How Did Australia Get so Many Marsupials?

Analysis by Dr. Karen Shaw Becker

marsupials found in australia

Story at-a-glance -

  • The oldest known marsupials resided in North America, “split off” from placental mammals at least 125 million years ago and evolved during the Cretaceous period
  • From the Latin word marsupium, meaning pouch, marsupials are relatively underdeveloped at birth and continue growing in their mothers’ pouches during a longer lactation period compared to other mammals
  • The millennia during which 15 or 20 different marsupial species flourished in North America, they populated what was then the “supercontinent” Laurasia
  • Around the time land-based dinosaurs died out, around 66 million years ago, marsupials made their way to South America via a land bridge or a series of islands, which may have connected North and South America
  • Experts say South America and Australia were both connected to Antarctica 40 million to 35 million years ago, which is why marsupial fossils are found on the frozen continent, but today about 250 marsupial species live in Australia

Australia is home to a number of fascinating marsupials, including kangaroos, koalas, wombats and Tasmanian devils. There are a number of lesser-known marsupials as well, namely Tasmanian and mountain pygmy possums, numbats, kowari, quolls, bilbies and bandicoots.

There are smaller kangaroo species with monikers to differentiate them, such as the musky rat kangaroo, which is the smallest macropod, or plant-eating marsupial, in Australia, and the red Kangaroo, which is the largest.

The term marsupial comes from the Latin word marsupium, meaning pouch. It denotes animals that, in comparison to other mammals, give birth to relatively underdeveloped offspring after a shorter gestation period. Babies continue getting the nutrients they need from their mothers inside the pouch, where they stay for a much longer lactation period — usually several months.

About 70% of the marsupials in the world reside on the Australian continent making it the “kingdom of marsupials.” But is this where these animals originated? The answer is an “un-koalafied” no.1

In fact, they’re immigrants, says Robin Beck, Ph.D., a lecturer in biology at the U.K.’s University of Salford, who says the oldest known marsupials first resided in North America, “split off” from placental mammals at least 125 million years ago and evolved during the Cretaceous period. New Guinea was part of Australia’s mainland until the relatively recent geological past.

“Basically, the species belonging to this infra-class of mammals evolved in North America. From there, they made their way to South America and eventually, to Australia, which was back then a part of the Gondwanaland. As the continents drifted apart, quite a few species adapted themselves to this region, while the related species in other regions became extinct.”2

For the millennia during which 15 or 20 different marsupial species apparently flourished in North America, they populated what was then the “supercontinent” Laurasia. However, those are all extinct, Beck says, adding that today it’s not clear why they did so well.

Quolls, Tigers and Bears, Oh My

Interestingly, it seems that right around the time land-based dinosaurs, excluding avian species, died out around 66 million years ago, marsupials made their way to South America. Experts surmise that a land bridge or a series of islands may have connected North and South America, making it possible for all kinds of animals to expand their territories.

For 2 million to 3 million years, marsupials and related species diversified and proliferated exponentially once they arrived in South America. Reportedly, some evolved into carnivores comparable in size to bears and weasels, and others evolved as fruit and seed eaters. One, the extinct Thylacosmilus, aka Tasmanian tiger or thylacine, evolved saber teeth. Sadly, the last of that species died in captivity in 1936.3

Thylacines were one of three suborders of carnivorous marsupials, but only two, the Dasyuridae, such as quolls (native cats), antechinus (marsupial mice) and dunnarts (marsupial rats),4 and the singular species Myrmecobiidae (numbat), an insectivore similar to an anteater) remain alive today.5 Herbivorous marsupials are placed in two categories: Vombatiformes and Phalangerida. Additionally:

“The Vombatiformes suborder comprises [three] extant species of wombats along with the koala. The Phalangerida suborder, on the other hand, comprises six families of possums and three families of macropods. As many as 26 species of possum are found in Australia today … as many as 51 species of macropods, most popular among which are kangaroos and wallabies, inhabit this island.”6

Possums and Opossums — Completely Different Animals

Beck maintains that the evolutionary “niches” filled by marsupials that thrived in South America at that early time likely kept pace with the number of placental animals in the northern continents. Notably, one South American marsupial, designated as the Virginia opossum, is the only one of its species north of the Mexican border.

As it stands now, South America still teems with marsupials, including more than 100 species of opossums, seven types of shrew opossums and the diminutive Dromiciops gliroides, aka monito del monte, meaning “little monkey of the mountain.”

Speaking of opossums, some believe the word “possum” is simply a misspelling of the word, but these are two different orders of animals. Closely related to kangaroos, possums (Diprotodontia) are native to Australia and New Guinea and have several anatomical differences, such as enlarged lower incisors, unlike the “American” opossum (Didelphimorphia).7

Possums are larger but are usually “less scary-looking.” They range from a few inches high to 4 feet in height and have bushy tails, fur that ranges between black, gray, silver and gold, and a musk gland behind their ears, giving them a rank, unpleasant smell. Opossums are relatively smaller, with coarse, gray-colored fur (ranging between pure white and all black) with black ears and feet. In comparison:

“Opossums are the scarier and uglier with large canines and small incisors. Besides the stretched snouts, they have a plantigrade posture, meaning they can walk as humans do with their heels touching the ground. Their tails, which are bare like those of rats, are capable of grabbing small objects for short periods.”8

For the record, I completely disagree with the “uglier” statement.

How Did Marsupials Get From the Americas to the ‘Land Down Under’?

According to Beck, both South America and Australia were connected to Antarctica 40 million to 35 million years ago, making it one gigantic land mass. Rather than being the ice-covered landscape it’s known as today, Antarctica was more like a temperate rainforest. You can even find fossils of marsupials and their relatives on Antarctica’s Seymour Island. However:

“The oldest fossil marsupials from Australia are found at a 55-million-year-old site called Tingamarra, near the town of Murgon in Queensland. Some of the fossil marsupials at Tingamarra are similar to those in South America.”9

An early, insect-eating Djarthia, may be the ancestor of all Australian marsupials, Beck notes, but then there’s a big gap in Australia’s fossil records, with the next oldest marsupial fossil being 25 million years old.

There’s some conjecture surrounding why marsupials thrived in Australia, but a simple answer might be because unlike mammals that had to wait to finish their gestation periods, which made life more dangerous, marsupials could travel easily and escape environmental and/or predator threats with their developing babies still in their pouches.

While today there’s one surviving marsupial in North America and 120 in South America, there are around 250 different marsupial species in Australia, including wombats, koalas and wallabies. It’s quite interesting that as an animal class, their ancestral geography has “flipped.”

In essence, Beck concludes, “That pattern is the complete reverse of the situation 125 million years ago. Where things are today is not necessarily an indication of where they were millions of years ago.”10

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