Marsupial#Evolutionary history

File:Koala climbing tree.jpg
(Phascolarctos cinereus)]]

Marsupials have typical mammalian characteristics—e.g., mammary glands, three middle ear bones, (and ears that usually have tragi,{{cite journal | vauthors = Stannard HJ, Dennington K, Old JM | title = The external ear morphology and presence of tragi in Australian marsupials | journal = Ecology and Evolution | volume = 10 | pages = 9853–9866 | year = 2020 | issue = 18 | doi = 10.1002/ece3.6634| pmid = 33005349 | pmc = 7520188 | bibcode = 2020EcoEv..10.9853S }} varying in hearing thresholds{{cite journal | vauthors = Old JM, Parsons CL, Tulk, ML | title = Hearing thresholds of small native Australian mammals - red-tailed phascogale (Phascogale calura), kultarr (Antechinomys laniger) and spinifex hopping-mice (Notomys Alexis) | journal = Zoological Journal of the Linnean Society | volume = 190 | pages = 342–351 | year = 2020 | doi = 10.1093/zoolinnean/zlaa003}}), true hair and bone structure.{{cite book|url={{Google books|plainurl=yes|id=udCnKce9hfoC|page=97}}| vauthors = Feldhamer GA, Drickamer LC, Vessey SH, Merritt JF, Krajewski C |year=2007|title=Mammalogy: Adaptation, Diversity, Ecology|edition=3rd|location=Baltimore|publisher=Johns Hopkins University Press|isbn=978-0-8018-8695-9|oclc=124031907}} However, striking differences including anatomical features separate them from eutherians.

Most female marsupials have a front pouch, which contains multiple nursing teats. Marsupials have other common structural features. Ossified patellae are absent in most modern marsupials (with exceptions){{cite journal | vauthors = Samuels ME, Regnault S, Hutchinson JR | title = Evolution of the patellar sesamoid bone in mammals | journal = PeerJ | volume = 5 | pages = e3103 | year = 2017 | pmid = 28344905 | pmc = 5363259 | doi = 10.7717/peerj.3103 | doi-access = free }} and epipubic bones are present. Marsupials (and monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.{{sfn|Nowak|1999}}

Marsupials exhibit distinct cranial features compared to placentals. 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 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 in monotremes. Marsupial reproductive organs differ from placentals. For them, the reproductive tract is doubled. Females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina.{{sfn|Nowak|1999}} In most species, males have a split or double penis lying in front of the scrotum, which is not homologous to the placental scrota.{{Cite book |last1=Armati |first1=Patricia J. |url=https://books.google.com/books?id=x3S5v971Nk0C&dq=scrotum&pg=PA115 |title=Marsupials |last2=Dickman |first2=Chris R. |last3=Hume |first3=Ian D. |date=2006-08-17 |publisher=Cambridge University Press |isbn=978-1-139-45742-2 |language=en}}

A pouch is present in most species. Many marsupials have a permanent bag, while in others such as the shrew 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 male water opossum has a pouch that protects his genitalia while swimming or running.

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Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the red kangaroo, grows up to {{convert|1.8|m|ftin}} in height and {{convert|90|kg|lbs}} in weight. Extinct genera, such as Diprotodon, were significantly larger and heavier. The smallest marsupials are the marsupial mice, which reach only {{convert|5|cm|in}} in body length.

Some species resemble placentals and are examples of convergent evolution. This convergence is evident in both brain evolution{{cite journal | vauthors = Todorov OS, Blomberg SP, Goswami A, Sears K, Drhlík P, Peters J, Weisbecker V | title = Testing hypotheses of marsupial brain size variation using phylogenetic multiple imputations and a Bayesian comparative framework | journal = Proceedings. Biological Sciences | volume = 288 | issue = 1947 | pages = 20210394 | date = March 2021 | pmid = 33784860 | pmc = 8059968 | doi = 10.1098/rspb.2021.0394 }} and behaviour.{{cite book | vauthors = Todorov OS |contribution=Marsupial Cognition |date=2019 | title = Encyclopedia of Animal Cognition and Behavior |pages=1–8 | veditors = Vonk J, Shackelford T |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-47829-6_1167-1 |isbn=978-3-319-47829-6 |s2cid=242256517 }} 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 placentals (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.

Marsupials, along with monotremes (platypuses and echidnas), typically have lower body temperatures than similarly sized placentals (eutherians),{{cite book| title=Abstract: Thermoregulation in marsupials and monotremes, chapter of Marsupials and monotremes: nature's enigmatic mammals| date=2015| author1=Gaughan, John B.|author2=Hogan, Lindsay A.|author3=Wallage, Andrea| publisher=Nova Science Publishers, Incorporated| url=https://espace.library.uq.edu.au/view/UQ:380292| isbn=9781634834872| accessdate=2022-04-20}} with the averages being {{convert|35|°C}} for marsupials and {{convert|37|°C}} for placentals.{{cite web |url=http://www.bio.davidson.edu/courses/anphys/1999/White/thermal.htm |author=White |title=Thermal Biology of the Platypus |publisher=Davidson College |year=1999 |access-date=2006-09-14 |df=dmy-all |url-status=dead |archive-url=https://web.archive.org/web/20120306024923/http://www.bio.davidson.edu/courses/anphys/1999/White/thermal.htm |archive-date=March 6, 2012}}{{cite web |url=http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./control%20systems%20part%202.pdf |title=Control Systems Part 2 |access-date=6 July 2016 |archive-date=8 October 2016 |archive-url=https://web.archive.org/web/20161008154505/http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./control%20systems%20part%202.pdf |url-status=dead }} Some species will bask to conserve energy {{cite journal|last1=Stannard|first1= Hayley J.|last2= Fabian|first2= Megan|last3= Old|first3= Julie M.|year=2015|title=To bask or not to bask: Behavioural thermoregulation in two species of dasyurid, Phascogale calura and Antechinomys laniger|journal= Journal of Thermal Biology|volume= 53|pages= 66–71| doi= 10.1016/j.jtherbio.2015.08.012|pmid= 26590457|bibcode= 2015JTBio..53...66S}}

{{see also|Kangaroo#Reproduction and life cycle}}{{anchor|Reproduction}}File:Kangaroo and joey03.jpg with a joey in her pouch]]

Marsupials' reproductive systems differ markedly from those of placentals.{{cite book| vauthors = Short RV, Balaban E |title=The Differences Between the Sexes|url={{google books |plainurl=y |id=zunYrumtsR8C}}|date=1994|publisher=Cambridge University Press|isbn=978-0-521-44878-9}} During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas.

Both sexes possess a cloaca, although modified by connecting to a urogenital sac and having a separate anal region in most species.{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 396–399|isbn= 978-0-03-910284-5}} The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.

File:The_cyclopædia_of_anatomy_and_physiology_(1847)_(20821803985).jpg]]

Most male marsupials, except for macropods{{cite book| vauthors = Staker L |title=Macropod Husbandry, Healthcare and Medicinals—Volumes One and Two|url={{google books|plainurl=y |id=37e1AwAAQBAJ}}|date=30 June 2014|publisher=Lynda Staker|isbn=978-0-9775751-2-1}} and marsupial moles,

[https://www.jstor.org/stable/pdf/2452234.pdf On the Habits and Affinities of the New Australian Mammal, Notoryctes typhlops]

E. D. Cope

The American Naturalist

Vol. 26, No. 302 (February 1892), pp. 121–128 have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.{{sfn|Nowak|1999}}{{cite web|url=http://www.biology.iastate.edu/InternationalTrips/1Australia/Australia%20papers/Discoveries%20about%20Marsupial%20Rep |publisher=Iowa State University Biology Dept. |title=Discoveries about Marsupial Reproduction | vauthors = King A |year=2001 |access-date=22 November 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120905123347/http://www.biology.iastate.edu/InternationalTrips/1Australia/Australia%20papers/Discoveries%20about%20Marsupial%20Rep |archive-date=5 September 2012 }}{{Cite book | doi=10.1007/978-1-349-02721-7_24|chapter = Reproduction in male marsupials|title = The Biology of Marsupials| pages=411–457|year = 1977| vauthors = Setchell BP | isbn=978-1-349-02723-1}}{{cite book| vauthors = Renfree M, Tyndale-Biscoe H |title=Reproductive Physiology of Marsupials|url={{google books |plainurl=y |id=HpjovN0vXW4C}}|date=1987|publisher=Cambridge University Press|isbn=9780521337922}} The penis is used only during copulation, and is separate from the urinary tract. It curves forward when erect,{{cite book| vauthors = Sadleir R |title=The Reproduction of Vertebrates|url={{google books |plainurl=y |id=PVP-wS9UXwoC}}|date=1973|publisher=Elsevier Science|isbn=978-0-323-15935-7}} and when not erect, it is retracted into the body in an S-shaped curve. Neither marsupials nor monotremes possess a baculum.{{sfn|Nowak|1999}} The shape of the glans penis varies among marsupial species.{{cite book|author=Australian Mammal Society|title=Australian Mammal Society|url={{google books |plainurl=y |id=N_ifwszrgFsC|page=73}}|date=1978|publisher=Australian Mammal Society|pages=73–}}{{cite book| vauthors = Osgood WH, Herrick CJ |title=A monographic study of the American marsupial, Caēnolestes ...|url={{google books |plainurl=y |id=8Ag9AAAAYAAJ|page=64}}|year=1921|publisher=University of Chicago|pages=64–}}{{cite book|title=The Urologic and Cutaneous Review|url={{google books |plainurl=y |id=nApEAAAAYAAJ|page=677}}|year=1920|publisher=Urologic & Cutaneous Press|pages=677–}}

The shape of the urethral grooves of the males' genitalia is used to distinguish between Monodelphis brevicaudata, M. domestica, and M. americana. The grooves form two channels that form the ventral and dorsal folds of the erectile tissue.{{cite journal|vauthors = Nogueira J, Castro AS, Câamara EC, Câmara BO |title= Morphology of the Male Genital system of Chironectes minimus and Comparison to other didelphid marsupials|journal= Journal of Mammalogy|volume= 85|issue= 5|pages= 834–841|doi=10.1644/207|year= 2004|s2cid= 85595933}} Several species of dasyurid marsupials can also be distinguished by their penis morphology.{{Cite journal| vauthors = Woolley PA, Westerman M, Krajewski C |date=December 2007|title=Interspecific Affinities within the Genus Sminthopsis (Dasyuromorphia: Dasyuridae) Based on Morphology of the Penis: Congruence with Other Anatomical and Molecular Data|journal=Journal of Mammalogy|volume=88|issue=6|pages=1381–1392|doi=10.1644/06-mamm-a-443r.1|issn=0022-2372|doi-access=free}} Marsupials' only accessory sex glands are the prostate and bulbourethral glands.{{cite journal| vauthors = Rodger JC, Hughes RL|title=Studies of the accessory glands of male marsupials|journal=Australian Journal of Zoology|year=1973|volume=21|issue=3|page=303|doi=10.1071/ZO9730303|hdl=1959.4/70011 |url=https://unsworks.unsw.edu.au/fapi/datastream/unsworks:71547/SOURCE01?view=true|hdl-access=free}} Male marsupials have one to three pairs of bulbourethral glands.{{Cite book| vauthors = Vogelnest L, Portas T |url=https://books.google.com/books?id=396VDwAAQBAJ&q=bulbourethral|title=Current Therapy in Medicine of Australian Mammals |date=2019-05-01|publisher=Csiro Publishing|isbn=978-1-4863-0753-1|language=en}} Ampullae of vas deferens, seminal vesicles or coagulating glands are not present.{{cite journal | vauthors = Rodger JC | title = Comparative aspects of the accessory sex glands and seminal biochemistry of mammals | journal = Comparative Biochemistry and Physiology. B, Comparative Biochemistry | volume = 55 | issue = 1 | pages = 1–8 | date = January 1976 | pmid = 780045 | doi = 10.1016/0305-0491(76)90164-4 }}{{cite book |url={{google books |plainurl=y |id=cESCLrRJGm0C}} |title=The Biology of Marsupials |vauthors=Hunsaker II D |publisher=Elsevier Science |year=1977 |isbn=978-0-323-14620-3}} The prostate is proportionally larger in marsupials than in placentals. During the breeding season, the male tammar wallaby's prostate and bulbourethral gland enlarge. However, the weight of the testes does not vary seasonally.{{cite journal | vauthors = Inns RW | title = Seasonal changes in the accessory reproductive system and plasma testosterone levels of the male tammar wallaby, Macropus eugenii, in the wild | journal = Journal of Reproduction and Fertility | volume = 66 | issue = 2 | pages = 675–680 | date = November 1982 | pmid = 7175821 | doi = 10.1530/jrf.0.0660675 | doi-access = free }}

{{see also|Birth#Marsupials}}

File:Cambridge Natural History Mammalia Fig 048.png

Female marsupials have two lateral vaginas, which lead to separate uteri, both accessed through the same orifice.{{Cite book |last=Nowak |first=Ronald M. |url=https://books.google.com/books?id=ldXtY8ppxSQC&pg=PA14 |title=Walker's Marsupials of the World |date=2005-09-12 |publisher=JHU Press |isbn=978-0-8018-8211-1 |language=en}} A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.{{sfn|Nowak|1999}} Some marsupial species store sperm in the oviduct after mating.{{cite book| vauthors = Plant TM, Zeleznik AJ |title=Knobil and Neill's Physiology of Reproduction|url=https://books.google.com/books?id=I1ACBAAAQBAJ|date=15 November 2014|publisher=Academic Press|isbn=978-0-12-397769-4}}

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 the marsupium, 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 several weeks. Offspring eventually leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.{{cite web|last1=Stannard|first1= Hayley J.|last2=Old |first2=Julie M. |title=Wallaby joeys and platypus puggles are tiny and undeveloped when born. But their mother's milk is near-magical|url=https://theconversation.com/wallaby-joeys-and-platypus-puggles-are-tiny-and-undeveloped-when-born-but-their-mothers-milk-is-near-magical-207726|publisher=The Conversation|year=2023}}{{cite journal|last1=Stannard|first1=Hayley J.|last2=Miller|first2=Robert D.|last3=Old|first3=Julie M.|title=Marsupial and monotreme milk – a review of its nutrients and immune properties|journal=PeerJ|year=2020|volume=8|page=e9335|doi=10.7717/peerj.9335 |pmid=32612884 |pmc=7319036 |doi-access=free }}

File:Jeune M. rufogriseus poche.jpg joey inside its mother's pouch]]

Gestation differs between marsupials and placentals. Key aspects of the first stages of placental embryo development, such as the inner cell mass and the process of compaction, are not found in marsupials.{{cite journal | vauthors = Frankenberg SR, de Barros FR, Rossant J, Renfree MB | title = The mammalian blastocyst | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 5 | issue = 2 | pages = 210–232 | year = 2016 | pmid = 26799266 | doi = 10.1002/wdev.220 | s2cid = 22001725 }} The cleavage stages of marsupial development are vary among groups and aspects of marsupial early development are not yet fully understood.

Marsupials have a short gestation period—typically between 12 and 33 days,{{cite journal |last1= Smith|first1=Kathleen K. |last2=Keyte |first2=Anna L. |date=2020 |title=Adaptations of the Marsupial Newborn: Birth as an Extreme Environment |url= |journal=The Anatomical Record |volume= 303|issue=2 |pages=235–249 |doi=10.1002/ar.24049 |pmid=30548826 |s2cid=56484546 |access-date=|doi-access=free }} but as low as 10 days in the case of the stripe-faced dunnart and as long as 38 days for the long-nosed potoroo.{{cite journal |last1=Drews |first1=Barbara |last2=Roellig |first2=Kathleen |last3= Menzies|first3=Brandon R.|last4=Shaw |first4=Geoff |last5=Buentjen |first5=Ina |last6=Herbert |first6=Catherine A. |last7=Hildebrandt |first7=Thomas B. |last8=Renfree |first8=Marilyn B.|date=15 March 2013 |title=Ultrasonography of wallaby prenatal development shows that the climb to the pouch begins in utero |url= |journal= Scientific Reports|volume=3 |issue= 1458|page=1458 |doi=10.1038/srep01458 |pmid=23492830 |pmc=3597997 |bibcode=2013NatSR...3.1458D |access-date=}} The baby (joey) is born in a fetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns: sizes range from 4-800g+. A newborn can be categorized in one of three grades of development. The least developed are found in dasyurids, intermediates are found in didelphids and peramelids, and the most developed are macropods.{{Cite journal|title=Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young - PMC|year=2017 |pmc=5472534 |last1=Schneider |first1=N. Y. |last2=Gurovich |first2=Y. |journal=Journal of Anatomy |volume=231 |issue=1 |pages=59–83 |doi=10.1111/joa.12621 |pmid=28620997 }} The newborn crawls across its mother's fur to reach the pouch,{{Cite book|url=https://books.google.com/books?id=rR9XPnaqvCMC&dq=%22The+pouch+is+like+an+external+womb%22&pg=PA224|title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution|first1=Richard|last1=Dawkins|first2=Yan|last2=Wong|date=9 February 2005|publisher=Houghton Mifflin Harcourt|isbn=9780618619160 |via=Google Books}} where it latches onto a teat. 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.{{cite journal | vauthors = Stannard HJ, Miller RD, Old JM | title = Marsupial and monotreme milk – a review of its nutrients and immune properties | journal = PeerJ | volume = 8 | pages = e9335 | date = 2020 | doi = 10.7717/peerj.9335 | pmid = 32612884 | pmc = 7319036 | doi-access = free }} Genes expressed in the eutherian placenta needed for the later stages of fetal development are expressed in females in their mammary glands during lactation.{{Cite web|url=http://med.stanford.edu/news/all-news/2017/09/baby-marsupials-drink-placenta-to-enhance-development.html|title=Baby marsupials 'drink' placenta to enhance development|first=Krista|last=Conger|website=Stanford Medicine News Center|date=12 September 2017}} 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 complex placenta to protect the embryo from its mother's immune 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 are altricial animals, needing intensive care following birth (cf. precocial). Newborns lack histologically mature immune tissues{{cite journal | vauthors = Old JM, Deane EM | title = The lymphoid and immunohaematopoietic tissues of the embryonic brushtail possum (Trichosurus vulpecula) | journal = Anatomy and Embryology | volume = 206 | issue = 3 | pages = 193–197 | date = 2003 | doi = 10.1007/s00429-002-0285-2| pmid = 12592570 | s2cid = 546795 }}{{cite journal | vauthors = Old JM, Selwood L, Deane EM | title = A histological investigation of the lymphoid and immunohaematopoietic tissues of the adult stripe-faced dunnart (Sminthopsis macroura) | journal = Cells Tissues Organs | volume = 173 | issue = 2 | pages = 115–121 | date = 2003 | doi = 10.1159/000068946| pmid = 12649589 | s2cid = 46354564 }}{{cite journal | vauthors = Old JM, Selwood L, Deane EM | title = Development of the lymphoid tissues of the stripe-faced dunnart (Sminthopsis macroura) | journal = Cells Tissues Organs | volume = 175 | issue = 4 | pages = 192–201 | date = 2003 | doi = 10.1111/j.0021-8782.2004.00310.x| pmid = 15255959 | pmc = 1571326 }} and are highly reliant on their mother's immune system for immunological protection.{{cite journal | vauthors = Old JM, Deane EM | title = Development of the immune system and immunological protection in marsupial pouch young | journal = Developmental and Comparative Immunology | volume = 24 | issue = 5 | pages = 445–454 | date = 2000 | doi = 10.1016/S0145-305X(00)00008-2| pmid = 10785270 }}

Newborns front limbs and facial structures are much more developed than the rest of their bodies at birth.{{cite journal | vauthors = Sears KE | title = Differences in the timing of prechondrogenic limb development in mammals: the marsupial-placental dichotomy resolved | journal = Evolution; International Journal of Organic Evolution | volume = 63 | issue = 8 | pages = 2193–2200 | date = August 2009 | pmid = 19453378 | doi = 10.1111/j.1558-5646.2009.00690.x | s2cid = 42635687 | doi-access = free }}{{cite journal | vauthors = Smith KK | title = Early development of the neural plate, neural crest and facial region of marsupials | journal = Journal of Anatomy | volume = 199 | issue = Pt 1-2 | pages = 121–131 | year = 2001 | pmid = 11523813 | pmc = 1594995 | doi = 10.1046/j.1469-7580.2001.19910121.x }} 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 into hooves, wings, or flippers. 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.Larry Vogelnest, Graeme Allan, Radiology of Australian Mammals

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 {{convert|30|-|32|C|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.{{cite journal | vauthors = Schneider NY | title = The development of the olfactory organs in newly hatched monotremes and neonate marsupials | journal = Journal of Anatomy | volume = 219 | issue = 2 | pages = 229–242 | date = August 2011 | pmid = 21592102 | pmc = 3162242 | doi = 10.1111/j.1469-7580.2011.01393.x }}{{Cite journal|title=Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype - PMC|year=2017 |pmc=5696127 |last1=Ferner |first1=K. |last2=Schultz |first2=J. A. |last3=Zeller |first3=U. |journal=Journal of Anatomy |volume=231 |issue=6 |pages=798–822 |doi=10.1111/joa.12689 |pmid=28960296 }}

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.

Europeans' first encounter with a marsupial 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".{{cite book| vauthors = Krause WJ, Krause WA |title=The Opossum: Its Amazing Story|date=2006|publisher=Dept. of Pathology of Anatomical Sciences, School of Medicine, University of Missouri|location=Columbia, US|isbn=978-0-9785999-0-4|page=6}}

The Portuguese first described Australasian marsupials: António Galvão, a Portuguese administrator in Ternate (1536–1540), wrote a detailed account of the northern common cuscus (Phalanger orientalis):

{{blockquote|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 like venison, and the stomach contained ginger leaves. This description appears to closely resemble the dusky pademelon (Thylogale brunii), the earliest European record of a member of the Macropodidae.{{cite book| vauthors = Dawson TJ |title=Kangaroos|date=2012|publisher=CSIRO Publishing|location=Collingwood, US|isbn=978-0-643-10625-3|page=181|edition=2nd}}

Marsupials are taxonomically identified as members of mammalian infraclass 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, which 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 Marsupialia.{{cite book| vauthors = Martin WC |title=A General Introduction to the Natural History of Mammiferous Animals|date=1841|publisher=Wright and Co. Printers|location=London, UK|pages=182–4|url=https://archive.org/stream/generalintroduct00martrich#page/182/mode/2up}}{{cite book| vauthors = Jackson S, Groves C |title=Taxonomy of Australian Mammals|date=2015|publisher=CSIRO Publishing|location=Australia|isbn=978-1-4863-0014-3|pages=82–3|url=https://books.google.com/books?id=jvznCQAAQBAJ&pg=PT82}} In 1997, researcher J. A. W. Kirsch and others accorded infraclass rank to Marsupialia.

With seven living orders in total,{{cite web|url= http://vertlife.org/data/mammals/ |title=Mammals |publisher=vertlife.org |accessdate=2024-11-12}} Marsupialia is further divided as follows:{{MSW3 Gardner|pages=3–21}}{{extinct}} – 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 Thylacomyidae: 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 {{extinct}} Diprotodontidae
• Family {{extinct}} Palorchestidae: marsupial tapirs
• Family {{extinct}} Thylacoleonidae: marsupial lions
• Suborder Phalangerida
• Infraorder Phalangeriformes – see list of phalangeriformes
• Family Acrobatidae: feathertail glider and feather-tailed possum
• Family Burramyidae: pygmy possums
• Family †Ektopodontidae: 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 {{extinct}} Balbaridae: basal quadrupedal kangaroos

{{See also|Metatheria|Evolution of Macropodidae|Evolution of mammals}}

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. 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.{{cite journal| vauthors = Szalay F | veditors = Archer M | title=A new appraisal of marsupial phylogeny and classification|journal=Carnivorous Marsupials|date=1982|volume=2|pages=621–40}} 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

|title=Cladogram of Marsupialia by Upham et al. 2019{{Cite journal |last1=Upham |first1=Nathan S. |last2=Esselstyn |first2=Jacob A. |last3=Jetz |first3=Walter |date=2019 |title=Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution and conservation |journal=PLOS Biol |volume=17 |issue=12 |pages=e3000494 |doi=10.1371/journal.pbio.3000494 |pmid=31800571 |pmc=6892540 |doi-access=free }}{{cite journal |last1=Upham |first1=Nathan S. |last2=Esselstyn |first2=Jacob A. |last3=Jetz |first3=Walter |year=2019 |title=DR_on4phylosCompared_linear_richCol_justScale_ownColors_withTips_80in |url=https://github.com/n8upham/MamPhy_v1/blob/master/Fig6_compare_tipDRs/DR_on4phylosCompared_linear_richCol_justScale_ownColors_withTips_80in.pdf |journal=PLOS Biology |volume=17 |issue=12 |pages= e3000494|doi=10.1371/journal.pbio.3000494 |doi-access=free |pmid=31800571 |pmc=6892540 }} & Álvarez-Carretero et al. 2022{{cite journal |last1=Álvarez-Carretero |first1=Sandra |last2=Tamuri |first2=Asif U. |last3=Battini |first3=Matteo |last4=Nascimento |first4=Fabrícia F. |last5=Carlisle |first5=Emily |last6=Asher |first6=Robert J. |last7=Yang |first7=Ziheng |last8=Donoghue |first8=Philip C.J. |last9=dos Reis |first9=Mario |year=2022 |title=A species-level timeline of mammal evolution integrating phylogenomic data |journal=Nature |volume= 602|issue=7896 |pages=263–267 |doi=10.1038/s41586-021-04341-1 |pmid=34937052 |bibcode=2022Natur.602..263A |s2cid=245438816 |url=https://qmro.qmul.ac.uk/xmlui/handle/123456789/75979|hdl=1983/de841853-d57b-40d9-876f-9bfcf7253f12 |hdl-access=free }}{{cite journal |last1=Álvarez-Carretero |first1=Sandra |last2=Tamuri |first2=Asif U. |last3=Battini |first3=Matteo |last4=Nascimento |first4=Fabrícia F. |last5=Carlisle |first5=Emily |last6=Asher |first6=Robert J. |last7=Yang |first7=Ziheng |last8=Donoghue |first8=Philip C.J. |last9=dos Reis |first9=Mario |year=2022 |title=4705sp_colours_mammal-time.tree |journal=Nature |volume= 602|issue=7896 |pages=263–267 |doi=10.1038/s41586-021-04341-1 |pmid=34937052 |bibcode=2022Natur.602..263A |s2cid=245438816 |url=https://figshare.com/articles/dataset/Data_for_A_Species-Level_Timeline_of_Mammal_Evolution_Integrating_Phylogenomic_Data_/14885691|hdl=1983/de841853-d57b-40d9-876f-9bfcf7253f12 |hdl-access=free }}|

{{clade|style=font-size:90%;line-height:80%;width:400px;

|label1=Marsupialia

|1={{clade

|1=Paucituberculata

|2={{clade

|1=Didelphimorphia

|2={{clade

|label1=Australidelphia

|1={{clade

|1=Microbiotheria

|2={{clade

|1={{clade

|label1=Agreodontia

|1={{clade

|1=Notoryctemorphia

|2={{clade

|1=Peramelemorphia

|2=Dasyuromorphia

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|2=Diprotodontia

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DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a single Gondwanan migration of marsupials from South America, across the Antarctic land bridge, to Australia.{{cite news| vauthors = Schiewe J |title=Australia's marsupials originated in what is now South America, study says |newspaper=Los Angeles Times |date=28 July 2010 |url=http://www.latimes.com/news/science/la-sci-marsupial-20100728,0,5549873.story |access-date=1 August 2010 |archive-url=https://web.archive.org/web/20100801001045/http://www.latimes.com/news/science/la-sci-marsupial-20100728%2C0%2C5549873.story |archive-date=1 August 2010 |url-status=live }}{{cite journal | vauthors = Nilsson MA, Churakov G, Sommer M, Tran NV, Zemann A, Brosius J, Schmitz J | title = Tracking marsupial evolution using archaic genomic retroposon insertions | journal = PLOS Biology | volume = 8 | issue = 7 | pages = e1000436 | date = July 2010 | pmid = 20668664 | pmc = 2910653 | doi = 10.1371/journal.pbio.1000436 | doi-access = free }} 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".

File:Djarthia murgonensis.jpg of Djarthia murgonensis, Australia's oldest marsupial fossils{{cite journal | vauthors = Beck RM, Godthelp H, Weisbecker V, Archer M, Hand SJ | title = Australia's oldest marsupial fossils and their biogeographical implications | journal = PLOS ONE | volume = 3 | issue = 3 | pages = e1858 | date = March 2008 | pmid = 18365013 | pmc = 2267999 | doi = 10.1371/journal.pone.0001858 | veditors = Hawks J | bibcode = 2008PLoSO...3.1858B | doi-access = free }}]]

File:Animaldentition macropusgiganteus.png

The relationships among the three extant divisions of mammals (monotremes, marsupials, and placentals) were long a matter of debate among taxonomists.{{cite book| vauthors = Moyal AM |title=Platypus: The Extraordinary Story of How a Curious Creature Baffled the World|publisher=The Johns Hopkins University Press|location= Baltimore|year=2004|isbn=978-0-8018-8052-0|url={{google books |plainurl=y |id=5DkezNMhSTYC}}}} Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems as well as most genetic and molecular evidence favors a closer evolutionary relationship between the marsupials and placentals than either has with the monotremes.{{cite journal | vauthors = van Rheede T, Bastiaans T, Boone DN, Hedges SB, de Jong WW, Madsen O | title = The platypus is in its place: nuclear genes and indels confirm the sister group relation of monotremes and Therians | journal = Molecular Biology and Evolution | volume = 23 | issue = 3 | pages = 587–597 | date = March 2006 | pmid = 16291999 | doi = 10.1093/molbev/msj064 | doi-access = free }}

File:Phylogenetic tree of marsupials derived from retroposon data - journal.pbio.1000436.g002.png of marsupials derived from retroposon data]]

The ancestors of marsupials, part of a larger group called metatherians, probably split from those of placentals (eutherians) during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time.{{cite journal | vauthors = Luo ZX, Yuan CX, Meng QJ, Ji Q | title = A Jurassic eutherian mammal and divergence of marsupials and placentals | journal = Nature | volume = 476 | issue = 7361 | pages = 442–445 | date = August 2011 | pmid = 21866158 | doi = 10.1038/nature10291 | s2cid = 205225806 | bibcode = 2011Natur.476..442L }} From DNA and protein analyses, the time of divergence of the two lineages has been estimated to be around 100 to 120 mya.{{cite book| vauthors= Tyndale-Biscoe H |title=Life of Marsupials|date=2004|publisher=CSIRO|location=Collingwood, Australia|isbn=978-0-643-06257-3}} 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.{{cite book| vauthors = Benton MJ |year=1997|title=Vertebrate Palaeontology|publisher=Chapman & Hall|location= London|page=306|isbn=978-0-412-73810-4}} Using this criterion, the earliest known metatherian was thought to be Sinodelphys szalayi, which lived in China around 125 mya.{{cite news| vauthors = Rincon P |url= http://news.bbc.co.uk/2/hi/science/nature/3311911.stm|title=Oldest Marsupial Ancestor Found|work= BBC News|date=12 December 2003|access-date=16 March 2010}}{{cite journal | vauthors = Luo ZX, Ji Q, Wible JR, Yuan CX | title = An Early Cretaceous tribosphenic mammal and metatherian evolution | journal = Science | volume = 302 | issue = 5652 | pages = 1934–1940 | date = December 2003 | pmid = 14671295 | doi = 10.1126/science.1090718 | s2cid = 18032860 | bibcode = 2003Sci...302.1934L }}{{cite journal | vauthors = Hu Y, Meng J, Li C, Wang Y | title = New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China | journal = Proceedings. Biological Sciences | volume = 277 | issue = 1679 | pages = 229–236 | date = January 2010 | pmid = 19419990 | pmc = 2842663 | doi = 10.1098/rspb.2009.0203 }} 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.{{cite journal | vauthors = Bi S, Zheng X, Wang X, Cignetti NE, Yang S, Wible JR | title = An Early Cretaceous eutherian and the placental-marsupial dichotomy | journal = Nature | volume = 558 | issue = 7710 | pages = 390–395 | date = June 2018 | pmid = 29899454 | doi = 10.1038/s41586-018-0210-3 | bibcode = 2018Natur.558..390B | s2cid = 49183466 }} Metatherians were widespread in North America and Asia during the Late Cretaceous, but suffered a severe decline during the end-Cretaceous extinction event.{{Cite journal |last1=Bennett |first1=C. Verity |last2=Upchurch |first2=Paul |last3=Goin |first3=Francisco J. |last4=Goswami |first4=Anjali |date=2018-02-06 |title=Deep time diversity of metatherian mammals: implications for evolutionary history and fossil-record quality |journal=Paleobiology |volume=44 |issue=2 |pages=171–198 |doi=10.1017/pab.2017.34 |bibcode=2018Pbio...44..171B |s2cid=46796692 |issn=0094-8373|doi-access=free |hdl=11336/94590 |hdl-access=free }}

Cladogram from Wilson et al. (2016)Wilson, G.P.; Ekdale, E.G.; Hoganson, J.W.; Calede, J.J.; Linden, A.V. (2016). "[https://www.nature.com/articles/ncomms13734 A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials]". Nature Communications. 7. {{doi|10.1038/ncomms13734}}.

{{clade| style=font-size:85%; line-height:85%

|label1=Metatheria

|1={{clade

|1=Holoclemensia

|2={{clade

|1={{clade

|1=Pappotherium

|2={{clade

|1=Sulestes

|2=Oklatheridium

|3={{clade

|1=Tsagandelta

|2={{clade

|1=Lotheridium

|2={{clade

|1=Deltatheroides

|2=Deltatheridium

|3={{clade

|1=Nanocuris

|2=Atokatheridium

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|label2=Marsupialiformes

|2={{clade

|1={{clade

|1=Gurlin Tsav skull

|2={{clade

|1=Borhyaenidae

|2={{clade

|1=Mayulestes

|2={{clade

|1=Jaskhadelphys

|2={{clade

|1=Andinodelphys

|2=Pucadelphys

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|2={{clade

|1=Asiatherium

|2={{clade

|1={{clade

|1=Iugomortiferum

|2=Kokopellia

}}

|2=Aenigmadelphys

|3=Anchistodelphys

|4={{clade

|label1=Glasbiidae

|1=Glasbius

|label2=Pediomyidae

|2=Pediomys

}}

|label5=Stagodontidae

|5={{clade

|1=Pariadens

|2={{clade

|1=Eodelphis

|2=Didelphodon

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|label6=Alphadontidae

|6={{clade

|1=Turgidodon

|2=Alphadon

|3=Albertatherium

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|7=Marsupialia

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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.{{cite journal |last1=Velazco |first1=Paúl M |last2=Buczek |first2=Alexandra J |last3=Hoffman |first3=Eva |last4=Hoffman |first4=Devin K |last5=O’Leary |first5=Maureen A |last6=Novacek |first6=Michael J |date=30 January 2022 |title=Combined data analysis of fossil and living mammals: a Paleogene sister taxon of Placentalia and the antiquity of Marsupialia |journal=Cladistics |volume=38 |issue=3 |pages=359–373 |doi=10.1111/cla.12499 |pmid=35098586 |s2cid=246429311 |url=https://discovery.ucl.ac.uk/id/eprint/10178921/ }} 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.{{cite book |last=Kemp |first=Thomas Stainforth |date=2005 |title= The origin and evolution of mammals|url=http://doc.rero.ch/record/200125/files/PAL_E3904.pdf |location=Oxford |publisher=Oxford University Press |page=217 |isbn=0-19-850760-7 }}{{cite journal | vauthors = Boschman LM, van Hinsbergen DJ, Torsvik TH, Spakman W, Pindell JL |date= 23 August 2014|title=Kinematic reconstruction of the Caribbean region since the Early Jurassic |journal=Earth-Science Reviews |volume=138 |pages=102–136 |doi=10.1016/j.earscirev.2014.08.007 |bibcode=2014ESRv..138..102B |citeseerx=10.1.1.727.4858 }}{{Cite journal |last1=Ali |first1=Jason R. |last2=Hedges |first2=S. Blair |date=November 2021 |editor-last=Hoorn |editor-first=Carina |title=Colonizing the Caribbean: New geological data and an updated land-vertebrate colonization record challenge the GAARlandia land-bridge hypothesis |journal=Journal of Biogeography |language=en |volume=48 |issue=11 |pages=2699–2707 |doi=10.1111/jbi.14234 |s2cid=238647106 |issn=0305-0270|doi-access=free |bibcode=2021JBiog..48.2699A }} Northern Hemisphere metatherians, which were of low morphological and species diversity compared to contemporary placental mammals, eventually became extinct during the Miocene epoch.{{Cite journal |last1=Eldridge |first1=Mark D B |last2=Beck |first2=Robin M D |last3=Croft |first3=Darin A |last4=Travouillon |first4=Kenny J |last5=Fox |first5=Barry J |date=2019-05-23 |title=An emerging consensus in the evolution, phylogeny, and systematics of marsupials and their fossil relatives (Metatheria) |url=https://academic.oup.com/jmammal/article/100/3/802/5498022 |journal=Journal of Mammalogy |language=en |volume=100 |issue=3 |pages=802–837 |doi=10.1093/jmammal/gyz018 |issn=0022-2372}}

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.{{cite journal | vauthors = Simpson GG | author-link = George Gaylord Simpson | title = History of the Fauna of Latin America | journal = American Scientist | volume = 38 | issue = 3 | pages = 361–389; see p. 368 | date = July 1950 | url = http://www.wku.edu/~smithch/biogeog/SIMP1950.htm | jstor = 27826322 | access-date = 2020-01-21 | archive-date = 15 June 2009 | archive-url = https://web.archive.org/web/20090615230938/http://www.wku.edu/~smithch/biogeog/SIMP1950.htm | url-status = dead }} 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,{{cite journal|title=The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?|journal=Journal of Mammalian Evolution|volume=20|pages=3–21|doi=10.1007/s10914-011-9175-9|year=2011| vauthors = Prevosti FJ, Forasiepi A, Zimicz N |hdl=11336/2663|s2cid=15751319|hdl-access=free}} while didelphimorphs (opossums) invaded Central America, with the Virginia opossum reaching as far north as Canada.

Marsupials reached Australia via the Antarctic Land Bridge during the Early Eocene, around 50 mya, shortly after Australia had split off.{{refn|group=n|This is supported by the find of Eocene fossil remains of an australidelphian, the microbiotherian Woodburnodon casei, on the Antarctic Peninsula,{{cite journal | vauthors = Goin FJ, Zimicz N, Reguero MA, Santillana SN, Marenssi SA, Moly JJ | title = New marsupial (Mammalia) from the Eocene of Antarctica, and the origins and affinities of the Microbiotheria | journal = Revista de la Asociación Geológica Argentina | volume = 62 | issue = 4 | pages = 597–603 | date = 2007 | url = https://www.researchgate.net/publication/262722641 | access-date = 2016-07-17 }}}}{{refn|group=n|Ratites may have similarly traveled overland from South America to colonise Australia;{{cite journal | vauthors = Yonezawa T, Segawa T, Mori H, Campos PF, Hongoh Y, Endo H, Akiyoshi A, Kohno N, Nishida S, Wu J, Jin H, Adachi J, Kishino H, Kurokawa K, Nogi Y, Tanabe H, Mukoyama H, Yoshida K, Rasoamiaramanana A, Yamagishi S, Hayashi Y, Yoshida A, Koike H, Akishinonomiya F, Willerslev E, Hasegawa M | display-authors = 6 | title = Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites | journal = Current Biology | volume = 27 | issue = 1 | pages = 68–77 | date = January 2017 | pmid = 27989673 | doi = 10.1016/j.cub.2016.10.029 | doi-access = free | bibcode = 2017CBio...27...68Y }} a fossil ratite is known from Antarctica,{{cite journal|vauthors= Tambussi CP, Noriega JI, Gaździcki A, Tatur A, Reguero MA, Vizcaíno SF|title= Ratite bird from the Paleogene La Meseta Formation, Seymour Island, Antarctica|journal= Polish Polar Research|volume= 15|issue= 1–2|pages= 15–20|date= 1994|url= http://polar.pan.pl/ppr15/1994-1-2_015-020.pdf|access-date= 28 December 2019|archive-date= 28 December 2019|archive-url= https://web.archive.org/web/20191228235043/http://polar.pan.pl/ppr15/1994-1-2_015-020.pdf|url-status= dead}} and South American rheas are more basal within the group than Australo-Pacific ratites.}} 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{{cite book | vauthors = Bond M, Reguero MA, Vizcaíno SF, Marenssi SA |year=2006 |chapter=A new 'South American ungulate' (Mammalia: Litopterna) from the Eocene of the Antarctic Peninsula | veditors = Francis JE, Pirrie D, Crame JA |title=Cretaceous-tertiary high-latitude palaeoenvironments: James Ross Basin, Antarctica |series=Geological Society, London, Special Publications |publisher=The Geological Society of London |volume= 258|issue=1 |pages=163–176 |doi= 10.1144/GSL.SP.2006.258.01.12 |contribution-url= https://www.researchgate.net/publication/249551916_A_new_%27South_American_ungulate%27_Mammalia_Litopterna_from_the_Eocene_of_the_Antarctic_Peninsula|bibcode=2006GSLSP.258..163B |s2cid=140546667 }}{{Cite journal | vauthors = Bond M, Kramarz A, Macphee RD, Reguero M | title = A new astrapothere (Mammalia, Meridiungulata) from La Meseta Formation, Seymour (Marambio) Island, and a reassessment of previous records of Antarctic astrapotheres | year = 2011 | journal = American Museum Novitates | issue = 3718 | pages = 1–16 | url = http://digitallibrary.amnh.org/dspace/bitstream/2246/6118/1/N3718.pdf | doi = 10.1206/3718.2 |doi-access=free | hdl = 11336/98139 | s2cid = 58908785 |url-status=dead |archive-url=https://web.archive.org/web/20120917125657/http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/6118/N3718.pdf;jsessionid=841BDC1267FD7DC21E8503D6930DA614?sequence=1 |archive-date= Sep 17, 2012 }}{{cite journal| vauthors = Gelfo JN, Mörs T, Lorente M, López GM, Reguero M |title=The oldest mammals from Antarctica, early Eocene of the La Meseta Formation, Seymour Island|journal= Palaeontology|volume= 58|issue=1|date= 2014-07-16|pages= 101–110|doi= 10.1111/pala.12121|url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-922|doi-access=free |url-status=live |archive-url=https://web.archive.org/web/20230405191308/http://nrm.diva-portal.org/smash/record.jsf?pid=diva2:768107 |archive-date= Apr 5, 2023 }} and xenarthran{{cite journal | vauthors = Davis SN, Torres CR, Musser GM, Proffitt JV, Crouch NM, Lundelius EL, Lamanna MC, Clarke JA | display-authors = 6 | title = New mammalian and avian records from the late Eocene La Meseta and Submeseta formations of Seymour Island, Antarctica | journal = PeerJ | volume = 8 | pages = e8268 | year = 2020 | pmid = 31942255 | pmc = 6955110 | doi = 10.7717/peerj.8268 | doi-access = free }} 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.{{cite book| vauthors = Dawkins R |title=The Ancestor's Tale : A Pilgrimage to the Dawn of Evolution|publisher=Mariner Books|location=Boston|year=2005|pages=223 |isbn=978-0-618-61916-0|url={{google books |plainurl=y |id=rR9XPnaqvCMC|page=223}}}}{{cite book| vauthors = Hand SJ, Long J, Archer M, Flannery TF |title=Prehistoric mammals of Australia and New Guinea: one hundred million years of evolution|publisher=Johns Hopkins University Press|location=Baltimore|year= 2002|isbn=978-0-8018-7223-5|url={{google books |plainurl=y |id=92yhnRHdxSoC}}}}{{cite book|author=Kemp, T.S.|title=The origin and evolution of mammals|publisher=Oxford University Press|location=Oxford [Oxfordshire]|year=2005|isbn=978-0-19-850761-1 }} 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.

In Australia, terrestrial placentals 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. Extant native Australian terrestrial placentals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.

Genetic analysis suggests a divergence date between the marsupials and the placentals at {{Ma|160}}.Graves JA, Renfree MB (2013)[https://www.researchgate.net/profile/Jennifer_Graves5/publication/247770436_Marsupials_in_the_Age_of_Genomics/links/5673df7708ae04d9b09be61a.pdf Marsupials in the age of genomics]. Annu Rev Genom Hum Genet 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 Djarthia 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.

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.{{Cite news |newspaper=The Guardian |language=en-GB |title=Meet Australia's first long-distance walker: a 250kg marsupial with 'heeled hands' |last=Lu |first=Donna |date=2023-05-30 |accessdate=2023-05-31 |url=https://www.theguardian.com/australia-news/2023/may/31/meet-australias-first-long-distance-walker-a-250kg-marsupial-with-heeled-hands}}

{{Portal|Mammals}}

• Marsupial lawn
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{{reflist|group=n}}

{{Reflist}}

{{refbegin|30EM}}

• {{cite book| veditors = Austin CR, Short RV |title=Reproduction in Mammals: Volume 4, Reproductive Fitness|url={{google books |plainurl=y |id=P5Al0X60UAIC&pg=PR4}}|date=21 March 1985|publisher=Cambridge University Press|isbn=978-0-521-31984-3|pages=4–}}
• {{cite book| vauthors = Bronson FH |title=Mammalian Reproductive Biology|url={{google books |plainurl=y |id=UmZmmW7DghMC}}|year=1989|publisher=University of Chicago Press|isbn=978-0-226-07559-4}}
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• {{cite book | vauthors = Hunsaker D | title = The Biology of Marsupials | location = New York | publisher = Academic Press | date = 1977 }}
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• {{cite encyclopedia| veditors = Knobill E, Neill JD |title= Encyclopedia of Reproduction |volume= 3 |location=New York |publisher=Academic Press |year=1998}}
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• {{cite book | vauthors = Nowak RM |title=Walker's Mammals of the World|url={{google books |plainurl=y |id=T37sFCl43E8C}}|date=7 April 1999|publisher=JHU Press|isbn=978-0-8018-5789-8}}
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{{Wikibooks|Dichotomous Key|Marsupialia}}

{{EB1911 poster|Marsupialia}}

• {{Cite web|title=Western Australian Mammal Species|url=http://members.iinet.net.au/~foconnor/mammals/mammals.htm|access-date=2021-06-28|website=members.iinet.net.au}}
• {{Cite web|title=Researchers Publish First Marsupial Genome Sequence|url=https://www.genome.gov/25521146/2007-release-researchers-publish-first-marsupial-genome-sequence|access-date=2021-06-28|website=Genome.gov|language=en}}
• [http://www.the-scientist.com/news/home/53187/ First marsupial genome released. Most differences between the opossom and placental mammals stem from non-coding DNA] {{Webarchive|url=https://web.archive.org/web/20110204033448/http://www.the-scientist.com/news/home/53187/ |date=4 February 2011 }}

{{Commons category|position=left|Marsupialia}}

{{Evolution of Marsupials}}

{{Metatheria|M.|state=autocollapse}}

{{mammals}}

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Category:Extant Paleocene first appearances