Why Are There So Many Marsupials in Australia?
These marsupials live Down
Under (the wallaby, tasmanian devil, wombat, kangaroo with its joey,
quokka and koala) but marsupials didn't actually begin their evolution
there.
Credit: Shutterstock
Australia is the kingdom of marsupials, home to furry kangaroos, koalas
and wombats. The continent has so many marsupials, it raises the
question: Did these pouch-bearing mammals arise Down Under?
The answer is an unqualified (or "un-koalafied") no. Marsupials were
around for at least 70 million years before they made it to Australia,
according to Robin Beck, a lecturer in biology at the University of
Salford in the United Kingdom.
"Marsupials absolutely categorically did not originate in Australia," Beck told Live Science. "They are immigrants." [Why Haven't All Primates Evolved into Humans?]
In comparison to most mammals, marsupials are odd. Unlike placental
mammals, such as humans, dogs and whales, marsupials give birth to
relatively underdeveloped young that continue to grow a ton in the
mother's pouch.
"The young are born alive, but they're very poorly developed," Beck
told Live Science. "They basically crawl to their mother's nipple, which is often in a pouch,
and they basically clamp on the nipple and stay there, feeding on their
mother's milk for long periods of time — usually, several months."
Marsupial homeland
And it turns out, the oldest known marsupials are actually from North
America, where they evolved during the Cretaceous period after splitting
off from placental mammals at least 125 million years ago, Beck said.
These ancient marsupials appeared to flourish in North America,
populating what was then the supercontinent Laurasia with about 15 to 20
different marsupial species, all of which are now extinct, Beck said.
It's unclear why these marsupials did well. But for some reason, at
about the time that the nonavian dinosaurs went extinct, about 66
million years ago, the marsupials made their way down to South America.
At that time, North and South America weren't connected as they are
today. But the two continents were very close, and a land bridge or a
series of islands may have linked them. This connection allowed all
kinds of animals to expand their stomping grounds.
Once in South America, marsupials and their close relatives had a field day, diversifying like crazy within
2 million to 3 million years after arriving, Beck said. For instance,
marsupials and their close relatives evolved into bear- and weasel-size
carnivores, and one even evolved saber teeth. Others evolved to eat
fruits and seeds.
"What's happening in South America is they're evolving to fill the
kinds of niches that in the northern continents certainly were filled by
placental mammals," Beck said.
The marsupial relative Thylacosmilus had saber teeth.
Credit: L. Gardner/Copyright AMNH
Many of these marsupials went extinct between then and now, but South
America is still a marsupial hotspot today. There are more than 100
species of opossums, seven species of shrew opossums and the adorable monito del monte (Dromiciops gliroides), whose Spanish name translates to "little monkey of the mountain."
On a side note, within the last 1 million years, one of South America's
opossums traveled north and now lives in North America. This is the
Virginia opossum (Didelphis virginiana), the only marsupial living north of Mexico, Beck said.
Also, opossums
belong to a different order than possums. Possums are native to
Australia and New Guinea, are closely related to kangaroos, and have a
number of anatomical differences, such as enlarged lower incisors, that
the South American opossum lacks, Beck said.
So, how did marsupials get from South America to Australia? [Will There Ever Be Another Pangea?]
Journey Down Under
Up until about 40 million to 35 million years ago, both South America
and Australia were connected to Antarctica, forming one giant land mass.
At that time, Antarctica wasn't covered with ice, but instead with a
temperate rainforest, and "it was not a bad place to live," Beck said.
It appears that marsupials and their relatives bounded down from South
America, strode across Antarctica and wound up in Australia, Beck said.
There's even fossil evidence: On Antarctica's Seymour Island, there are
fossils of marsupials and their relatives, including a close relative of
the monito del monte, Beck said.
The oldest fossil marsupials
from Australia are found at a 55-million-year-old site called
Tingamarra, near the town of Murgon in Queensland, Beck said. Some of
the fossil marsupials at Tingamarra are similar to those in South
America. For instance, the ancient and tiny fruit-eating marsupial Chulpasia from Peru is a close relative of another fossil marsupial found at Tingamarra, Beck said.
Yet another Tingamarra marsupial, the insect-eating Djarthia, may be the ancestor of all living Australian marsupials, Beck said.
Then, there's a big gap in the Australian fossil record. After
Tingamarra, the next oldest marsupial fossils on record are 25 million
years old. "What we see then is clearly there's been a huge amount of
diversification within Australia," Beck said. "By that time we see koalas, we see relatives of wombats,
we see relatives of bandicoots." Basically, all of the major Australian
marsupial groups are present by 25 million years ago, he said.
Again, it's unclear why marsupials thrived in Australia. But one idea
is that when times were tough, marsupial mothers could jettison any
developing babies they had in their pouches, while mammals had to wait
until gestation was over, spending precious resources on their young,
Beck said.
Another idea is that there were no placental mammals competing with the
marsupials in Australia. But this idea is now contradicted, by a fossil
tooth that belongs to a placental mammal or a placental-mammal relative
discovered at Tingamarra. This indicates that placental mammals were on
the continent as far back as 55 million years ago, Beck said.
Today, there are about 250 marsupial species alive in Australia, around
120 marsupial species in South America and just one (the Virginia
opossum) living in North America. In essence, the marsupials' ancestral
geography has flipped.
"That pattern is the complete reverse of the situation 125 million
years ago," Beck said. "Where things are today is not necessarily an
indication of where they were millions of years ago."
