Marsupials are an infraclass of mammals, characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy.

History

Isolated petrosals of Djarthia murgonensis, Australia’s oldest marsupial fossils

It was once commonly believed that marsupials were a primitive forerunner of modern placental mammals, but fossil evidence, first presented by researcher M.J. Spechtt in 1982, conflicts with this assumption. Instead, both main branches of the mammal tree appear to have evolved concurrently toward the end of the Mesozoic era. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs.

Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago. This makes it almost contemporary to the earliest placental fossils, which have been found in the same area.

The discovery of Chinese marsupials appears to support the idea that marsupials reached Australia via Southeast Asia. There are a few species of marsupials still living in Asia, especially in the Sulawesi region of Indonesia. These marsupials coexist with primates, hooved mammals and other placentals. However, due to the fact that Australia and China were separated by the wide Tethys Sea in the early Cretaceous into the Northern continent of Laurasia and Southern continent of Gondwana, marsupials had to take a much longer route around. From their origin in East Laurasia (modern day China), they spread westwards into modern North America (still attached to Eurasia) and skipped across to South America, which was connected to North America up until around 65MYA. Here they radiated into Borhyaenids and Shrew Opossums, creating a unique fauna found in South America and Antarctica (which were connected until 35MYA). Marsupials reached Australia via Antarctica about 50MYA just after Australia had split off, suggesting a single dispersion event of several of just one species, related to South America’s Monito del Monte (Microbiothere), rafted across the widening, but still narrow gap between Australia and Antarctica at that time. In Australia, being the only mammals present (except a few Austrosphenids like echidnas and platypuses) they radiated into the wide varieties we see today, even island hopping some way through the Indonesian archipelagos, almost completing a circumnavigation back to their homeland in China.

On most continents, placental mammals were much more successful and no marsupials survived, though in South America the opossums retained a strong presence, and the Tertiary saw the genesis of marsupial predators such as the borhyaenids and the saber-toothed Thylacosmilus. In Australia, however, marsupials displaced placental mammals entirely, and have since dominated the Australian ecosystem. Marsupial success over placental mammals in Australia has been attributed to their comparatively low metabolic rate, a trait which would prove helpful in the hot Australian climate. As a result, native Australian placental mammals (such as hopping mice) are more recent immigrants.

Description

An early birth removes a developing marsupial from its parent’s body much sooner than in placental mammals, and thus marsupials have not developed a complex placenta to protect the embryo from its mother’s immune system. Though early birth places the tiny newborn marsupial at a greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to full-term in bad seasons.

Because newborn marsupials must climb up to their mother’s nipples, their front limbs are much more developed than the rest of the body at the time of birth. It is possible that this requirement has resulted in the limited range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop a grasping forepaw during their early youth, making the transition from this limb into a hoof, wing, or flipper, as some groups of placental mammals have done, far more difficult.

There are about 334 species of marsupial, and over 200 are native to Australia and neighboring northern islands. There are also 100 extant American species; these are centered mostly in South America, but the Great American Interchange has provided Central America with 13 species, and North America with one (the Virginia Opossum).

A feature of marsupials (and also monotremes) is that they don’t have a gross communication (corpus callosum) between the right and left brain hemisphere.

Reproductive system

Marsupials’ reproductive systems differ markedly from those of placental mammals (Placentalia). Females have two lateral vaginas, which lead to separate uteruses, but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.^[11] The males generally have a two-pronged penis, which corresponds to the females’ two vaginas.^[12] The penis is used only for discharging semen into females, and there is instead a urogenital sac used to store waste before expulsion.

Pregnant females develop a kind of yolk sac in their wombs, which delivers nutrients to the embryo. Marsupials give birth at a very early stage of development (about 4–5 weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located inside the marsupium. There they remain for a number of weeks, attached to the nipple. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection and nourishment.