Marsupialia: Marsupial Mammals

What Are The Characteristics of Marsupial Mammals?

Marsupials exhibit distinct reproductive, anatomical, and physiological features that set them apart from other mammals.

Reproductive Traits

• Short gestation, long lactation – The young are born very early (some as soon as 12 days after fertilization) and develop externally, latching onto the mother’s teats inside a protective pouch.
• Altricial birth – Marsupial newborns are extremely tiny and underdeveloped, often blind and hairless at birth.
• Bifurcated reproductive system – Female marsupials have two uteri and two vaginas, while males often have a bifurcated (forked) penis.

Skeletal and Anatomical Features

• Epipubic bones – Found in both males and females, these bones support the marsupium and are absent in placental mammals.
• Different tooth replacement pattern – Marsupials usually replace only one set of teeth (the third premolar), whereas placental mammals replace multiple teeth over their lifetime.
• Smaller brains compared to placentals – Marsupials have relatively smaller and less convoluted brains, but some species show complex problem-solving abilities.

Metabolic and Physiological Aspects

• Lower metabolic rate – Many marsupials have a slower metabolic rate than placental mammals, which contributes to their energy-efficient lifestyles.
• Powerful hind limbs – Many species, especially kangaroos and wallabies, have specialized jumping adaptations, allowing them to travel long distances.
• Diverse dietary adaptations – Marsupials include carnivores (Tasmanian devils), omnivores (opossums), herbivores (kangaroos), and insectivores (numbats).

Major Groups of Marsupialia

Ameridelphia (American Marsupials)

This group includes the marsupials of South and Central America, such as opossums and shrew-like caenolestids. These species are considered more primitive, retaining many ancestral traits of early marsupials. Most American marsupials are small, nocturnal, and opportunistic omnivores, capable of adapting to various environments. Unlike their Australian relatives, they have fewer specialized adaptations and face more competition from placental mammals.

Fun fact: The Virginia opossum (Didelphis virginiana) is the only marsupial found in North America, making it an exception in a continent dominated by placental mammals.

Australidelphia (Australian Marsupials)

This group includes the majority of marsupials, which diversified in Australia, New Guinea, and nearby islands. Due to geographic isolation, Australian marsupials evolved into a wide variety of ecological niches, including large herbivores (kangaroos), arboreal folivores (koalas), carnivores (Tasmanian devils), and even burrowing species (wombats).

Unlike their American relatives, Australian marsupials fill many of the ecological roles occupied by placental mammals elsewhere, making them a dominant group in their habitats.

Fun fact: Despite being an Australian group, the Monito del Monte (Dromiciops gliroides), a small tree-dwelling marsupial in South America, is genetically closer to Australian marsupials than to American opossums.

Marsupialia vs. Metatheria: What’s the Difference?

The term Metatheria is sometimes used interchangeably with Marsupialia, but there is a key distinction:

• Metatheria includes both modern marsupials AND their extinct relatives (some dating back to the Cretaceous).
• Marsupialia refers only to living marsupials.

Fossil evidence suggests that early metatherians once lived in North America, Asia, and Europe, but went extinct, while their marsupial descendants flourished in South America and Australia.

Animals in the infraclass Marsupialia

Northern Pig-footed Bandicoot

Southern Pig-footed Bandicoot

Koala