Marsupials have no placenta, so their cubs are very small at birth.
Australia is a kingdom of marsupials, where about 250 species of marsupials such as koalas, kangaroos and wombats live. Why are there so many marsupials in Australia? Could marsupials originate in Australia? Scientists have discovered that the ancestors of marsupials did not live in Australia at first, but most of the species later migrated to Australia.
Why are there bags?
Marsupials are a very old branch of Mammalia, and their body structure is different from the placental mammals that appeared later. Mammals such as humans, dogs, and whales have placentas, while marsupials do not. As an organ that protects the fetus from the mother’s immune system, the placenta plays a very important role in mammalian fetal development. Because there is no placenta structure, the pups of marsupials cannot fully develop in the mother’s body. They are completely “premature babies” when they are born. Therefore, these cubs need to enter the mother’s “pouch” where they continue to develop. The cubs can draw milk from the mother’s nipple in the nursery bag. Under normal circumstances, the pups of marsupials will grow up in the pouch for several months before they can move independently on the ground.
Take the red kangaroo as an example, its cub weighs less than 1 gram at birth, just like a peanut, and the weight of an adult red kangaroo can easily reach hundreds of kilograms. These cubs were weak when they were born, their eyes and ears were not yet developed, and they could only rely on their physical sensations, like a worm, instinctively crawled into the nursery bag along a groove leading to the nursery bag that the mother licked on her abdomen with her tongue. . After living there for about 6 months, they will be able to work independently.
Since Australia is not home to marsupials, where did they come from in the first place?
The appearance of the placenta is one of the major events in the evolutionary history of mammals.
Tracing the hometown of marsupials
Studies have shown that the oldest marsupial mammals actually came from the North American continent. In the Cretaceous period about 125 million years ago, some mammals evolved placental structures. They parted ways with marsupials, gradually evolving into Artiodactyla (bovine etc.), Rodent (rabbit etc.) and Pterodactyla. (Bats, etc.) and other placental mammals, while marsupials have embarked on another evolutionary path.
The skull of the marsupial tiger (an extinct marsupial).
These ancient marsupials developed 15-20 species on today’s North American continent (then it was part of the northern Laoaki continent). For some reason, around the time when the North American dinosaurs went extinct, 66 million years ago, they moved south and settled in today’s South American continent. How did marsupials survive the disaster that led to the complete extinction of the dinosaurs? There is currently no conclusion. At that time, although the South American continent and the North American continent were not connected as they are today, the two continents were very close to each other. There should be land bridges or islands connecting them, allowing ancient marsupials to migrate from the North American continent to the South American continent.
After the ancestors of marsupials entered the South American continent, they immediately began to mate with their relatives. In the next 2 to 3 million years, the marsupials that entered the South American continent evolved a wealth of species: some were large in size. Like bears, some are as small as mongooses, some have evolved saber teeth, and some give up eating meat and feed on fruits and seeds. This multi-niche evolution strategy allows marsupials to firmly occupy the previously unoccupied niche of the South American continent. On the North American continent, the niche occupied by marsupials is vacant, which was previously mainly occupied by placental mammals.
Although many marsupial species have become extinct, the South American continent is still one of the hot spots for marsupials today. There are more than 100 South American opossums, 7 shrew opossums and the cute “Nanya”. In addition, in the past 1 million years, a species of opossum from the South American continent has continuously migrated north and settled in the North American continent, becoming the only marsupial in North and Central America.
Primitive marsupials that lived in North America 125 million years ago.
In addition, South American possums and Australian possums are different. The Australian possum (possum) is a species native to Australia and New Guinea, and is closer to the kangaroo in evolution. Australian possums and South American possums also have many differences in body structure. For example, only the former has enlarged lower incisors. Therefore, Australian possums are not the offspring of South American possums that followed modern humans to Australia.
Australia’s oldest marsupial.
Journey to Australia
How did marsupials migrate from the South American continent to Australia? Between 40 and 35 million years ago, the South American continent and Australia were connected by the Antarctic continent. At that time, the Antarctic continent had not moved to where it is today, so there were many temperate rain forests on the Antarctic continent at that time, and the environment was sufficient to provide food for many animals. Some marsupials and their relatives migrated all the way south from the South American continent, across the polar continent, and finally settled and settled in Australia. Scientists once found fossils of marsupials and their relatives on Seymour Island in Antarctica.
Australia’s oldest marsupial fossils were found in Tingamara, a 55 million-year-old structure. Some of these fossils are very close to those found in the South American continent. For example, a fruit-eating marsupial from Peru is a close relative of a fossil animal in Tingamala. Scientists even believe that among the fossils found in Tingamara, a marsupial that feeds on insects may be the common ancestor of all marsupials in Australia.
However, there is a 30 million-year fossil gap in Australia’s fossil record. After the Tingamara fossil, the second oldest marsupial fossil is only 25 million years old. Since then, the diversity of Australian marsupials has increased sharply, and marsupials such as koalas, close relatives of wombats, and close relatives of bandicoots have gradually appeared. It can be said that all major marsupial species in Australia at present appeared in the past 25 million years.
What is not clear is how these marsupials successfully survived and thrived in Australia. There is a hypothesis that in a difficult period and lack of food, female marsupials in parenting can abandon the underdeveloped cubs in the pouch, while placental mammals have to continue to allocate precious nutrient resources to them. The fetus in the abdomen. There is also a guess that in the past, Australia did not have placental mammals to compete with marsupials, so marsupials can occupy so many niches in Australia. However, this view is now overturned by fossils of close relatives of placental mammals unearthed in Tingamara. These fossils indicate that placental mammals began to multiply in Australia 55 million years ago.
From North America to South America to Australia, the main location of marsupial colony habitats has undergone tremendous changes. With geological changes, the geographical distribution of organisms can produce such an astonishing flip. It can be seen that a species living somewhere today does not mean that it also lived here in the past. The migration distance of species on the earth is indeed amazing at times.
Nan Yi’s appearance looks a bit like a mouse. Nanya is mainly distributed in Chile in the South American continent. It is the smallest marsupial in the world, with a body length of only 12 cm in adulthood. Nan Yi likes to be active at night, and his main food is not fruits and seeds, but insects and other arthropods. Nan Yi’s tail is thick and can store fat to provide energy during hibernation. Due to changes in the ecological environment, Nanyi has been on the verge of extinction.
The red kangaroo is widely distributed in Australia and is the largest marsupial in the world. Its male body color is red or reddish-brown, and its female sex is blue-gray. There are black whiskers on both sides of the nostrils. They are good at jumping. The distance of each jump is 1.2 to 1.9 meters when traveling slowly, and the distance of each jump is up to 9 meters when running and jumping. They can mate all year round, one per litter. Newborn cubs are less than 1 gram, and they can stick their heads out of the nursery bag after two months of birth, and move out of the bag independently after half a year. The hallmark feature of the red kangaroo is its strong muscles. Because red wallabies lack myostatin, their muscles are prone to overgrowth. Even if they do not deliberately exercise, they also have huge muscles that many fitness enthusiasts envy.
North American Possum
North American opossums are mainly distributed in southern North America and Central America. Their size is very different: the small ones are only the size of voles, and the large ones are close to medium-sized dogs. Their ability to pretend to be dead is amazing, and each suspended animation can last for several hours, during which they will release a rancid smell to drive away predators. The North American possum is very cute and a very good pet rat. Although they are docile in temperament, they have poor eyesight and tend to bite their owner’s fingers as food by mistake, so be careful when feeding.
Possum (Australian possum)
Possum is a representative species of marsupial possum family, mainly distributed in Australia, New Guinea, Indonesia and other places. They generally live in forests or grasses, feed on various plants, and compete with monkeys. In ancient times, there was a very fierce carnivore under the superfamily of possums-marsupial lion. The brushtail possum is a common possum. They burrow in hollow tree trunks and also settle on the beams of houses. The abandoned hare hole is sometimes used as a residence by the brushtail possum. Broomtail possums often live in human settlements, and the noise they make during courtship or running at night often makes local residents unable to sleep.
Koalas are also called koalas, and they are Australia’s national treasures. In 1816, they had their own scientific name-“Grizzly Bear” for the first time. Although there is a “bear” in their name, they are not bears at all, nor are they close relatives of bears, but a precious and primitive arboreal marsupial. Koalas almost never drink water, but get nutrients and water from eucalyptus trees. Koalas belong to nocturnal animals and only wake up to search for food at night. During the day, the koala spends 90% of the time resting on the tree, only occasionally eating eucalyptus leaves or twigs. Because of chewing a lot of rough eucalyptus leaves every day, the crowns of koalas are easy to wear, so many elderly koalas with excessive wear of their teeth starve to death because they cannot eat.
The thylacine has a slender body and a fox-like face. It is the largest marsupial carnivore in modern times (kangaroo is a herbivore). It was once widely distributed in the tropical rainforests of New Guinea and the grasslands of Australia, but has now become extinct. The last thylacine died in a zoo on Tasmania on September 7, 1936. In the first half of the 20th century, Australian herders suspected that the thylacine had killed their herding sheep (in fact, the real murderer was the Australian dingo), so they killed the thylacine. However, some scientists have successfully extracted their genetic material from the thylacine specimens in the museum. Perhaps with the help of future genetic technology, we can once again see their style.