeth, skull, and reproductive system. Metatherians typically have a dental formula that includes more incisors than eutherians. Their skulls have large openings (fenestrae) behind the eye sockets, and their reproductive systems are bifurcated, with females possessing two uteri and males having a split or double penis.

The development of metatherian young is significantly different from that of placental mammals. After a short gestation period, a very small and underdeveloped offspring is born. This offspring then typically crawls into the mother's pouch, where it attaches to a nipple and continues to develop. Not all metatherians have a physical pouch, but the developmental process is similar across the group.

Evolution

The earliest known metatherians date back to the Late Cretaceous period, making them contemporary with some of the last dinosaurs. The lineage leading to modern marsupials diverged from that of placental mammals in the late Jurassic or early Cretaceous. Metatherians were once more widespread and diverse, with fossils found in North America, South America, Europe, and Asia. However, their diversity in the Northern Hemisphere declined significantly by the end of the Eocene epoch, likely due to climate change and competition with placental mammals.

Today, marsupials are most diverse in Australia and South America, with one family, the Didelphidae (opossums), native to North America. The Australian marsupials, in particular, have evolved a wide range of forms that occupy ecological niches similar to those of placental mammals elsewhere.

Classification

Metatheria is divided into several orders, with the majority of living species classified within the order Diprotodontia, which includes kangaroos, wallabies, and koalas, and the order Dasyuromorphia, which includes the Tasmanian devil and quolls. Other notable orders include Peramelemorphia (bandicoots and bilbies) and Notoryctemorphia (marsupial moles).

Conservation

Many metatherian species are threatened by habitat destruction, introduced species, and other human activities. Conservation efforts are underway in various parts of the world to protect these unique animals and their habitats.