Kangaroos hop freely in Australia. Their ancestors hopped all over the world.
Australia has the weirdest distribution of mammals in the whole world. While placental mammals are the most numerous on other continents, marsupials form the most numerous in Australia followed by placentals and then the monotremes. Why is Australia so unique in terms of terrestrial mammal populations?
Placental mammals are animals that give birth to live young who look like the adults, except smaller and include animals like bats and the dingo. Marsupials also give live birth, but the young are born prematurely and migrate to their mother's pouch, like a kangaroo or a koala. Monotremes on the other hand, lay eggs, like a platypus or an echidna. How do all of them come together in Australia?
From Continent Hoppers to the Kangaroo
In 2003, a fossil of a peculiar rat-like creature was discovered in the Liaoning province of China. It was named Sinodelphys. Small and nimble, it scurried on trees feeding on leaves and seeds while also avoiding the dinosaurs that reigned on the ground.
Sinodelphys, the marsupial ancestor.
Sinodelphys's name means ‘Chinese womb’ and is derived from the Greek word Delphinus for womb, and this root is used several times in reference to Marsupials. For example, there are two subdivisions of these pouch-bearing beasts - Ameridelphia and Australidelphia - with the suffixes indicating their continent of habitation.
These earliest fossils were undoubtedly found in Asia. But how did these beasts travel from Asia to Australia? During the Cretaceous period, China, North America, South America, Antarctica and Australia are known to have been connected physically. The metatherians of that period migrated to North America where they gave rise to modern marsupial groups (represented by Didelphodon, the biggest mammal who ever lived along the dinosaurs). The extinction of the dinosaurs provided several opportunities for the marsupials to diversify, as it did for the other surviving mammals and birds.
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Didelphodon
North and South America might have been connected during this period and would have allowed marsupial migration to and from the continent. In South America, marsupials are today represented by about ninety species of opossums. Unfortunately, the native North American marsupials have gone extinct and have no descendents in the present day.
Soon after the Dinosaurs extinction, Australia started drifting apart from Antarctica whereas it remained connected to South America. Paleontologists place the arrival date of marsupials in Australia around this period. The marsupial fossil called Djarthia has been dated to about 55 million years and has been found in Southern Australia. Australia is an isolated island, but still large enough so as to provide ample opportunities for these continent hoppers to diversify, eventually leading to the iconic herbivorous hopper itself, the Kangaroo.
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Hip bones of Djarthia
North American marsupials did not die in vain however. When the two American were connected again via the formation of the Isthmus of Panama about three million years ago, some opossums invaded North America with one species (the Virginia Opossum) going as far north as Canada. After a gap of about sixty million years, North America had marsupials again.
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Didelphodon would be proud.
Homebodies
When the first Djarthias entered Australia, it might have come across a perplexing creature - an animal with a fur, a beak and... lays eggs? What in the world might this creature be? Is it a mammal, a bird, or a reptile?
Humans had similar questions when the first such animals were dissected for scientific studies. Platypus is a monotreme - a branch of mammals which lays eggs as opposed to giving live birth. Monotremes are distantly related to the marsupials and placentals. The lineage leading to monotremes broke off from the lineage leading to the marsupials and placentals further back.
Unlike marsupials which moved continents to arrive in Australia, it appears that monotremes have literally been present on the Australian continent forever. All, except one, monotreme fossils have been found on the Australian continent. Steropodon, an ancient platypus relative, is about 110 million years old and is known only from its partial lower jaw. Early monotreme fossils are mostly known from teeth, molars and lower jaw remains, and it is quite difficult to draw a consensus about their evolution. However, they do show the presence of bills for detecting the movements of their prey in water.
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It is highly likely that platypus and their extinct relatives have led a similar lifestyle, performing the same function in their environments for millions of years. Platypuses today occupy a semiaquatic niche along the river's edge, feeding on insect larvae, shrimps and arthropods. Their adaptations have made them especially successful in this environment: webbed feet for swimming, electroreceptors on their bill for detecting muscle movements of its prey, and poison glands on their feet to be used either in self-defense or for hunting.
Echidnas are another closely related monotreme group that evolved from a platypus like ancestor; however they are nowhere close in uniqueness to the icon that is the duck-billed platypus.
Until you consider the next group of animals on this list.
Island hoppers
Sinodelphys had a mammalian contemporary called Eomaia, which also lived quite a similar lifestyle to the marsupial ancestor. It was also small, rat like, lived on the trees and probably fed on arboreal sources. Unlike Sinodelphys, it however was an ancestor to the placental mammals like you and me. Both fossils were found in the same rock formation and would have crossed paths several times in life.
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Placental and marsupial ancestors coexisted in Asia along with the dinosaurs. Since the continents were connected, they would have also travelled to the Americas. However, placentals seemed to have stopped somewhere at the end of South America. After the end of the dinosaurs, placentals are largely absent from the Australian mainland except for the bats, rodents and dingos. A large reason for this is its geographic isolation during much of this period.
Bats are represented by the 55 million year old fossil of Australonycteris, who arrived on the island continent not too long after bats evolved on earth. It is known from jaw bones and molars, and may have fed on a marine diet of shellfish and fish. It might have entered the continent from Asia by flight, especially since Australia was close to Antarctica during this stage in Earth's history. It roughly corresponded with the time when Djarthia was entering the continent through the southern coast. Other bat fossils belonging to the genus Icarops have also been found on the continent, dating from about 34 million years ago to the present.
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As Australia drifted northwards towards Asia, opportunities were created for other placentals to come over. Both the rodents and dingos followed similar routes for entering Australia: through the northern archipelagos of Indonesia and New Guinea. Rodents entered approximately six million years ago, while dingos accompanied sea-faring humans about five thousand years ago. Rodents currently occupy several niches in the Australian mainland and are represented by fifty species. Ironically, some of them have also evolved the hopping trait unique to Kangaroos. Dingos on the other hand have been in Australia for a really short time, but they were already giving a tough competition to marsupial apex predators like the Tasmanian wolf.
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In fact, by the time Europeans arrived on the scene, the Tasmanian wolf had already become extinct and only existed on the neighboring island of Tasmania. Placental diversity has, in fact, exploded since the arrival of Europeans on the scene. Red foxes now prowl the Australian wastes, providing tough competition to the dingo. Rodents like rats, mice and squirrels, and lagomorphs like rabbits and bunnies dot the landscape and eat into the resources of the native marsupials. Efforts are underway to conserve these rare creatures, but it's a race against the clock that any non-human animal has little hope for.
Hopping from the frying pan into the fire.
Unfortunately the isolation that provides unique opportunities for species to diversify also makes island ecosystems very fragile. Recently fires raging on the Eastern Australian coast have caused the debts of marsupials in the billions. The World Wildlife Fund is helping the local authorities reinstate the conditions back to normal. I encourage the reader to donate whatever they can to the fund in order to save the lives of these unique and rare animals.
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