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Edits to Metatherians
https://en.wikipedia.org/wiki/Metatheria

Metatherians are one of three main classes of extant mammals (egg laying mammals like the platypus and the echidna), metatheria (or marsupials which includes the 3 American orders: didelphimorphia, paucituberculata and microbiotheres, and the four Australasian orders: notoryctemorphia, dasyuromorphia, peramelemorphia and diprotodontia and finally the eutherians (or placental mammals consisting of four orders) . Metatherians belong to subgroups of the northern tribosphenic mammal clade or Boreosphenida . They differ from all other mammals in certain morphologies like their dental formula which includes about five upper and four lower incisors, a canine, three premolars, and four molars . Other morphologies include skeletal and anterior dentition such as wrist and ankle apomorphies; all metatherians share derived pedal characters and calcaneal features. Another trait that all marsupials share is their life histories. Around the end of the Triassic period, the Therapsida, a member of a reptilian order, developed traits or characters that are diagnostic of the class Mammalia. This class gave rise to Multituberculata (herbivorous mammals), Triconodonta and Symmetrodonta (carnivorous and insectivorous mammals). However, these are not seen after the end of the Early Cretaceous and by the Late Cretaceous marsupials and placentals had evolved from a common eupantotherian ancestor molars. The Mammalia class probably saw its first eutherian in the early Cretaceous Jehol biota in China called the Acristatherium yanesis. This eutherian was determined the most basal based on a phylogenetic analysis that used a data matrix of many other species. Metatherians probably evolved to take advantage of open arboreal niches. Adaptive radiation of marsupials excluded competition with their terrestrial placental counterparts. This 2003 study presents a new fossil from the late Cretaceous Yixian formation in China called the Sinodelphys szalayi that gives enough morphological data to possibly be a basal metatherian in its didelphid-like morphology; it shares derived traits such as dental formation and wrist and ankle structures. The fossil is about 125 million years old, making it one of the oldest metatherian fossils found and gives claim that Asia was probably the center for diversification during the early Cretaceous. The researchers hypothesize that the divergence of metatheria from eutheria occurred in Asia no later than 125 million years ago followed by the evolution of deltatheroidian-like taxa in Asia and North America about 120-100 million years ago and then the Paleocene diversification of relatives to the crown marsupials in South America.

Final Draft Starts Here
Introduction: Metatherians, or marsupials, are one of three main classes of extant mammals (2); monotremata (egg laying mammals like the platypus and the echidna), metatheria (or marsupials which includes the 3 American orders: didelphimorphia, paucituberculata and microbiotheres, and the four Australasian orders: notoryctemorphia, dasyuromorphia, peramelemorphia and diprotodontia (3)(1) and finally the eutherians (or placental mammals consisting of four orders) (5). The evolution of extant marsupials has been debated for many years; who came first: the placentals, the monotremes, or the marsupials? How did marsupials evolve and when did they split from the other lineages of mammals? Where did marsupials originate and how did they disperse? And, finally, what is a marsupial; what makes them distinct from other mammals and are these distinctions an evolutionary advantage? This paper aims to answer all of these questions and examine how recent studies have formed hypotheses and phylogenies about the evolution of the metatherians.

What are Metatherians? Metatherians belong to subgroups of the northern tribosphenic mammal clade or Boreosphenida (2). They differ from all other mammals in certain morphologies like their dental formula which includes about five upper and four lower incisors, a canine, three premolars, and four molars (4). Other morphologies include skeletal and anterior dentition such as wrist and ankle apomorphies; all metatherians share derived pedal characters and calcaneal features. Another trait that all marsupials share is their life histories. Most marsupials breed seasonally with some who breed opportunistically (7). Females have a “yolk sac” in their womb that nourishes the fetus for about four to five weeks until it will crawl into the mothers pouch and latches on to the mothers nipple inside the pouch. The amount of time the baby stays inside its mothers pouch varies between all marsupials as does which direction the pouch faces. In some marsupials, like the Tasmanian devil, the pouch faces towards the rear which makes the trip from the birth canal to the pouch much shorter, but in others, like the kangaroos, the pouch faces upwards making the trip into the pouch longer for the baby but is necessary for the pouch to face upwards in an animal that stands upright most of the time in order for the baby to not fall out of the pouch. The trip from the birth canal to the pouch requires the fetus to have accelerated development of their forearms and facial structures but at the cost of the deceleration of their hind limb and brain development. Females also have two lateral vaginas which eventually separate uteri and both open externally through the same orifice and a third orifice is the median vagina which serves as the birth canal. Male marsupials have a bifurcated penis which has two ends to correspond with the females two vaginas. Both penis’ are only used to inseminate the female. Males do not have pouches. (8) Marsupials invest a relatively small amount of energy in early pregnancy which could be an advantage in variable environments however they usually have periods of conception to weaning about two times as long as eutherians of similar size. Comparatively low basal metabolic rates could contribute to the slow rates of development at low resource levels for marsupials which is therefore less energetically demanding (7).

How did Metatheria originate? Around the end of the Triassic period, the Therapsida, a member of a reptilian order, developed traits or characters that are diagnostic of the class Mammalia (4). This class gave rise to Multituberculata (herbivorous mammals), Triconodonta and Symmetrodonta (carnivorous and insectivorous mammals) (4). However, these are not seen after the end of the Early Cretaceous and by the Late Cretaceous marsupials and placentals had evolved from a common eupantotherian ancestor (4). The Mammalia class probably saw its first eutherian in the early Cretaceous Jehol biota in China called the Acristatherium yanesis (6). This eutherian was determined the most basal based on a phylogenetic analysis that used a data matrix of many other species (6). Metatherians probably evolved to take advantage of open arboreal niches. Adaptive radiation of marsupials excluded competition with their terrestrial placental counterparts (4). Much debate has surrounded the exact time of metatherian divergence and where they originated and how they dispersed.

When and where did Metatheria originate? A study done in 1968 postulates that Peratherium, the oldest marsupial found in the early Eocene of Europe at the time, had expanded its range from North America to Europe to Asia during the late Paleocene or early Eocene periods and could be the ancestor to all of the Australian marsupials. This study also postulates that marsupials might have reached the Cretaceous or early Cenozoic coasts of Asia by dispersal across the Bering Straits. The study denies that Metatheria could have originated from Asia due to the lack of fossil evidence present. (4). On the other hand, a 2003 study presents a new fossil from the late Cretaceous Yixian formation in China called the Sinodelphys szalayi that gives enough morphological data to possibly be a basal metatherian in its didelphid-like morphology; it shares derived traits such as dental formation and wrist and ankle structures. The fossil is about 125 million years old, making it one of the oldest metatherian fossils found and gives claim that Asia was probably the center for diversification during the early Cretaceous. The researchers hypothesize that the divergence of metatheria from eutheria occurred in Asia no later than 125 million years ago followed by the evolution of deltatheroidian-like taxa in Asia and North America about 120-100 million years ago and then the Paleocene diversification of relatives to the crown marsupials in South America. (2). Conversely, in 2008, researchers found a fossil from the early Eocene Tinganarra fauna in southeaster Queensland called Djarthia murgonensis. Researchers claimed this fossil was the oldest known crown group marsupial in the world based off of its dental, cranial, and post-cranial remains. Because of this evidence, Djarthia could be the ancestral morphotype of the Australian marsupials. The clade Australidelphia includes the South American microbiotheres, therefore if Djarthia is indeed the oldest known crown group marsupial, this could mean that the microbiotheres are a result of a back-dispersal from eastern Gondwana (which includes modern day Antarctica, South America, Africa, some of Asia, Australia, and Madagascar). Researchers postulate that this could mean Australidelphia is about 75.1 million years old. Yet, there is no mention of Sinodelphys in this article’s phylogenetic trees. (1). A 2010 study confirms Djarthia as the oldest member of Australidelphia but disagrees that the microbiotheres were a result of a back-dispersal. A group of researchers used archaic genomic retroposon insertions to place the much debated microbiotheres evolutionary relationship to Australidelphia once and for all. There are three American marsupial orders (Didelphimorhpia, Paucituberculata, and microbiotheres) of which the microbiotheres had been previously placed in the Australidelphia orders (notoryctemorphia, dasyuromorphia, peramelemorphia and diprotodontia). The DNA data gathered from the archaic genomic retroposon insertions and the morphological data from the skull and post-cranium both supports Didelphimorphia is the sister to all marsupials and should be placed first on the phylogenetic tree for metatheria. This means that there was only one effective migration from South America to Antarctica to Australia. Molecular estimates from this study have also placed the earliest divergences of marsupials in the late Cretaceous period, about 85 million years ago. (3).

Conclusion: Many studies have been conducted for decades in effort to pin down exactly when, where, and how marsupials diverged from placental mammals. Most of them disagree on finer points of the divergence of metatheria from eutheria, but it seems that a divergence date of the late Cretaceous has been agreed upon throughout most studies conducted. (1)(2)(3)(4)(6). Thanks to the advancement of technology, researches can dig further back into the past and deeper into organisms DNA make up giving us the power to discover more about the world around us. Using these resources, many researchers have found that Eutheria and Metatheria diverged in Asia with relatives in the Americas (2)(6). Early metatherians dispersed from South America into Antarctica then to Australia (2)(3) and then went through adaptive radiation and millions of years of evolution to become the species we have come to appreciate today (4).

Sources: (1)	Beck RMD, Godthelp H, Weisbecker V, Archer M, Hand SJ (2008) “Australia’s Oldest Marsupial Fossils and their Biogeographical Implications”. PLoS ONE 3(3): e1858. doi:10.1371/journal.pone.0001858 (2)	Zhe-Xi Luo et. al., (2003) “An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution”. Science 302 (3)	Nilsson MA, Churakov G, Sommer M, Tran NV, Zemann A, et al. (2010) “Tracking Marsupial Evolution Using Archaic Genomic Retroposon Insertions”. PLoS Biol 8(7): e1000436. doi:10.1371/journal.pbio.1000436 (4)	Clemens WA (1968) “Origin and Early Evolution of Marsupials”. Evolution 22(1) PP1-18 (5)	 Groves, C. P. (2005). Wilson, D. E.; Reeder, D. M, eds. Mammal Species of the World (3rd ed.). Baltimore: Johns Hopkins University Press. pp. 22–70. (6)	Hu, Yaoming Hu; Meng, Jin Meng Chuankui Li; Li, Yuanqing; Wang, Y. (2010). "New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China". Proceedings of the Royal Society B 277 (1679): 229–236. (7)	Morton, S. R.; Reecher, H. F.; Thompson, S. D.; Braithwaite, R. W. (1982) “Comments on the relative advantages of marsupial and eutherian reproduction”. The American Naturalist 120(1) pp. 128-134 (8)	Renfree, Marilyn; Hugh Tyndale-Biscoe (1987). Reproductive Physiology of Marsupials. Cambridge University Press

https://en.wikipedia.org/wiki/Marsupial

Citations Needed
Multiple statements need citations in this article suck as the Jelly bean statement in reference to the size of a newborn marsupial. Also the statement "except for the largest species of kangaroo; the eastern grey and red kangaroos" in reference to the male reproductive system. Secondly, Citations 9 and 16 are BBC news articles which does not serve as a reliable source for scientific material. Should insert a primary research article that has been peer reviewed.

Sentence Structure
Multiple sentences should be edited or rearranged so they are more understandable. For example: "Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes, as does most genetic and molecular evidence" Edited: "Most morphological, genetic, and molecular evidence comparing traits such as number and arrangement of teeth, structure of the reproductive, and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes."

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.

How did marsupials evolve? Beck RMD, Godthelp H, Weisbecker V, Archer M, Hand SJ (2008) Australia’s Oldest Marsupial Fossils and their Biogeographical Implications. PLoS ONE 3(3): e1858. doi:10.1371/journal.pone.0001858 ---Cranial and post-cranial marsupial fossils found in Australia from the early Eocene Tingamarra Local Fauna indicates that Djarthia murgonesis is a member of Australidelphia; a pan-Gondwanan clade consisting of all Australian marsupials and South American microbiotheres. This would make Djarthia the oldest known Australian marsupial by 30 million years and the most plesiomorphic known Australidelphia. Because of this, Djarthia may be the ancestrial morphotype of the Australian marsupial radiation. This also indicates that the South American microbiotheres may have come from eastern Gondwana instead of the original hypothesis that Australian marsupials evolved from ancestors from South America via Antarctica during the late Cretaceous.

Clemens WA (1968) Origin and Early Evolution of Marsupials. Evolution 22(1) PP1-18 ---This paper goes into the first fossils found of the Marsupialia as well as where they were found. It attempts to answer some of the questions that evolutionary scientists and zoologists had at the time such as where did marsupialia originate and when considering that fossil records of this class where unknown in Asia and Africa.(This will later be refuted in 2003 when an important fossil was found in Asia suggesting that radiation may have originated there) He attempts to answer these questions by describing the origin of marsupials, the fossil records, and the ancestors known at the time and the radiation of marsupials.

Nilsson MA, Churakov G, Sommer M, Tran NV, Zemann A, et al. (2010) Tracking Marsupial Evolution Using Archaic Genomic Retroposon Insertions. PLoS Biol 8(7): e1000436. doi:10.1371/journal.pbio.1000436 ---By using the retroposons insertion pattern in a paleobiogeographic context,these researchers are able to indicate that a single marsupial migration from South America to Australia is probably responsible for the spread of marsupials today. This research also reinforces the hypothesis that the order Microbiotheria belonged with the taxonomic group Australidelphia and even provides evidences for basal relationships between these orders; the four Australasian orders share one origin with Microbiothera which establishes a clear divergence.

Lilligraven JA, (1969) LATEST CRETACEOUS MAMMALS OF UPPER PART OF EDMONTON FORMATION OF ALBERTA, CANADA, AND REVIEW OF MARSUPIAL-PLACENTAL DICHOTOMY IN MAMMALIAN EVOLUTION. The University of Kansas Paleontological Contributions (50). pp.87-108 ---There are three "groups" of Mammilia; Protothera (monotremes), Metathera (marsupials), and Eutheria (placentals). Lilligraven uses this section of his paper to enforce evidence that marsupial anatomy evolved as a specialization instead of prototypal of placentals. He uses comparative anatomy and physiology, fossil records, and explains the significance of the marsupial pouch as evidence to his claim.

Zhe-Xi Luo et. al., (2003) An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution. Science 302 ---This artical introduces a new boreosphenidan mammal (dating back 125 million years ago) from the lower cretaceous Yixian Formation of China. Derived features of this mammal suggests that it is closely related to metatherians. The fossil has foot structures that is only from tree climbing and tree living mammals as well as many other marsupial like apomorphies in the skeleton and anterior dentition. Because of this find, it is likely that Asia may have been the "center for diversification of the earliest metatherians during the early cretaceous".