Marsupials are an extant clade from the Therians of which comprises all extant mammals excluding monotremes. As an infraclass of the mammals, they are predominantly characterized by giving birth to altricial young which often reside in a pouch for a certain amount of time after birth, like other mammals in Metatheria. Most marsupials mainly inhabit the Southern Hemisphere; commonly the well known marsupials such as kangaroos, koalas, possums, wombats, and the Tasmanian Devil. Approximately 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the New World; Primarily South America, but with 13 in Central America and 1 in North America (Cracraft, 1974).
The origin of marsupials is unclear to this date however fossil evidence suggests little inheritance of morphological change has occurred since the earliest mammals (Clemens, 1968). Due to little knowledge of marsupial history and an unrepresentative fossil record, interrelations between marsupials and placentals were not fully established. Jurassic strata however uncovered evidence suggesting both were descendants of Eupantotheria. From these few segments of marsupial history, zoologists pieced together that the American Opossum (Didelphis) was the closest living relic to the earliest marsupial, citing it as a 'living fossil'. The oldest marsupial fossil evidence obtained to date is of the Synodelphys szalayi which lived in China around 125 million years ago (Benton, 1997).
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The remaining three extant divisions of mammals have created great debate among the scientific community. Marsupials, placental mammals, and monotremes all share common mammalian characteristics such as high internal temperatures, vertically orientated limbs, a diaphragm separating the thorax and abdomen, a four chambered heart and erythrocytes without nuclei for example. Their interconnecting relatedness was difficult to find however. Analysing fossils can be problematic as fossilisation only portrays the hard tissues such as bone. Soft tissue characteristics such as the pouch and reproductive system are lost during the transformation to fossil, therefore morphological evidence and comparison of traits such as arrangement of denture and number of teeth was observed to allow scientists to conclude marsupials and placental mammals were more closely related than either to monotremes (Zhe-Xi et al. 2011) (Gill, 1872) (Huxley, 1880). Furthermore genetic and molecular data suggests placental mammals and marsupials can form a separate clade of their own, a subclass known as Theria (Van Rheede et al. 2006).
The divergence of marsupials and placentals encompasses adaptive traits namely scansorial or arboreal adaptations to allow Therians to exploit arboreal localities. Approximately 50 million years ago, marsupials reached Australia after populations spread aided by continental drift (Cracraft, 1974). From the earliest Metatherians predicted to have inhabited China first, it is suggested they moved west into what is North America today, after Pangaea split into Laurasia (the northern super continent) and Gondwana (the southern supercontinent). Following this, migrations to South America occurred. Marsupials that were left behind in Laurasia are thought to have been wiped out due to competition, most likely from placental mammals (Zhe-Xi, 2003). From South America it is widely accepted that they reached the Australian
Fig. 1 The divergence of marsupials to placental mammals occurred approximately 100 million years ago. These species are proposed to be essentially identical to their counterpart. Picture taken from http://www.nwcreation.net/marsupials.html
continent via Antarctica and therefore the Australian marsupials are descendents of the ones found in the Americas. Continental drift evidence implies Australia and South America were still in the southern hemisphere and part of the super continent Gondwana; joined by the land mass Antarctica as we know it today. Using both pieces of evidence, scientists have constructed a theory suggesting marsupials crossed over from South America to Australia approximately 40 million years ago (Xhe-Xi, 2003).
Two main radiations of marsupials have occurred since the divergence of Eutherians from Metatherians and placental mammals approximately 130-140 million years ago (Renfree and Pask, 2011). The first of the two was the American radiation giving rise to American carnivores and opossums, the second was the Australian radiation giving rise to bandicoots, kangaroos, wombats, and Australian carnivores.
Didelphis is the name of one marsupial species meaning 'two wombs'. This is one of the defining differences between the two mammal groups placentals and a marsupials. When South America rejoined with North America approximately 3 million years ago, Didelphis is predicted to have migrated up through the continents via the newly formed Ismuth of Panama, where placentals had completely replaced the original northern hemisphere marsupials.
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Fig. 2. Marsupial female reproductive organ illustrating dual vaginal tracts and pseudovaginal canal. Picture taken from http://people.eku.edu/ritchisong/342notes10.htmlMarsupial and Placental Divergence
Marsupials and placental mammals are said to have diverged from a common ancestor in Asia, around 130 million years ago according to fossil and genetic evidence. Many scientists believe the reproductive organ differences within placentals and marsupials holds the potential key to explaining their divergence, as such they have been widely studied (Low, 1978). The marsupial Tammar Wallaby is the most frequent marsupial of choice to study due to its clear developmental pattern, ability to breed well in a captive environment, and its comfort during handling (Renfree and Pask, 2011). Placentals and marsupials supposedly are descendents of the Eutherians with allantoic placenta, more specifically, marsupials gained their viviparous reproduction through full viviparity displayed in past Eutherians. This line of theory was studied and it is observed that reproductive tracts develop simultaneous to the kidney ducts and it is this relationship during early development that separates marsupials from true mammals (Sharman, 1970). The developmental movement of ureters is the overall difference between marsupials and eutherians, with a discernibly large difference in ureter origin within the adult but only small within the embryo. Due to said ureter movements, marsupials obtain two lateral vaginas, which lead to separate uteri, due to the vas deferens having to loop over the ureter opposed to the vas deferens being outside the ureter in mammals consequently allowing fusion of Mullerian duct derivatives- the uterus and vagina. If this were to occur in marsupials the ureters would be impeded, hence fusion does not occur. Both vaginal tracts are utilised during copulation due to the male marsupials bifid penis (Woolley et al. 2007). The pseudovaginal canal is a midline passage in which births occurs. In ancestral marsupials, birthing is thought to have occurred in the lateral vaginal canals. This route is the homolog of the eutherian midline vagina (Sharman, 1970).
The birth canal (pseudovaginal canal) as a rule closes up following parturition (Tyndale-Biscoe, 1968). This however is not the case for macropods (e.g. kangaroos).
The split womb makes development of large foetuses difficult and may furthermore explain the short gestation period and immaturity at which the young are delivered (after approximately 4-5 weeks) (Renfree, 2009).
Placental mammals possess a more complex mechanism allowing longer residence of the foetus within the womb. A longer lasting placenta acts as a transporter for essential materials between the mother and young. The placenta offers a source of nutrition as well as relieving the foetus of waste products. Marsupials also have a yolky placenta in early uterine stage however it is far less developed and shorter lived. Some marsupials such as bandicoots have a more complex and developed placenta however and are born with their umbilical cords still attached.
Since the divergence of these two mammal groups from their common ancestor, further characteristics familiar to mammals have adapted individually. The dentition of marsupials and placentals is notably different; marsupials have four molars in contrast to only three in placentals. Jaw articulation and bone structure also vary such as the epipubic bone. Additional differences are the lack of corpus collosum within marsupials along with their lower basal body temperature, furthermore the absence of nipples on male marsupials REF.
Fossils found in the lower cretaceous Yixian Formation of China have presented a closer relationship to metatherians than to eutherians. Dating back 125 million years, it displays many foot structures akin to climbing and tree-living observed in extant mammals. This evidence suggests early crown therians exploited diverse niches and that diversification of the earliest metatherians during the early cretaceous occurred in Asia (Zhe- Xi et al, 2003). Due to extensive tectonic plate movement during the mid tertiary period, Australia moved away from Antarctica isolating many species. It is thought one species made the migration onto the Australian continent before it split apart. This movement opened up a vast array of different environments and ecological niches needed to be filled. As the Australian plate drifted north, the cooling drying climate exacerbated the drive for new species to immigrate (Clemens, 1968). Tectonic plate movement incurred the same fate for South America and now today it has over 70 closely related species related to the Australian marsupials (Cox, 2000). Exceptional similarities have been identified between these related species even though they are separated over a large temporal and geographic scale; the Dromociops gliroides a species from South America and Australasian marsupials have sperm which swim individually where as the sperm of all American marsupials are conjoined at the head when they swim. This is backed by genomics suggesting it is more closely related to Australia.
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Further differences between placentals and marsupials is the variation in offspring development. Placentals give birth to more developed offspring due to increased gestation periods (Selwood, 1980). The marsupials give birth to altricial young reliant on the mothers pouch for a long period of time before being able to detach from the mum (How, 1976).
Marsupial and Placental Convergence
Both placental and marsupial mammals display clues of convergent evolution over the millenia. Due to occupancy of similar niches worldwide, morphological traits aiding adaptation to their natural surroundings i.e. climate, food, and locomotion have occurred, resulting in convergence of both mammalian subclasses. Both lineages evolved independently following separation of a common ancestor approximately 100 million years ago, however despite the vast difference in geographical location between North American placentals and Australian marsupials, similar body plan, feeding habits, and locomotion methods have emerged (McNab, 1978).
Marsupials present themselves as a formidable infraclass of mammals existing on the planet for hundreds of millions of years. Their original diversification in China lead to subsequent successful migrations around the globe culminating in populations on nearly every continent at present. Their parallel evolution with placental mammals has evoked many similarities and competition between the therians and evidence suggests this has persisted over the years since the splitting of Pangaea and separation from their common ancestor. Morphological adaptations as well as instinctual habits such as feeding and locomotion methods have shown to converge over the more recent years due to similar niche fulfilments regardless of the great distance they are often separated by.