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Description

When one talk about the marsupials, the most know about them are related to all species that have evolved in isolation from the rest of the world on the Australian continent, which show great diversity both totally unique species that do not exist elsewhere in the planet as different examples of convergences by niches or behaviors that they have.

However, these are not just limited to the small continent, as a separate group in America has been largely achieved great adaptability to different environments and has been able to deal with placental mammals, these are the opossums (Didelphidae), the strange archaic-looking marsupial mammals, which with an unspecialized biology, flexible diet and reproductive habits, have guaranteed them to exist for a long time since the late Cretaceous. With such features and evolutionary history, gives the chance and turn a hope that in long-term marsupials could last longer and even survive the future potential disasters.

Although that humanity showed to be a serious threat to several species belonging to this group for several hundred years, these managed to endure until the end of its era. So, what would be its next step on the far future?

The resurgence

The history of this group around the future starts a little slow, but still with some important changes. A few million years after the vanishing of humans, two different varieties of this group were formed by the disappearance of the isthmus the complete separation of America.

During the rest of the Cenozoic, the South American didelphids took temporary roles subsequent to the mass extinction of the quaternary and all the Agresti period, developing average lengths and becoming terrestrial and arboreal carnivorous, however, were overcome by more specialized varieties of placentary predators and crocodilians. While in the north, some species became predators or arboreal frugivorous, with the formation of Beringia between Asia and America led to the expansion of this group to the Asian territories, stopping only for competition among other mammals. With the destructive end of the Cenozoic and the beginning of Therozoic marked a huge change, however, with unfavorable results; most of the north species died, and although they survived the catastrophe by some species in the south, they had to stay for another million years in the shadow of the new dominant fauna conformed by the crocodilian descendants, but just for a while more. After the great changes around the middle Therozoic, the archosaurs and others dominant groups were reduced greatly, and thus it caused an ecological void, which with the time, were filled by a new kind of animals, and in this case, the marsupials find a chance.

The next order is not supposed to be the only living marsupials, but one of the most prominent groups for the period.

Order Magallanotheria
The magallanotherids were the one of the rising groups of mammals, which succeeded to take most of the niches around South America and even the Antarctica after de extinction of the great mammals. Something like Australidelphia, they managed to evolve around the end of the Agerogene period, and take most of the great herbivores and carnivore role around the Metamonus. Are characterized by being digitigrade, most of them quadrupeds and few semi-bipedal species. Its name comes from where they appeared, a closed region between South America (Patagonia) and the Antarctic which is called "The Magallanes” before the territory rises and causes the formation of the mountain range with the same name.

-Family Crassypedae
These are a prominent of great herbivorous that spread during the Metamonus around Antarctica, being one of the biggest terrestrial marsupials since the Cenozoic. Its sizes varied from 30 cm to 4 m long, most of them are terrestrial or semiaquatic browsers that inhabit forests and swamps, feeding on bushes, low vegetation and aquatic plants. It has a strange variety of bony structures like horns that emerge at the top of the skull; these are used as a weapon, most to confront between them or to deal with the predators around. Most of them have a very simplified dentition, with only the molars; however, these ones continually grow and are replaced.

Dihyoceras macrogastrus
The Dihyoceras is a great cow-like crassyped that inhabits the Antarctic woodlands, has a length of 4 meters long and weighing approximately 200 kg. Its digestive system has evolved so that they can digest different types of plant material from tender and nutritious plants to fibrous, the stomach are divided into two special cameras which allow to ferment and take the necessary nutrients.

-Family Venarodonidae
This is a clade conformed by carnivora-like forms, extended for all Antarctica and part of South America. Most of them have 2 pairs of incisors and two pairs of sharp canines, the molars in many species are moldable and change its shape depending of the diet. Most of them are terrestrials, covering different niches, most prominents in main meat-eaters.

Melecephalus vulgaris
This badger-like species is a little omnivorous venarodonid which inhabit for most part of South America regions, being a kind of small robustly being, with short legs but powerful enough to dig and climb trees with a semi-cylindrical body, it has a very muscular and little elongated head with a bite force quite high.

Macrocynops regines
This heavy predator of about 2 meters long gregarious species, inhabitant of Antarctic savannas, It is a primarily diurnal predator, able to blend in with their environment to stay out of sight of their prey to hunt, he is capable of running at up to 40 k/ph. It uses both the incisors and canines to tear the flesh from their prey, and their molars to cut it.

Aepyhyomimus gracilis
A tall omnivore marsupial of about 1.6 meters tall is a gregarious and cursorial species inhabitant of the territories compounds by great and tall grass-like plants. It feed mainly on small mammals, birds, sauropsids, archosaurs, tubers, and fallen fruit from nearby trees. The main feature of this species are the hypertrophied limbs, which give it a better view on the upper of the tall plants, an advantage to achieve spot any movement around and avoid predators.

-Family Allocaudophorae
Maybe the most prominent and successful form of the Magallanotheria, is a clade of hoofed animals evolved during the Metamonus (266-349 m.y). Most of the members are exclusively herbivores, with some insectivorous and scavenger species, they are extended around all South America and Antarctic region during part of the Metamonus and were able to expand far of these areas to most of the continents around the Retrogeian period (349-470 m.y.), surviving until the early Bathovician (527 m.y.), making them one of the last living groups of the Magallanotheria after the therozoic. As the ungulates, many of the members of this groups possess horns, some conformed by bones and other by keratin.

Flavucapricoida laticaudus
This species belongs to a very bizarre kind of herbivores, because unlike any other mammal of the same shape that has existed before, because these have developed a huge flexible tail which they use as an extra limb to reach the branches of trees or cutting plants floor. It has a length of about 2 meters (including the tail), 1.7 meters tall and with a weight of about 60 kg. Its horns are smooth texture, hollow but structured to withstand heavy blows; these animals inhabit rugged areas, mainly cliffs and mountainous places.

Trispirocornus gnoumimma
Are a more generic species of Allocaudids which have evolved convergently with the placentary ruminant bovids, are one of the most common varieties of herbivorous mammals, from the southwestern hemisphere of the supercontinent hysterea. They move in large hordes of hundreds of individuals across hundreds of kilometers in search of places to graze and drink. They have a fairly heterogeneous dentition, with fairly short upper incisors, quite small and completely atrophied canines and molars reinforced to digest hard plants.

Fall and start up again
During the late part of the Therozoic and the beginning of the Phinizoic, the marsupials stood as one of several dominant groups on the planet, however, the earth began to gather all its land masses again and to unite them in one single supercontinent, hysterea. This tectonic event caused several havoc to the terrestrial and marine biosphere, destroying many food chains, homogenizing groups and weakening the habitable conditions, this leading to a mass extinction event, wiping out most of the fauna, including great part of the marsupials groups in the moment. Although that some lineages like the Magallanotheria managed to survive, these at the time gradually declined, until they become extinct millions of years after, and with the, the remaining marsupials, but not without a successor.

Class Opsiapheria
The Opsiapheians (Late separation) turn out to be a rather strange new group of amniotes that evolve from the metatherian around 350 to 400 million years in the future as a branch of semiaquatic marsupials, and after evolve into a monophyletic clade. This clade differed from mammals for the development of different methods of reproduction apart of the living birth, metabolism as active animals, morphology for the new dozens of shapes and thermoregulation for became into ectotherms. Unlike marsupials or common mammals, are not reproduced in a viviparous way, many primitive members of this lineage are reproduced in a oviparous way, however, these oviparous are quite unique, since they do not develop a shell and those eggs are not kept on land. The eggs are covered by a transparent but impermeable membrane, which allows gas exchange between the embryo and water when they are born, these amounts on water and begin to breathe air. But how can an amniote has developed the ability to stay in their larva stage underwater?
The transitional forms had to deal with huge and long periods of dormancy under water to survive in the harsh environments, these clades began to develop alternatives take the oxygen from the water as through the cloaca, the dermal “breathing”, and development of a specialized area in the nasal cavity. The last two ones were developed, and evolve into a "pseudo-gill" which in a rudimentary way, was able to get the oxygen in the water. This proved very useful not for adults, but for the young that depend on the development in the bag of mothers, helped to overcome the problem of drowning. The oviparous species appeared after with the time, being more common and expanding for 200 million years.

Order Metanascia
The most basal family of this clade, they showed the strange transition between the unchanged marsupials to the derived and strange forms, most of them have a cylindrical body with four limbs that in advanced species have become developed fins and a long flat tail. They have a fairly simple dentition, made up of dozens of long, sharp teeth.

Family homeolepidae
These are characterized by having long bodies and leathered skin covered by rugose integument and fur in some basal species. All are carnivores, being most of them piscivores and few species being more generalists on eating others beings as crustaceans, insects and mollusks. Most of the families in this group have two form of regulation of temperature, from endotherm poikilothermic (body temperature of their bodies fluctuates in conjunction with their environment) to ectothermic, the reasons are for their ancestral forms that had a metabolism slow suitable to withstand periods of great shortages of food, coupled with the stable climate that prevailed hysterea for millions of years and led to the expansion of these forms.
Memoreplatypi vulgaris
Maybe a little reminder of the extinct monotremes, this species is formed by benthonic eaters that have a jaw covered by hardened structure similar to a beak, devoid of teeth in the front of the mouth with some quite small molars in the back, which is bordered by small electroreceptors to detect prey under the mud and sand. What makes it stand out among others of their relative is its skin covered by a thick coat, part of their ancestral mammalian roots.
Gnathidirus suchocephala
A very specialized ambush predator, it lives around the rivers of the tropical zones, is completely aquatic, only able to peek out of the water to capture large prey. It has a relatively short body, with a long and robust head, capable of exerting a bite force strong enough to break bones. This species live in large groups of dozens of individuals, agglomerates near the banks of rivers, to take advantage of this opportunity to get together a big prey.
Brevisarchus thalassus
This is one of the few species of this lineage that has managed to colonize salt water areas, living near the coast, in shallow water, about 20 to 40 meters underwater. It has a length of 1.8 meters, being the males slightly lower, feed mainly on pelagic animals around, and being agile, quite efficient swimmers. Something characteristic of this group is that at the time of reproduction, these migrate from the sea into rivers at a point of the rainy season to spawn in fresh water. Males and females mate near the coast, then only females traveling on tens of kilometers to find suitable places to nest and lay their eggs.

Family Stringeriscelidae
These Metanascia are a strange group of long hind-limbed small sizes beings, which evolved around the Klastogeian and be extinct around the middle of the Bathovician (590-600 m.y.), being a very ephemeral clade, but with a significant role for the formation of a new lineage derived from them. The most prominent feature in this clade is the appearance, has small front limbs but huge and long hind legs, which offer great propulsion. Unlike other Metanascia, they have begun to exhibit a noticeable change in their way of reproduction, became more ovoviviparous, keep their offspring inside until these born.
Gavialutroida polius
It is a medium-sized carnivore with 1.8 meters long, has small forelegs, without claws and with only one finger, which only have a function of grip when are copulating. Otherwise, this has hypertrophied hind legs that help a lot with underwater propulsion together with the long tail. This is a quite intelligent species, live in small groups of five to nine individuals. It feeds mainly on fish, arthropods and molluscs.
Miceolutroida natodermus
This little Stringeriscelid is an omnivorous species that has developed a type of mutuality with different varieties of large aquatic animals, cleaning the skin and mouths of these with the benefit of eating parasites and dead skin. Around the mouth has developed large lips with rough surfaces that allows to scrape the skin of their "customers", the frontal limbs are below of the chest, barely visible, and the hind limbs have evolve into rounded shaped fins, that have suction pads to properly hold the animals when are cleaning them.

Order Monadipoda
This is a derived branch of the Metanascia which existed around the end of the Klastogeian, probably from the Stringeriscelids, these have developed a more divergent morphology, reducing their forelimbs to make them disappear and keeping the hind limbs. Most of the members of this group are fully aquatic, except for some amphibian types that returned to land. This is a fairly exclusive group of freshwater environments around the globe which had expanded million years before the supercontinent break up, with several species that have managed to colonize brackish environments or of shallow water areas. Unlike the primitives’ Metanascia, they have developed a more specialized form of reproduction, not depending on spawn eggs and turning back to viviparism. However, these species have developed a fairly basic larval form being a four limbed and so similar to its mammalian ancestral forms, this also has more specialized "pseudo-gill" to stay underwater so they can develop through a metamorphosis, in which they atrophied and lose their forelimbs and when those are grow as adults, are able to breathe through the of lungs.

Family Gaiopodae
These forms are to be oddities of this water order since one way or another have managed to colonize the land again, turning them in a kind of secondarily terrestrial group of animals. These species inhabit only sub-continents or islands where terrestrial competition is almost nonexistent. Despite returning to land, its anamniotes condition beings forced to return to the water to give birth and maintain its young forms until they grow, except for some more specialized species that managed to deal with this.
Phocinosoma titanognata
This pinniped-like gaiopode is one of the biggest amphibian species of this clade, with 3.5 meters long and with a weight of about 300 kg. This massive animal has a triangular head with a huge “V” shaped mouth, a strong neck which actually is comprised of the ribcage, without front legs and long hind legs as a fin followed by a huge paddle-shaped tail. These are capable of producing severe and quite loud snorts both to communicate and to warn.
Penguipes panpottamus
This is probably the strangest forms of future marsupial mammals, with a head of a hippopotamus, bipedal, semi-aquatic with a long flat tail, and capable to give birth underwater, is a rare form of gaiopode that inhabits the rivers around a subcontinent near to the North Pole. These, unlike the hippos, are not herbivores, but piscivores. These have evolved a very interesting way of breeding, because instead of back into the water, they dig a nest at a considerable distance from the coast, which filled with water, and there, the female deposits the hatchlings and these remain safe until they develop.

Family Hallucigotherae
No doubt this is one of the most impressive extremes that this group can reach, most members of this clade are slow herbivorous inhabitants of tropical areas of the planet, unlike other exposed groups, these have atrophied or modified the huge paddle-shaped tail, and modifying its body to adapt well to any of the marine environments where they evolve. The most interesting of this group is to preserve the "pseudo-gill" of the larval form, to stay under water for long periods of time, but in a moment these need to return to the surface for air.

Figoequu leptocaudus
One of several inhabitants of a strange ecosystem, consisting of huge and large plants like mangroves, which have huge and deep roots that reach depths of between 40 to 60 meters, forming strange root-forests atolls. This strange sea horse like creature is one of several specialized herbivores that feed on algae and small plants that grow on the roots use its long jaw full of small teeth on the front and little molars on the back. It's a pretty slow swimmer, remaining always clung to the roots through its prehensile tail and using the round fins to propel and stabilize. They are a very social species, congregating in dozens of individuals around of a single tree. Males have a thick brow, which is colored a shade of orange. This use it in rituals challenge where both males intertwine with their tails, and they start beating his head until one of them loses.
Segnocrassus specesirenus
This is a species that lives in more open areas, being similar to a manatee with a length of about 1.9 meters, is a grazing herbivore that inhabits near the shallow water plains covered by algae and a variety of sea grass like plants that grows on the sands. They have a stocky body, barrel-shaped, which drive through two long fins. Like the Figoequu relative, this has a row of teeth in the front of the mouth, however, this is followed by a set of 4 specialized molars, which allows it to grind plants and facilitate the digestion.

Family Chimeropterae
This lineage is better designed for fast and efficient swimming, having a torpedo-shaped body many members of this group are found in freshwater, with a few species adapted to salt water. Similar to the way odontocetes, several species developed a melon-like structure that allows them to use echolocation, in some species, use this as a weapon.
Delphifronts basilus
This is a social species of chimeropterid, it has a size of about 5 meters long, inhabits the great rive systems of the continent Ereboreus. It is one of the largest river predators on the planet, able to hunt in groups of five a different variety of prey, from large land animals that pass by their areas to shoals of fish that lurk between the murky waters.
Rhyniprion piloserrata
This saltwater species is a medium size carnivore that possesses a long, thin structure like a saw is covered with small structures like teeth composed of keratin. These ones possess a modified melon to their river relatives because it extends not only from the front of the head, but also along the saw. When this is producing clicks, the melon extended in the saw drives those below the sand to locate any potential prey. The preys are not limited only in creatures under the sand; it can also hunt animals like fish or nektonic invertebrates. To do this, it moves the saw violently near the prey, inflicting an enormous damage. When these born, the larvae lack this structure, and usually grow in a period of about 3 months. This species is quite solitary, only gathering in one point to procreate.

Fatal fate
Throughout the biological history of the earth, not always all organisms will be able to have the ability to face all the changes that the world can get to throw, and at some point, they must perish, leaving nothing to continue their lineage and just disappear for others to take their place, and unfortunately, the far descendants of the opossums will not last against the great mother of all the extinctions at the end of the Phanerozoic, 690 million years in the future.
There is not much to lament about the end that they suffered, in a memorable way, they accomplished what few groups could make in the last 600 million years, thrive and spread across the world and time.

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I must be frank about this last group I added, there is a high probability that such beings can not exist in any known future and are implausible in a way, however, I must emphasize that this is the distant future hundreds of millions of years, so the possibilities for new and aberrant features in certain groups to arise are high. Not with this I will assume that this is going to be completely plausible or probable, but just possible.

Description

When one talk about the marsupials, the most know about them are related to all species that have evolved in isolation from the rest of the world on the Australian continent, which show great diversity both totally unique species that do not exist elsewhere in the planet as different examples of convergences by niches or behaviors that they have.

However, these are not just limited to the small continent, as a separate group in America has been largely achieved great adaptability to different environments and has been able to deal with placental mammals, these are the opossums (Didelphidae), the strange archaic-looking marsupial mammals, which with an unspecialized biology, flexible diet and reproductive habits, have guaranteed them to exist for a long time since the late Cretaceous. With such features and evolutionary history, gives the chance and turn a hope that in long-term marsupials could last longer and even survive the future potential disasters.

Although that humanity showed to be a serious threat to several species belonging to this group for several hundred years, these managed to endure until the end of its era. So, what would be its next step on the far future?

The resurgence

The history of this group around the future starts a little slow, but still with some important changes. A few million years after the vanishing of humans, two different varieties of this group were formed by the disappearance of the isthmus the complete separation of America.

During the rest of the Cenozoic, the South American didelphids took temporary roles subsequent to the mass extinction of the quaternary and all the Agresti period, developing average lengths and becoming terrestrial and arboreal carnivorous, however, were overcome by more specialized varieties of placentary predators and crocodilians. While in the north, some species became predators or arboreal frugivorous, with the formation of Beringia between Asia and America led to the expansion of this group to the Asian territories, stopping only for competition among other mammals. With the destructive end of the Cenozoic and the beginning of Therozoic marked a huge change, however, with unfavorable results; most of the north species died, and although they survived the catastrophe by some species in the south, they had to stay for another million years in the shadow of the new dominant fauna conformed by the crocodilian descendants, but just for a while more. After the great changes around the middle Therozoic, the archosaurs and others dominant groups were reduced greatly, and thus it caused an ecological void, which with the time, were filled by a new kind of animals, and in this case, the marsupials find a chance.

The next order is not supposed to be the only living marsupials, but one of the most prominent groups for the period.

Order Magallanotheria
The magallanotherids were the one of the rising groups of mammals, which succeeded to take most of the niches around South America and even the Antarctica after de extinction of the great mammals. Something like Australidelphia, they managed to evolve around the end of the Agerogene period, and take most of the great herbivores and carnivore role around the Metamonus. Are characterized by being digitigrade, most of them quadrupeds and few semi-bipedal species. Its name comes from where they appeared, a closed region between South America (Patagonia) and the Antarctic which is called "The Magallanes” before the territory rises and causes the formation of the mountain range with the same name.

-Family Crassypedae
These are a prominent of great herbivorous that spread during the Metamonus around Antarctica, being one of the biggest terrestrial marsupials since the Cenozoic. Its sizes varied from 30 cm to 4 m long, most of them are terrestrial or semiaquatic browsers that inhabit forests and swamps, feeding on bushes, low vegetation and aquatic plants. It has a strange variety of bony structures like horns that emerge at the top of the skull; these are used as a weapon, most to confront between them or to deal with the predators around. Most of them have a very simplified dentition, with only the molars; however, these ones continually grow and are replaced.

Dihyoceras macrogastrus
The Dihyoceras is a great cow-like crassyped that inhabits the Antarctic woodlands, has a length of 4 meters long and weighing approximately 200 kg. Its digestive system has evolved so that they can digest different types of plant material from tender and nutritious plants to fibrous, the stomach are divided into two special cameras which allow to ferment and take the necessary nutrients.

-Family Venarodonidae
This is a clade conformed by carnivora-like forms, extended for all Antarctica and part of South America. Most of them have 2 pairs of incisors and two pairs of sharp canines, the molars in many species are moldable and change its shape depending of the diet. Most of them are terrestrials, covering different niches, most prominents in main meat-eaters.

Melecephalus vulgaris
This badger-like species is a little omnivorous venarodonid which inhabit for most part of South America regions, being a kind of small robustly being, with short legs but powerful enough to dig and climb trees with a semi-cylindrical body, it has a very muscular and little elongated head with a bite force quite high.

Macrocynops regines
This heavy predator of about 2 meters long gregarious species, inhabitant of Antarctic savannas, It is a primarily diurnal predator, able to blend in with their environment to stay out of sight of their prey to hunt, he is capable of running at up to 40 k/ph. It uses both the incisors and canines to tear the flesh from their prey, and their molars to cut it.

Aepyhyomimus gracilis
A tall omnivore marsupial of about 1.6 meters tall is a gregarious and cursorial species inhabitant of the territories compounds by great and tall grass-like plants. It feed mainly on small mammals, birds, sauropsids, archosaurs, tubers, and fallen fruit from nearby trees. The main feature of this species are the hypertrophied limbs, which give it a better view on the upper of the tall plants, an advantage to achieve spot any movement around and avoid predators.

-Family Allocaudophorae
Maybe the most prominent and successful form of the Magallanotheria, is a clade of hoofed animals evolved during the Metamonus (266-349 m.y). Most of the members are exclusively herbivores, with some insectivorous and scavenger species, they are extended around all South America and Antarctic region during part of the Metamonus and were able to expand far of these areas to most of the continents around the Retrogeian period (349-470 m.y.), surviving until the early Bathovician (527 m.y.), making them one of the last living groups of the Magallanotheria after the therozoic. As the ungulates, many of the members of this groups possess horns, some conformed by bones and other by keratin.

Flavucapricoida laticaudus
This species belongs to a very bizarre kind of herbivores, because unlike any other mammal of the same shape that has existed before, because these have developed a huge flexible tail which they use as an extra limb to reach the branches of trees or cutting plants floor. It has a length of about 2 meters (including the tail), 1.7 meters tall and with a weight of about 60 kg. Its horns are smooth texture, hollow but structured to withstand heavy blows; these animals inhabit rugged areas, mainly cliffs and mountainous places.

Trispirocornus gnoumimma
Are a more generic species of Allocaudids which have evolved convergently with the placentary ruminant bovids, are one of the most common varieties of herbivorous mammals, from the southwestern hemisphere of the supercontinent hysterea. They move in large hordes of hundreds of individuals across hundreds of kilometers in search of places to graze and drink. They have a fairly heterogeneous dentition, with fairly short upper incisors, quite small and completely atrophied canines and molars reinforced to digest hard plants.

Fall and start up again
During the late part of the Therozoic and the beginning of the Phinizoic, the marsupials stood as one of several dominant groups on the planet, however, the earth began to gather all its land masses again and to unite them in one single supercontinent, hysterea. This tectonic event caused several havoc to the terrestrial and marine biosphere, destroying many food chains, homogenizing groups and weakening the habitable conditions, this leading to a mass extinction event, wiping out most of the fauna, including great part of the marsupials groups in the moment. Although that some lineages like the Magallanotheria managed to survive, these at the time gradually declined, until they become extinct millions of years after, and with the, the remaining marsupials, but not without a successor.

Class Opsiapheria
The Opsiapheians (Late separation) turn out to be a rather strange new group of amniotes that evolve from the metatherian around 350 to 400 million years in the future as a branch of semiaquatic marsupials, and after evolve into a monophyletic clade. This clade differed from mammals for the development of different methods of reproduction apart of the living birth, metabolism as active animals, morphology for the new dozens of shapes and thermoregulation for became into ectotherms. Unlike marsupials or common mammals, are not reproduced in a viviparous way, many primitive members of this lineage are reproduced in a oviparous way, however, these oviparous are quite unique, since they do not develop a shell and those eggs are not kept on land. The eggs are covered by a transparent but impermeable membrane, which allows gas exchange between the embryo and water when they are born, these amounts on water and begin to breathe air. But how can an amniote has developed the ability to stay in their larva stage underwater?
The transitional forms had to deal with huge and long periods of dormancy under water to survive in the harsh environments, these clades began to develop alternatives take the oxygen from the water as through the cloaca, the dermal “breathing”, and development of a specialized area in the nasal cavity. The last two ones were developed, and evolve into a "pseudo-gill" which in a rudimentary way, was able to get the oxygen in the water. This proved very useful not for adults, but for the young that depend on the development in the bag of mothers, helped to overcome the problem of drowning. The oviparous species appeared after with the time, being more common and expanding for 200 million years.

Order Metanascia
The most basal family of this clade, they showed the strange transition between the unchanged marsupials to the derived and strange forms, most of them have a cylindrical body with four limbs that in advanced species have become developed fins and a long flat tail. They have a fairly simple dentition, made up of dozens of long, sharp teeth.

Family homeolepidae
These are characterized by having long bodies and leathered skin covered by rugose integument and fur in some basal species. All are carnivores, being most of them piscivores and few species being more generalists on eating others beings as crustaceans, insects and mollusks. Most of the families in this group have two form of regulation of temperature, from endotherm poikilothermic (body temperature of their bodies fluctuates in conjunction with their environment) to ectothermic, the reasons are for their ancestral forms that had a metabolism slow suitable to withstand periods of great shortages of food, coupled with the stable climate that prevailed hysterea for millions of years and led to the expansion of these forms.
Memoreplatypi vulgaris
Maybe a little reminder of the extinct monotremes, this species is formed by benthonic eaters that have a jaw covered by hardened structure similar to a beak, devoid of teeth in the front of the mouth with some quite small molars in the back, which is bordered by small electroreceptors to detect prey under the mud and sand. What makes it stand out among others of their relative is its skin covered by a thick coat, part of their ancestral mammalian roots.
Gnathidirus suchocephala
A very specialized ambush predator, it lives around the rivers of the tropical zones, is completely aquatic, only able to peek out of the water to capture large prey. It has a relatively short body, with a long and robust head, capable of exerting a bite force strong enough to break bones. This species live in large groups of dozens of individuals, agglomerates near the banks of rivers, to take advantage of this opportunity to get together a big prey.
Brevisarchus thalassus
This is one of the few species of this lineage that has managed to colonize salt water areas, living near the coast, in shallow water, about 20 to 40 meters underwater. It has a length of 1.8 meters, being the males slightly lower, feed mainly on pelagic animals around, and being agile, quite efficient swimmers. Something characteristic of this group is that at the time of reproduction, these migrate from the sea into rivers at a point of the rainy season to spawn in fresh water. Males and females mate near the coast, then only females traveling on tens of kilometers to find suitable places to nest and lay their eggs.

Family Stringeriscelidae
These Metanascia are a strange group of long hind-limbed small sizes beings, which evolved around the Klastogeian and be extinct around the middle of the Bathovician (590-600 m.y.), being a very ephemeral clade, but with a significant role for the formation of a new lineage derived from them. The most prominent feature in this clade is the appearance, has small front limbs but huge and long hind legs, which offer great propulsion. Unlike other Metanascia, they have begun to exhibit a noticeable change in their way of reproduction, became more ovoviviparous, keep their offspring inside until these born.
Gavialutroida polius
It is a medium-sized carnivore with 1.8 meters long, has small forelegs, without claws and with only one finger, which only have a function of grip when are copulating. Otherwise, this has hypertrophied hind legs that help a lot with underwater propulsion together with the long tail. This is a quite intelligent species, live in small groups of five to nine individuals. It feeds mainly on fish, arthropods and molluscs.
Miceolutroida natodermus
This little Stringeriscelid is an omnivorous species that has developed a type of mutuality with different varieties of large aquatic animals, cleaning the skin and mouths of these with the benefit of eating parasites and dead skin. Around the mouth has developed large lips with rough surfaces that allows to scrape the skin of their "customers", the frontal limbs are below of the chest, barely visible, and the hind limbs have evolve into rounded shaped fins, that have suction pads to properly hold the animals when are cleaning them.

Order Monadipoda
This is a derived branch of the Metanascia which existed around the end of the Klastogeian, probably from the Stringeriscelids, these have developed a more divergent morphology, reducing their forelimbs to make them disappear and keeping the hind limbs. Most of the members of this group are fully aquatic, except for some amphibian types that returned to land. This is a fairly exclusive group of freshwater environments around the globe which had expanded million years before the supercontinent break up, with several species that have managed to colonize brackish environments or of shallow water areas. Unlike the primitives’ Metanascia, they have developed a more specialized form of reproduction, not depending on spawn eggs and turning back to viviparism. However, these species have developed a fairly basic larval form being a four limbed and so similar to its mammalian ancestral forms, this also has more specialized "pseudo-gill" to stay underwater so they can develop through a metamorphosis, in which they atrophied and lose their forelimbs and when those are grow as adults, are able to breathe through the of lungs.

Family Gaiopodae
These forms are to be oddities of this water order since one way or another have managed to colonize the land again, turning them in a kind of secondarily terrestrial group of animals. These species inhabit only sub-continents or islands where terrestrial competition is almost nonexistent. Despite returning to land, its anamniotes condition beings forced to return to the water to give birth and maintain its young forms until they grow, except for some more specialized species that managed to deal with this.
Phocinosoma titanognata
This pinniped-like gaiopode is one of the biggest amphibian species of this clade, with 3.5 meters long and with a weight of about 300 kg. This massive animal has a triangular head with a huge “V” shaped mouth, a strong neck which actually is comprised of the ribcage, without front legs and long hind legs as a fin followed by a huge paddle-shaped tail. These are capable of producing severe and quite loud snorts both to communicate and to warn.
Penguipes panpottamus
This is probably the strangest forms of future marsupial mammals, with a head of a hippopotamus, bipedal, semi-aquatic with a long flat tail, and capable to give birth underwater, is a rare form of gaiopode that inhabits the rivers around a subcontinent near to the North Pole. These, unlike the hippos, are not herbivores, but piscivores. These have evolved a very interesting way of breeding, because instead of back into the water, they dig a nest at a considerable distance from the coast, which filled with water, and there, the female deposits the hatchlings and these remain safe until they develop.

Family Hallucigotherae
No doubt this is one of the most impressive extremes that this group can reach, most members of this clade are slow herbivorous inhabitants of tropical areas of the planet, unlike other exposed groups, these have atrophied or modified the huge paddle-shaped tail, and modifying its body to adapt well to any of the marine environments where they evolve. The most interesting of this group is to preserve the "pseudo-gill" of the larval form, to stay under water for long periods of time, but in a moment these need to return to the surface for air.

Figoequu leptocaudus
One of several inhabitants of a strange ecosystem, consisting of huge and large plants like mangroves, which have huge and deep roots that reach depths of between 40 to 60 meters, forming strange root-forests atolls. This strange sea horse like creature is one of several specialized herbivores that feed on algae and small plants that grow on the roots use its long jaw full of small teeth on the front and little molars on the back. It's a pretty slow swimmer, remaining always clung to the roots through its prehensile tail and using the round fins to propel and stabilize. They are a very social species, congregating in dozens of individuals around of a single tree. Males have a thick brow, which is colored a shade of orange. This use it in rituals challenge where both males intertwine with their tails, and they start beating his head until one of them loses.
Segnocrassus specesirenus
This is a species that lives in more open areas, being similar to a manatee with a length of about 1.9 meters, is a grazing herbivore that inhabits near the shallow water plains covered by algae and a variety of sea grass like plants that grows on the sands. They have a stocky body, barrel-shaped, which drive through two long fins. Like the Figoequu relative, this has a row of teeth in the front of the mouth, however, this is followed by a set of 4 specialized molars, which allows it to grind plants and facilitate the digestion.

Family Chimeropterae
This lineage is better designed for fast and efficient swimming, having a torpedo-shaped body many members of this group are found in freshwater, with a few species adapted to salt water. Similar to the way odontocetes, several species developed a melon-like structure that allows them to use echolocation, in some species, use this as a weapon.
Delphifronts basilus
This is a social species of chimeropterid, it has a size of about 5 meters long, inhabits the great rive systems of the continent Ereboreus. It is one of the largest river predators on the planet, able to hunt in groups of five a different variety of prey, from large land animals that pass by their areas to shoals of fish that lurk between the murky waters.
Rhyniprion piloserrata
This saltwater species is a medium size carnivore that possesses a long, thin structure like a saw is covered with small structures like teeth composed of keratin. These ones possess a modified melon to their river relatives because it extends not only from the front of the head, but also along the saw. When this is producing clicks, the melon extended in the saw drives those below the sand to locate any potential prey. The preys are not limited only in creatures under the sand; it can also hunt animals like fish or nektonic invertebrates. To do this, it moves the saw violently near the prey, inflicting an enormous damage. When these born, the larvae lack this structure, and usually grow in a period of about 3 months. This species is quite solitary, only gathering in one point to procreate.

Fatal fate
Throughout the biological history of the earth, not always all organisms will be able to have the ability to face all the changes that the world can get to throw, and at some point, they must perish, leaving nothing to continue their lineage and just disappear for others to take their place, and unfortunately, the far descendants of the opossums will not last against the great mother of all the extinctions at the end of the Phanerozoic, 690 million years in the future.
There is not much to lament about the end that they suffered, in a memorable way, they accomplished what few groups could make in the last 600 million years, thrive and spread across the world and time.

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I must be frank about this last group I added, there is a high probability that such beings can not exist in any known future and are implausible in a way, however, I must emphasize that this is the distant future hundreds of millions of years, so the possibilities for new and aberrant features in certain groups to arise are high. Not with this I will assume that this is going to be completely plausible or probable, but just possible.