Marsupials represent the clade originating from the last common ancestor of extant metatherians, the group containing all mammals more closely related to marsupials than to placentals. Placentals and marsupials diverged from each other at least 125 million years ago, and possibly over 160 million years ago, during the Middle Jurassic-Early Cretaceous. Marsupials give birth to relatively undeveloped young that often reside in a pouch located on their mothers' abdomen for a certain amount of time. Close to 70% of the 334 extant species occur on the Australian continent (the mainland, Tasmania, New Guinea and nearby islands). The remaining 30% are found in the Americas—primarily in South America, thirteen in Central America, and one species, the Virginia opossum, in 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 μάρσιπποςmársippos, meaning "pouch".
Marsupials have the typical characteristics of mammals—e.g., mammary glands, three middle ear bones, 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. Ossifiedpatellae are absent in most modern marsupials (though a small number of exceptions are reported)[1] and epipubic bones are present. Marsupials (and monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.[2]
Skull and teeth
The skull has peculiarities in comparison to placental mammals. In general, the skull is relatively small and tight. Holes (foramen lacrimale) are located in the front of the orbit. The cheekbone is enlarged and extends farther to the rear. The angular extension (processus angularis) of the lower jaw is bent toward the center. The hard palate has more openings than placental mammals' foramina. The teeth also differ from that of placental mammals. For example, all Australia marsupials not in the order Diprotodontia have a different number of incisors in the upper and lower jaws.[3] The early marsupials had a dental formula from 5.1.3.44.1.3.4, that is, per quadrant; they have five (maxillary) or four (mandibular) incisors, one canine, three premolars and four molars, for a total of 50 teeth. Some taxa, such as the opossum, have the original number of teeth. In other groups the number of teeth is reduced. The dental formula for Macropodidae (kangaroos and wallabies etc.) is 3/1 – (0 or 1)/0 – 2/2 – 4/4. Marsupials in many cases have 40 to 50 teeth, significantly more than most placental mammals. The second set of teeth grows in only at the 3rd premolar site and back; all teeth more anterior to that 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.[2] The males have a split or double penis lying in front of the scrotum.[4]
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.
Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the red kangaroo, grows up to 1.8 metres (5 ft 11 in) in height and 90 kilograms (200 lb) in weight, but extinct genera, such as Diprotodon, were significantly larger and heavier. The smallest members of this group are the marsupial mice, which often reach only 5 centimetres (2.0 in) in body length.
Some species resemble placental mammals and are examples of convergent evolution. This convergence is evident in both brain evolution[5] and behaviour.[6] 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.
The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction, have engaged discussion since the end of the 19th century. Both sexes possess a cloaca,[12] which is connected to a urogenital sac used to store waste before expulsion. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.[12]
The male thylacine had a pouch that acted as a protective sheath, covering his external reproductive organs while running through thick brush.[23]
The shape of the urethral grooves of the males' genitalia is used to distinguish between Monodelphis brevicaudata, Monodelphis domestica, and Monodelphis americana. The grooves form 2 separate channels that form the ventral and dorsal folds of the erectile tissue.[24] Several species of dasyurid marsupials can also be distinguished by their penis morphology.[25]
The only accessory sex glands marsupials possess are the prostate and bulbourethral glands.[26] Male marsupials have 1-3 pairs of bulbourethral glands.[27] There are no ampullae of vas deferens, seminal vesicles or coagulating glands.[28][15] The prostate is proportionally larger in marsupials than in placental mammals.[4] 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.[29]
Female reproductive anatomy of several marsupial species
Female marsupials have two lateral vaginas, which lead to separate 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.[2] Some marsupial species are able to store sperm in the oviduct after mating.[30]
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.[31][32]
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.[33] 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 gestation period—usually between 12.5 and 33 days,[34] but as low as 10.7 days in the case of the stripe-faced dunnart and as long as 38 days for the long-nosed potoroo.[35] The joey is born in an essentially fetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns with sizes ranging from 4g to over 800g.[34] A newborn marsupial can be arranged into one of three grades of developmental complexity. Those who are the least developed at birth are found in dasyurids, intermediate ones are found in didelphids and peramelids, and the most developed are in macropods.[36] Despite the lack of development it crawls across its mother's fur to make its way into the pouch, which acts like an external womb,[37] where it latches onto a teat for food. It will not re-emerge for several months, during which time it is fully reliant on its mother's milk for essential nutrients, growth factors and immunological defence.[38] Genes expressed in the 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.[39] 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 placenta to protect the embryo from its mother's immune system. Though early birth puts the tiny newborn marsupial at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to a full term in bad seasons. Marsupials are extremely altricial animals, needing to be intensely cared for immediately following birth (cf.precocial). Newborn marsupials lack histologically mature immune tissues [40][41][42] and are highly reliant on their mother's immune system for immunological protection.,[43] as well as the milk.[31][32]
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.[44][45][40] This requirement has been argued to have resulted in the limited range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop grasping forepaws during their early youth, making the evolutive transition from these limbs into hooves, wings, or flippers, as some groups of placental mammals have done, more difficult. However, several marsupials do possess atypical forelimb morphologies, such as the hooved forelimbs of the pig-footed bandicoot, suggesting that the range of forelimb specialization is not as limited as assumed.[46]
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.[47][48]
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 opossumDidelphis 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 common opossum. Vicente Yáñez Pinzón, commander of the Niña on Christopher 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 the Spanish monarchs, though by then the young were lost and the female had died. The animal was noted for its strange pouch or "second belly", and how the offspring reached the pouch was a mystery.[49][50]
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):[49]
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).[51][49]
Taxonomy
Marsupials are taxonomically identified as members of mammalianinfraclass Marsupialia, first described as a family under the order Pollicata by German zoologist Johann Karl Wilhelm Illiger in his 1811 work Prodromus Systematis Mammalium et Avium. However, James Rennie, author of The 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, that were placed under the order Salientia) – under a single order (Pollicata) did not appear to have a strong justification. In 1816, French zoologist George Cuvier classified all marsupials under the order Marsupialia.[52][53] In 1997, researcher J. A. W. Kirsch and others accorded infraclass rank to Marsupialia.[53]
Classification
Marsupialia is further divided as follows:[54]
† – Extinct
Comprising over 300 extant species, several attempts have been made to accurately interpret the phylogenetic relationships among the different marsupial orders. Studies differ on whether Didelphimorphia or Paucituberculata is the sister group to all other marsupials.[55] Though the order Microbiotheria (which has only one species, the monito del monte) is found in South America, morphological similarities suggest it is closely related to Australian marsupials.[56] 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[57][58] & Álvarez-Carretero et al. 2022[59][60]
DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a single Gondwanan migration of marsupials from South America, across Antarctica, to Australia.[61][62] There are many small arboreal species in each group. The term "opossum" is used to refer to American species (though "possum" is a common abbreviation), while similar Australian species are properly called "possums".
Isolated petrosals of Djarthia murgonensis, Australia's oldest marsupial fossils[63]Dentition of the herbivorous eastern grey kangaroo, as illustrated in Knight's Sketches in Natural History
The ancestors of marsupials, part of a larger group called metatherians, probably split from those of placental mammals (eutherians) during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time.[66] From DNA and protein analyses, the time of divergence of the two lineages has been estimated to be around 100 to 120 mya.[49] Fossil metatherians are distinguished from eutherians by the form of their teeth; metatherians possess four pairs of molar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.[67] Using this criterion, the earliest known metatherian was thought to be Sinodelphys szalayi, which lived in China around 125 mya.[68][69][70] However Sinodelphys was later reinterpreted as an early member of Eutheria. The unequivocal oldest known metatherians are now 110 million years old fossils from western North America.[71] Metatherians were widespread in North America and Asia during the Late Cretaceous, but suffered a severe decline during the end-Cretaceous extinction event.[72]
Marsupials spread to South America from North America during the Paleocene, possibly via the Aves Ridge.[75][76][77] Northern Hemisphere metatherians, which were of low morphological and species diversity compared to contemporary placental mammals, eventually became extinct during the Miocene epoch.[78]
In South America, the opossums evolved and developed a strong presence, and the Paleogene also saw the evolution of shrew opossums (Paucituberculata) alongside non-marsupial metatherian predators such as the borhyaenids and the saber-toothed Thylacosmilus. South American niches for mammalian carnivores were dominated by these marsupial and sparassodont metatherians, which seem to have competitively excluded South American placentals from evolving carnivory.[79] 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 of native placental ungulates (now extinct) and xenarthrans (whose largest forms are also extinct). South America and Antarctica 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 the Isthmus of Panama formed. This led to the Great 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 like procyonids and canines, and disappeared long before the arrival of macropredatory forms like felines,[80] while didelphimorphs (opossums) invaded Central America, with the Virginia 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'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[84][85][86] and xenarthran[87] 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.[88][89][90] A 2010 analysis of retroposoninsertion 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.[61][62]
In Australia, terrestrial placental mammals disappeared early in the Cenozoic (their most recent known fossils being 55 million-year-old teeth resembling those of condylarths) for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem.[88] Extant native Australian terrestrial placental mammals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.[89]
Genetic analysis suggests a divergence date between the marsupials and the placentals at 160 million years ago.[91] The ancestral number of chromosomes has been estimated to be 2n = 14.
A new hypothesis suggests that South American microbiotheres resulted from a back-dispersal from eastern Gondwana due to new cranial and post-cranial marsupial fossils from the Djarthia murgonensis from the early Eocene Tingamarra Local Fauna in Australia that indicate the Djarthia murgonensis is the most plesiomorphic, the oldest unequivocal australidelphian, and may be the ancestral morphotype of the Australian marsupial radiation.[63]
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.[92]
^This is supported by the find of Eocene fossil remains of an australidelphian, the microbiotherian Woodburnodon casei, on the Antarctic peninsula,[81]
^Ratites may have similarly traveled overland from South America to colonise Australia;[82] a fossil ratite is known from Antarctica,[83] and South American rheas are more basal within the group than Australo-Pacific ratites.[82]
^Stannard, Hayley J.; Fabian, Megan; Old, Julie M. (2015). "To bask or not to bask: Behavioural thermoregulation in two species of dasyurid, Phascogale calura and Antechinomys laniger". Journal of Thermal Biology. 53: 66–71. doi:10.1016/j.jtherbio.2015.08.012. PMID26590457.
^Sharman GB, Pilton PE (1964). "The life history and reproduction of the red kangaroo (Megaleia rufa)". Proceedings of the Zoological Society of London. 142 (1): 29–48. doi:10.1111/j.1469-7998.1964.tb05152.x.
^Sadleir RM (1965). "Reproduction in two species of kangaroo (Macropus robustus and Megaleia rufa in the arid Pilbara region of Western Australia". Proceedings of the Zoological Society of London. 145 (2): 239–261. doi:10.1111/j.1469-7998.1965.tb02016.x.
^Paddle R (2002). The last Tasmanian tiger : the history and extinction of the thylacine (Paperback ed.). Port Melbourne, Vic.: Cambridge University Press. ISBN978-0-521-53154-2.
^Nogueira J, Castro AS, Câamara EC, Câmara BO (2004). "Morphology of the Male Genital system of Chironectes minimus and Comparison to other didelphid marsupials". Journal of Mammalogy. 85 (5): 834–841. doi:10.1644/207. S2CID85595933.
^Rodger JC (January 1976). "Comparative aspects of the accessory sex glands and seminal biochemistry of mammals". Comparative Biochemistry and Physiology. B, Comparative Biochemistry. 55 (1): 1–8. doi:10.1016/0305-0491(76)90164-4. PMID780045.
^kristac@stanford.edu, <img src='https://med stanford edu/news/media-contacts/krista_conger/_jcr_content/image img 620 high jpg/conger-krista-90 jpg' alt='Krista Conger'> Krista Conger Krista Conger is a senior science writer in the Office of Communications Email her at. "Baby marsupials 'drink' placenta to enhance development". News Center.((cite web)): CS1 maint: numeric names: authors list (link)
^ abOld JM, Deane EM (2003). "The lymphoid and immunohaematopoietic tissues of the embryonic brushtail possum (Trichosurus vulpecula)". Anatomy and Embryology. 206 (3): 193–197. doi:10.1007/s00429-002-0285-2. PMID12592570. S2CID546795.
^Old JM, Selwood L, Deane EM (2003). "A histological investigation of the lymphoid and immunohaematopoietic tissues of the adult stripe-faced dunnart (Sminthopsis macroura)". Cells Tissues Organs. 173 (2): 115–121. doi:10.1159/000068946. PMID12649589. S2CID46354564.
^Old JM, Deane EM (2000). "Development of the immune system and immunological protection in marsupial pouch young". Developmental and Comparative Immunology. 24 (5): 445–454. doi:10.1016/S0145-305X(00)00008-2. PMID10785270.
^ abcdTyndale-Biscoe H (2004). Life of Marsupials. Collingwood, Australia: CSIRO. ISBN978-0-643-06257-3.
^Krause WJ, Krause WA (2006). The Opossum: Its Amazing Story. Columbia, US: Dept. of Pathology of Anatomical Sciences, School of Medicine, University of Missouri. p. 6. ISBN978-0-9785999-0-4.
^Prevosti FJ, Forasiepi A, Zimicz N (2011). "The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?". Journal of Mammalian Evolution. 20: 3–21. doi:10.1007/s10914-011-9175-9. hdl:11336/2663. S2CID15751319.
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. ISBN9780802143716.
Flannery TF (2005). Country : a continent, a scientist & a kangaroo (2nd ed.). Melbourne: Text Pub. ISBN978-1-920885-76-2.
Frith, H. J. and J. H. Calaby. Kangaroos. New York: Humanities Press, 1969.
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.
First Europeans visit Australia in 1606, settlements begin in 1788. Dingo introduced 3500-4000 ya. Thylacine and Tasmanian 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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