Marsupial

Marsupials Temporal range: Paleocene–Recent PreꞒ Ꞓ O S D C P T J K Pg N Possible Late Cretaceous records
Clockwise from left: Didelphimorphia, Peramelemorphia, Microbiotheria and Dasyuromorphia respectively
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Clade:	Marsupialiformes
Infraclass:	Marsupialia Illiger, 1811
Orders
Didelphimorphia Paucituberculata Australidelphia Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia † Yalkaparidontia †Polydolopimorphia?
Present-day distribution of marsupials   Introduced   Native

Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are primarily found in Australasia, Wallacea, and the Americas. One of the defining features of marsupials is their unique reproductive strategy, where the young are born in a relatively undeveloped state and then nurtured within a pouch.

Living marsupials encompass a wide range of species, including kangaroos, koalas, opossums, Tasmanian devils, wombats, wallabies, and bandicoots, among others. Each species exhibits distinct adaptations and behaviors suited to its particular ecological niche.

Marsupials constitute a clade stemming from the last common ancestor of extant metatherians, which encompasses all mammals more closely related to marsupials than to placentals. This evolutionary split between placentals and marsupials occurred at least 125 million years ago, possibly dating back over 160 million years to the Middle Jurassic-Early Cretaceous period.

Characteristically, marsupials give birth to relatively underdeveloped offspring that typically reside in a pouch situated on their mothers' abdomen for a period of time. Presently, close to 70% of the 334 extant species of marsupials are concentrated on the Australian continent, including mainland Australia, Tasmania, New Guinea, and nearby islands. The remaining 30% are distributed across the Americas, primarily in South America, with thirteen species in Central America and a single species, the Virginia opossum, inhabiting North America north of Mexico.

The word marsupial comes from marsupium, the technical term for the abdominal pouch. It, in turn, is borrowed from the Latin marsupium and ultimately from the ancient Greek μάρσιππος mársippos, meaning "pouch".

Anatomy

Marsupials have the typical characteristics of mammals—e.g., mammary glands, three middle ear bones, (and ears with to without tragi,^[1] varying in hearing thresholds^[2]) and true hair. There are, however, striking differences as well as a number of anatomical features that separate them from eutherians.

Most female marsupials have a front pouch, which contains multiple teats for the sustenance of their young. Marsupials also have other common structural features. Ossified patellae are absent in most modern marsupials (though a small number of exceptions are reported)^[3] and epipubic bones are present. Marsupials (and monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.^[4]

Skull and teeth

Marsupials exhibit distinct cranial features compared to placental mammals. Generally, their skulls are relatively small and compact. Notably, they possess frontal holes known as foramen lacrimale situated at the front of the orbit. Marsupials also have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension (processus angularis) is bent inward toward the center. The hard palate of marsupials contains more openings compared to placental mammals.

Teeth in marsupials also differ significantly from those in placental mammals. For instance, most Australian marsupials outside the order Diprotodontia have a varying number of incisors between their upper and lower jaws. Early marsupials had a dental formula of 5.1.3.4/4.1.3.4 per quadrant, consisting of five (maxillary) or four (mandibular) incisors, one canine, three premolars, and four molars, totaling 50 teeth. While some taxa, like the opossum, retain this original tooth count, others have reduced numbers.

For instance, members of the Macropodidae family, including kangaroos and wallabies, have a dental formula of 3/1 – (0 or 1)/0 – 2/2 – 4/4. Many marsupials typically have between 40 and 50 teeth, which is notably more than most placental mammals. Notably, in marsupials, the second set of teeth only grows in at the site of the third premolar and posteriorly; all teeth anterior to this erupt initially as permanent teeth.

Torso

Few general characteristics describe their skeleton. In addition to unique details in the construction of the ankle, epipubic bones (ossa epubica) are observed projecting forward from the pubic bone of the pelvis. Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs differ from the placental mammals. For them, the reproductive tract is doubled. The females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina.^[4] In most species, males have a split or double penis lying in front of the scrotum.^[5]

A pouch is present in most, but not all, species. Many marsupials have a permanent bag, whereas in others the pouch develops during gestation, as with the shrew opossum, where the young are hidden only by skin folds or in the fur of the mother. The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours have the opening in the back. Usually, only females have a pouch, but the male water opossum has a pouch that is used to accommodate his genitalia while swimming or running.

General and convergences

The sugar glider, a marsupial, (left) and flying squirrel, a rodent, (right) are examples of convergent evolution.

Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the

Some species resemble placental mammals and are examples of convergent evolution. This convergence is evident in both brain evolution^[6] and behaviour.^[7] The extinct thylacine strongly resembled the placental wolf, hence one of its nicknames "Tasmanian wolf". The ability to glide evolved in both marsupials (as with sugar gliders) and some placental mammals (as with flying squirrels), which developed independently. Other groups such as the kangaroo, however, do not have clear placental counterparts, though they share similarities in lifestyle and ecological niches with ruminants.

Body temperature

Marsupials, along with

Marsupials' reproductive systems differ markedly from

The evolution of reproduction in marsupials, and speculation about the ancestral state of

Most male marsupials, except for

The male thylacine had a pouch that acted as a protective sheath, covering his external reproductive organs while running through thick brush.^[24]

The shape of the urethral grooves of the males' genitalia is used to distinguish between

The only accessory sex glands marsupials possess are the prostate and

seminal vesicles or coagulating glands.^[29][16] The prostate is proportionally larger in marsupials than in placental mammals.^[5] During the breeding season, the male tammar wallaby's prostate and bulbourethral gland enlarge. However, there does not appear to be any seasonal difference in the weight of the testes.^[30]

Female reproductive system

See also: Birth § Marsupials

Female marsupials have two lateral

uteri, 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.^[4] Some marsupial species are able to store sperm in the oviduct after mating.^[31]

Marsupials give birth at a very early stage of development; after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a teat, which is located on the underside of the mother, either inside a pouch called the marsupium, or open to the environment. Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase chances of making it into the marsupium. There they remain for a number of weeks, attached to the teat. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.^[32][33]

Early development

Prenatal development differs between marsupials and placental mammals. Key aspects of the first stages of placental mammal embryo development, such as the inner cell mass and the process of compaction, are not found in marsupials.^[34] The cleavage stages of marsupial development are very variable between groups and aspects of marsupial early development are not yet fully understood.

An infant marsupial is known as a joey. Marsupials have a very short

eutherian placenta that are important for the later stages of fetal development are in female marsupials expressed in their mammary glands during their lactation period instead.^[40]

After this period, the joey begins to spend increasing lengths of time out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it will seek refuge in its mother's pouch for safety.

An early birth removes a developing marsupial from its mother's body much sooner than in placental mammals; thus marsupials have not developed a complex

precocial). Newborn marsupials lack histologically mature immune tissues ^[41][42][43] and are highly reliant on their mother's immune system for immunological protection.,^[44] as well as the milk.^[32][33]

Newborn marsupials must climb up to their mother's teats and their front limbs and facial structures are much more developed than the rest of their bodies at the time of birth.

pig-footed bandicoot, suggesting that the range of forelimb specialization is not as limited as assumed.^[47]

Joeys stay in the pouch for up to a year in some species, or until the next joey is born. A marsupial joey is unable to regulate its body temperature and relies upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature of 30–32 °C (86–90 °F) must be constantly maintained.

Joeys are born with "oral shields", which consist of soft tissue that reduces the mouth opening to a round hole just large enough to accept the mother's teat. Once inside the mouth, a bulbous swelling on the end of the teat attaches it to the offspring till it has grown large enough to let go. In species without pouches or with rudimentary pouches these are more developed than in forms with well-developed pouches, implying an increased role in maintaining the young attached to the mother's teat.[48]^[49]

Geography

In Australasia, marsupials are found in Australia, Tasmania and New Guinea; throughout the Maluku Islands, Timor and Sulawesi to the west of New Guinea, and in the Bismarck Archipelago (including the Admiralty Islands) and Solomon Islands to the east of New Guinea.

In the Americas, marsupials are found throughout South America, excluding the central/southern Andes and parts of Patagonia; and through Central America and south-central Mexico, with a single species (the Virginia opossum Didelphis virginiana) widespread in the eastern United States and along the Pacific coast.

Interaction with Europeans

The first American marsupial (and marsupial in general) that a European encountered was the

On the other hand, it was the Portuguese who first described Australasian marsupials. António Galvão, a Portuguese administrator in Ternate (1536–1540), wrote a detailed account of the northern common cuscus (Phalanger orientalis):^[50]

Some animals resemble ferrets, only a little bigger. They are called Kusus. They have a long tail with which they hang from the trees in which they live continuously, winding it once or twice around a branch. On their belly they have a pocket like an intermediate balcony; as soon as they give birth to a young one, they grow it inside there at a teat until it does not need nursing anymore. As soon as she has borne and nourished it, the mother becomes pregnant again.

From the start of the 17th century, more accounts of marsupials arrived. For instance, a 1606 record of an animal, killed on the southern coast of New Guinea, described it as "in the shape of a dog, smaller than a greyhound", with a snakelike "bare scaly tail" and hanging testicles. The meat tasted like venison, and the stomach contained ginger leaves. This description appears to closely resemble the dusky pademelon (Thylogale brunii), in which case this would be the earliest European record of a member of the kangaroo family (Macropodidae).^[52][50]

Taxonomy

Marsupials are taxonomically identified as members of

Marsupialia is further divided as follows:[55] † – Extinct

• Superorder Ameridelphia (American marsupials)
  • Order
    Didelphimorphia (93 species) – see list of didelphimorphs
    • Family
      Didelphidae
      : opossums
  • Order Paucituberculata (seven species)
    • Family
      Caenolestidae: shrew opossums
• Superorder Australidelphia (Australian marsupials)
  • Order Microbiotheria (one extant species)
    • Family Microbiotheriidae: monitos del monte
  • Order †
    Yalkaparidontia
    (incertae sedis)
  • Grandorder
    Agreodontia
    • Order Dasyuromorphia (73 species) – see list of dasyuromorphs
      • Family †Thylacinidae: thylacine
      • Family Dasyuridae: antechinuses, quolls, dunnarts, Tasmanian devil, and relatives
      • Family
        Myrmecobiidae: numbat
    • Order
      Notoryctemorphia
      (two species)
      • Family Notoryctidae: marsupial moles
    • Order Peramelemorphia (27 species)
      • Family
        bilbies
      • Family †Chaeropodidae:
        pig-footed bandicoots
      • Family Peramelidae: bandicoots and allies
  • Order Diprotodontia (136 species) – see list of diprotodonts
    • Suborder Vombatiformes
      • Family
        Vombatidae: wombats
      • Family Phascolarctidae: koalas
      • Family † Diprotodontidae: giant wombats
      • Family † Palorchestidae: marsupial tapirs
      • Family † Thylacoleonidae: marsupial lions
    • Suborder Phalangerida
      • Infraorder Phalangeriformes – see list of phalangeriformes
        • Family Acrobatidae: feathertail glider and feather-tailed possum
        • Family
          Burramyidae: pygmy possums
        • Family †
          sprite possums
        • Family Petauridae: striped possum, Leadbeater's possum, yellow-bellied glider, sugar glider, mahogany glider, squirrel glider
        • Family Phalangeridae: brushtail possums and cuscuses
        • Family Pseudocheiridae: ringtailed possums and relatives
        • Family
          Tarsipedidae: honey possum
      • Infraorder Macropodiformes – see list of macropodiformes
        • Family Macropodidae: kangaroos, wallabies, and relatives
        • Family Potoroidae: potoroos, rat kangaroos, bettongs
        • Family Hypsiprymnodontidae: musky rat-kangaroo
        • Family † Balbaridae: basal quadrupedal kangaroos

Evolutionary history

Comprising over 300 extant species, several attempts have been made to accurately interpret the

Molecular analyses in 2010 and 2011 identified Microbiotheria as the sister group to all Australian marsupials. However, the relations among the four Australidelphid orders are not as well understood.

Cladogram of Marsupialia by Upham et al. 2019[58][59] & Álvarez-Carretero et al. 2022[60][61]

Marsupialia Paucituberculata Didelphimorphia Australidelphia Microbiotheria Agreodontia Notoryctemorphia Peramelemorphia Dasyuromorphia Diprotodontia

Cladogram of Marsupialia by Gallus et al. 2015[56]
Marsupialia Didelphimorphia Paucituberculata Australidelphia Microbiotheria Diprotodontia Notoryctemorphia Peramelemorphia Dasyuromorphia New World marsupials Australasian marsupials

DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a single

The relationships among the three extant divisions of mammals (

The ancestors of marsupials, part of a larger group called

Cladogram from Wilson et al. (2016)[74]

In 2022, a study provided strong evidence that the earliest known marsupial was Deltatheridium known from specimens from the Campanian age of the Late Cretaceous in Mongolia.^[75] This study placed both Deltatheridium and Pucadelphys as sister taxa to the modern large American opossums.

Marsupials spread to South America from North America during the Paleocene, possibly via the Aves Ridge.^[76][77][78] Northern Hemisphere metatherians, which were of low morphological and species diversity compared to contemporary placental mammals, eventually became extinct during the Miocene epoch.^[79]

In South America, the

Marsupials reached Australia via Antarctica during the Early Eocene, around 50 mya, shortly after Australia had split off.[n 1]^{[n 2]} This suggests a single dispersion event of just one species, most likely a relative to South America's monito del monte (a microbiothere, the only New World australidelphian). This progenitor may have rafted across the widening, but still narrow, gap between Australia and Antarctica. The journey must not have been easy; South American ungulate^[85][86][87] and xenarthran^[88] remains have been found in Antarctica, but these groups did not reach Australia.

In Australia, marsupials radiated into the wide variety seen today, including not only omnivorous and carnivorous forms such as were present in South America, but also into large herbivores. Modern marsupials appear to have reached the islands of New Guinea and Sulawesi relatively recently via Australia.^[89][90][91] A 2010 analysis of retroposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials. This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off.^[62][63]

In Australia, terrestrial placental mammals disappeared early in the

Genetic analysis suggests a divergence date between the marsupials and the placentals at 160 million years ago.^[92] The ancestral number of chromosomes has been estimated to be 2n = 14.

A recent hypothesis suggests that South American microbiotheres resulted from a back-dispersal from eastern Gondwana. This interpretation is based on new cranial and post-cranial marsupial fossils of

In 2023, imaging of a partial skeleton found in Australia by paleontologists from Flinders University led to the identification of Ambulator keanei, the first long-distance walker in Australia.^[93]

See also

• Marsupial lawn
• Metatheria
• List of mammal genera
• List of recently extinct mammals
• List of prehistoric mammals

Notes

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Further reading

• Austin CR, Short RV, eds. (21 March 1985). Reproduction in Mammals: Volume 4, Reproductive Fitness. Cambridge University Press. pp. 4–.
  ISBN 978-0-521-31984-3
  .
• Bronson FH (1989). Mammalian Reproductive Biology. University of Chicago Press.
  ISBN 978-0-226-07559-4
  .
• Dawson TJ (1995). Kangaroos: Biology of the Largest Marsupials. Cornell University Press.
  ISBN 978-0-8014-8262-5
  .
• ISBN 978-0-8021-3943-6
  .
• Flannery TF (2008). Chasing kangaroos : a continent, a scientist, and a search for the world's most extraordinary creature (1st American ed.). New York: Grove.
  ISBN 9780802143716
  .
• Flannery TF (2005). Country : a continent, a scientist & a kangaroo (2nd ed.). Melbourne: Text Pub.
  ISBN 978-1-920885-76-2
  .
• Frith, H. J. and J. H. Calaby. Kangaroos. New York: Humanities Press, 1969.
• McKay G (2006). The Encyclopedia of MAMMALS. Weldon Owen.
  ISBN 978-1-74089-352-7
  .
• Hunsaker D (1977). The Biology of Marsupials. New York: Academic Press.
• Johnson MH, Everitt BJ (1988). Essential Reproduction. Blackwell Scientific.
  ISBN 978-0-632-02183-3
  .
• Jones M, Dickman C, Archer (2003). Predators with pouches : the biology of carnivorous marsupials. Collingwood, Victoria: Australia).
  ISBN 9780643066342
  .
• Knobill E, Neill JD, eds. (1998). Encyclopedia of Reproduction. Vol. 3. New York: Academic Press.
• McCullough DR, McCullough Y (2000). Kangaroos in Outback Australia: Comparative Ecology and Behavior of Three Coexisting Species. Columbia University Press.
  ISBN 978-0-231-11916-0
  .
• Nowak RM (7 April 1999). Walker's Mammals of the World. JHU Press.
  ISBN 978-0-8018-5789-8
  .
• Taylor AC, Taylor P (1997). "Sex of Pouch Young Related to Maternal Weight in Macropus eugeni and M. parma". Australian Journal of Zoology. 45 (6): 573–578.
  doi:10.1071/ZO97038
  .

External links

The Wikibook Dichotomous Key has a page on the topic of: Marsupialia

Wikisource has the text of the 1911 Encyclopædia Britannica article "Marsupialia".

• "Western Australian Mammal Species". members.iinet.net.au. Retrieved 28 June 2021.
• "Researchers Publish First Marsupial Genome Sequence". Genome.gov. Retrieved 28 June 2021.
• First marsupial genome released. Most differences between the opossom and placental mammals stem from non-coding DNA Archived 4 February 2011 at the Wayback Machine

Wikimedia Commons has media related to Marsupialia.