Marsupials are a diverse group ofmammals belonging to theinfraclassMarsupialia. They are natively found inAustralasia,Wallacea, and theAmericas. One of marsupials' unique features is their reproductive strategy: the young are born in a relatively undeveloped state and then nurtured within a pouch on their mother's abdomen.
Marsupials constitute a clade stemming from the last common ancestor of extantMetatheria, which encompasses all mammals more closely related to marsupials than toplacentals. The evolutionary split between placentals and marsupials occurred 125-160million years ago, in theMiddle Jurassic-Early Cretaceous period.
Presently, close to 70% of the 334 extant marsupial species 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 wordmarsupial comes frommarsupium, the technical term for the abdominal pouch. It, in turn, is borrowed from the Latinmarsupium and ultimately from the ancient Greekμάρσιπποςmársippos, meaning "pouch".
Marsupials have typical mammalian characteristics—e.g., mammary glands, threemiddle ear bones, (and ears that usually havetragi,[3] varying in hearing thresholds[4]), truehair and bone structure.[5] However, striking differences including anatomical features separate them fromeutherians.
Most female marsupials have a frontpouch, which contains multiple nursingteats. Marsupials have other common structural features.Ossifiedpatellae are absent in most modern marsupials (with exceptions)[6] andepipubic bones are present. Marsupials (andmonotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.[7]
Marsupials exhibit distinct cranial features compared to placentals. Generally, their skulls are relatively small and compact. Notably, they possess frontal holes known asforamen lacrimale situated at the front of the orbit. Marsupials have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension (processus angularis) is bent toward the center. The hard palate of marsupials contains more openings than that of placentals.
Teeth differ significantly. 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, more than most placentals. 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.
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 inmonotremes. Marsupial reproductive organs differ from placentals. For them, the reproductive tract is doubled. Females have twouteri and twovaginas, and before birth, a birth canal forms between them, the median vagina.[7] In most species, males have a split or double penis lying in front of the scrotum,[8] which is nothomologous to the placental scrota.[9]
A pouch is present in most species. Many marsupials have a permanent bag, while in others such as theshrew opossum the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur. 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 open in the back. Usually, only females have a pouch, but the malewater opossum has a pouch that protects his genitalia while swimming or running.
Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, thered kangaroo, grows up to 1.8 metres (5 ft 11 in) in height and 90 kilograms (200 lb) in weight. Extinct genera, such asDiprotodon, were significantly larger and heavier. The smallest marsupials are themarsupial mice, which reach only 5 centimetres (2.0 in) in body length.
Some species resemble placentals and are examples ofconvergent evolution. This convergence is evident in both brain evolution[10] and behaviour.[11] The extinctthylacine strongly resembled the placental wolf, hence one of its nicknames "Tasmanian wolf". The ability to glide evolved in both marsupials (as withsugar gliders) and some placentals (as withflying 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 withruminants.
Marsupials, along withmonotremes (platypuses andechidnas), typically have lower body temperatures than similarly sizedplacentals (eutherians),[12] with the averages being 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placentals.[13][14] Some species will bask to conserve energy[15]
Both sexes possess acloaca,[17] although modified by connecting to a urogenital sac and having a separate anal region in most species.[18] Thebladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.[17]
Most male marsupials, except formacropods[19] andmarsupial moles,[20] have abifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.[7][17][21][8] The penis is used only duringcopulation, and is separate from theurinary tract.[8][17] It curves forward when erect,[22] and when not erect, it is retracted into the body in an S-shaped curve.[8] Neither marsupials nor monotremes possess abaculum.[7] The shape of theglans penis varies among marsupial species.[8][23][24][25]
The shape of the urethral grooves of the males' genitalia is used to distinguish betweenMonodelphis brevicaudata,M. domestica, andM. americana. The grooves form two channels that form the ventral and dorsal folds of the erectile tissue.[26] Several species ofdasyurid marsupials can also be distinguished by their penis morphology.[27] Marsupials' only accessory sex glands are theprostate andbulbourethral glands.[28] Male marsupials have one to three pairs of bulbourethral glands.[29]Ampullae of vas deferens,seminal vesicles or coagulating glands are not present.[30][31] The prostate is proportionally larger in marsupials than in placentals.[8] During the breeding season, the maletammar wallaby's prostate and bulbourethral gland enlarge. However, the weight of the testes does not vary seasonally.[32]
Female reproductive anatomy of several marsupial species
Female marsupials have two lateralvaginas, which lead to separateuteri, both accessed through the same orifice.[33] A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.[7] Some marsupial speciesstore sperm in theoviduct after mating.[34]
Marsupials give birth very early in gestation; after birth, newborns crawl up their mothers' bodies and attach themselves to a teat, which is located on the underside of the mother, either inside a pouch called themarsupium, or externally. Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase their chances of reaching the marsupium. There they remain for a weeks. Offspring eventually leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.[35][36]
Gestation differs between marsupials andplacentals. Key aspects of the first stages of placental embryo development, such as theinner cell mass and the process of compaction, are not found in marsupials.[37] Thecleavage stages of marsupial development are vary among groups and aspects of marsupial early development are not yet fully understood.
Marsupials have a shortgestation period—typically between 12 and 33 days,[38] but as low as 10 days in the case of thestripe-faced dunnart and as long as 38 days for thelong-nosed potoroo.[39] The baby (joey) is born in afetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns: sizes range from 4-800g+.[38] A newborn can be categorized in one of three grades of development. The least developed are found indasyurids, intermediates are found indidelphids andperamelids, and the most developed aremacropods.[40] The newborn crawls across its mother's fur to reach thepouch,[41] where it latches onto ateat. It does not emerge for several months, during which time it relies on its mother's milk for essential nutrients, growth factors and immunological defence.[42] Genes expressed in theeutherian placenta needed for the later stages of fetal development are expressed in females in their mammary glands during lactation.[43] After this period, the joey spends increasing periods out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it seeks refuge in its mother's pouch.
An early birth removes a developing marsupial from its mother's body much sooner than in placentals; thus marsupials lack a complexplacenta to protect theembryo from its mother'simmune system. Though early birth puts the newborn at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as the fetus cannot compromise the mother in bad seasons. Marsupials arealtricial animals, needing intensive care following birth (cf.precocial). Newborns lack histologically mature immune tissues[44][45][46] and are highly reliant on their mother's immune system for immunological protection.[47]
Newborns front limbs and facial structures are much more developed than the rest of their bodies at birth.[48][49][44] This requirement has been argued to have limited the range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop grasping forepaws early, complicating the evolutive transition from these limbs intohooves,wings, orflippers. However, several marsupials do possess atypical forelimb morphologies, such as the hooved forelimbs of thepig-footed bandicoot, suggesting that the range of forelimb specialization is not as limited as assumed.[50]
Joeys stay in the pouch for up to a year or until the next joey arrives. Joeys are unable to regulate their body temperature and rely 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", soft tissue that reduces the mouth opening to a round hole just large enough to accept the 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 ensuring that the young remain attached to the teat.[51][52]
In the Americas, marsupials are found throughout South America, excluding the central/southernAndes and parts ofPatagonia; and through Central America and south-central Mexico, with a single species (theVirginia opossumDidelphis virginiana) widespread in the eastern United States and along the Pacific coast.
Europeans' first encounter with a marsupial was thecommon opossum.Vicente Yáñez Pinzón, commander of theNiña onChristopher Columbus'first voyage in the late fifteenth century, collected a female opossum with young in her pouch off the South American coast. He presented them to theSpanish monarchs, though by then the young were lost and the female had died. The animal was noted for its strange pouch or "second belly".[53][54]
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.
In the 17th century, more accounts of marsupials emerged. 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 likevenison, and the stomach contained ginger leaves. This description appears to closely resemble thedusky pademelon (Thylogale brunii), the earliest European record of a member of theMacropodidae.[55][53]
Marsupials are taxonomically identified as members ofmammalianinfraclassMarsupialia, firstdescribed as a family under the order Pollicata by German zoologistJohann Karl Wilhelm Illiger in his 1811 workProdromus Systematis Mammalium et Avium. However, James Rennie, author ofThe Natural History of Monkeys, Opossums and Lemurs (1838), pointed out that the placement of five different groups of mammals –monkeys,lemurs,tarsiers,aye-ayes and marsupials (with the exception of kangaroos, which were placed under the orderSalientia) – under a single order (Pollicata) did not appear to have a strong justification. In 1816, French zoologistGeorge Cuvier classified all marsupials under Marsupialia.[56][57] In 1997, researcher J. A. W. Kirsch and others accorded infraclass rank to Marsupialia.[57]
Comprising over 300 extant species, several attempts have been made to accurately interpret thephylogenetic relationships among the different marsupial orders. Studies differ on whether Didelphimorphia or Paucituberculata is thesister group to all other marsupials.[60] Though the orderMicrobiotheria (which has only one species, themonito del monte) is found in South America, morphological similarities suggest it is closely related to Australian marsupials.[61] 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[62][63] & Álvarez-Carretero et al. 2022[64][65]
DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a singleGondwanan migration of marsupials from South America, across Antarctica, to Australia.[66][67] There are many smallarboreal species in each group. The term "opossum" is used to refer to American species (though "possum" is a common abbreviation), whilesimilar Australian species are properly called "possums".
Isolatedpetrosals ofDjarthia murgonensis, Australia's oldest marsupial fossils[68]Dentition of the herbivorous eastern grey kangaroo, as illustrated in Knight'sSketches in Natural History
The ancestors of marsupials, part of a larger group calledmetatherians, probably split from those of placentals (eutherians) during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time.[71] From DNA and protein analyses, the time of divergence of the two lineages has been estimated to be around 100 to 120mya.[53] Fossil metatherians are distinguished from eutherians by the form of their teeth; metatherians possess four pairs ofmolar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.[72] Using this criterion, the earliest known metatherian was thought to beSinodelphys szalayi, which lived in China around 125 mya.[73][74][75] HoweverSinodelphys was later reinterpreted as an early member ofEutheria. The unequivocal oldest known metatherians are now 110 million years old fossils from western North America.[76] Metatherians were widespread in North America and Asia during the Late Cretaceous, but suffered a severe decline during theend-Cretaceous extinction event.[77]
Marsupials spread to South America from North America during thePaleocene, possibly via theAves Ridge.[80][81][82] Northern Hemisphere metatherians, which were of low morphological and species diversity compared to contemporary placental mammals, eventually became extinct during theMiocene epoch.[83]
In South America, theopossums evolved and developed a strong presence, and thePaleogene also saw the evolution ofshrew opossums (Paucituberculata) alongside non-marsupial metatherian predators such as theborhyaenids and the saber-toothedThylacosmilus. South American niches for mammalian carnivores were dominated by these marsupial andsparassodont metatherians, which seem to havecompetitively excluded South American placentals from evolving carnivory.[84] While placental predators were absent, the metatherians did have to contend with avian (terror bird) and terrestrial crocodylomorph competition. Marsupials were excluded in turn from large herbivore niches in South America by the presence ofnative placental ungulates (now extinct) andxenarthrans (whose largest forms are also extinct). South America andAntarctica remained connected until 35 mya, as shown by the unique fossils found there. North and South America were disconnected until about three million years ago, when theIsthmus of Panama formed. This led to theGreat American Interchange. Sparassodonts disappeared for unclear reasons – again, this has classically assumed as competition from carnivoran placentals, but the last sparassodonts co-existed with a few small carnivorans likeprocyonids and canines, and disappeared long before the arrival of macropredatory forms like felines,[85] while didelphimorphs (opossums) invaded Central America, with theVirginia opossum reaching as far north as Canada.
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'smonito del monte (amicrobiothere, the only New Worldaustralidelphian). This progenitor may haverafted across the widening, but still narrow, gap between Australia and Antarctica. The journey must not have been easy; South American ungulate[89][90][91] and xenarthran[92] 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 ofNew Guinea andSulawesi relatively recently via Australia.[93][94][95] A 2010 analysis ofretroposoninsertion sites in thenuclear 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 mostbasal 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.[66][67]
In Australia, terrestrial placentals disappeared early in theCenozoic (their most recent known fossils being 55 million-year-old teeth resembling those ofcondylarths) for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem.[93] Extant native Australian terrestrial placentals (such ashopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.[94]
Genetic analysis suggests a divergence date between the marsupials and the placentals at160 million years ago.[96] 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 ofDjarthia murgonensis from the early Eocene Tingamarra Local Fauna in Australia that indicate this species is the most plesiomorphic ancestor, the oldest unequivocal australidelphian, and may be the ancestral morphotype of the Australian marsupial radiation.[68]
In 2023, imaging of a partial skeleton found in Australia by paleontologists fromFlinders University led to the identification ofAmbulator keanei, the first long-distance walker in Australia.[97]
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First Europeans visit Australia in 1606, settlements begin in 1788. Dingo introduced 3500-4000 ya.Thylacine andTasmanian devil subsequently disappear from Australian mainland.
High marsupial diversity in South America. Appearance of the oldest Australian marsupial in late Paleocene. Dinosaurs are wiped off the Earth after an asteroid collision.
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