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  • It's the world's most remote and isolated continent.

  • It's home to glaciers, mountains, plants, and penguins, but today, Antarctica is also

  • noteworthy for what it doesn't have.

  • In modern Antarctica, there aren't any trees, or any native terrestrial mammals, reptiles,

  • or amphibians!

  • At all!

  • But, it wasn't always like this.

  • Thanks to plate tectonics, Antarctica has been connected to lots of other continents

  • at various points in deep time.

  • As a matter of fact, before the start of the Eocene Epoch about 56 million years ago--Antarctica

  • was still joined to both Australia and South America.

  • And the fossil record tells us that, in the early Eocene, Antarctica was a warm, forested

  • place, very different from the continent we know today.

  • Palm trees thrived there, as did flowering plants, dung beetles, and even a number of

  • hoofed mammals and marsupials.

  • And because of the way it was situated, Antarctica probably served as an important migration

  • path for the ancestors of some of the southern hemisphere's most charismatic mammals, like

  • wallabies and kangaroos.

  • Eventually, of course, the lush environment of Eocene Antarctica transitioned into the

  • cold, glacier-covered landmass that it is today, isolated from the rest of the world

  • by the most powerful ocean currents on the planet.

  • But it turns out that a lot of what we recognize about the southern hemisphere -- including

  • those famously unique animals of Australia -- can be traced back to that time when Antarctica

  • was green.

  • If you could travel back in time and visit Antarctica in the Eocene Epoch, the first

  • thing you'd notice would probably be the greenery.

  • Off the coast of Wilkes Land, in eastern Antarctica, scientists have discovered sporomorphs--fossilized

  • pollen and spores -- from ancient palm trees and ferns.

  • And they've also found pollen from other plants that often live in tropical environments

  • today.

  • The traces of these warm-weather plants can tell us a lot about what Antarctica was like

  • back then.

  • Since these palms and other trees can't tolerate the cold very well, paleontologists

  • think that, in the early Eocene, the coast of Wilkes Land experienced very mild winters,

  • with little to no frost.

  • By one estimate, the mean annual temperature of that part of Antarctica was around 16 degrees

  • Celsius, with an average winter temperature around 11 degrees Celsius.

  • So, how could ancient Antarctica have been so warm?

  • Well, for one thing, the Eocene wasn't the first time that Antarctica's climate was

  • so mild.

  • Scientists have found sporomorphs and other fossils from warm-weather plants in Antarctica

  • that date way back to the Devonian Period, more than 358 million years ago.

  • And in the early Jurassic Period, about 190 million years ago, Antarctica was a temperate

  • home for dinosaurs like the long-necked Glacialisaurus and Cryolophosaurus, a crested carnivore.

  • In those days, Antarctica was just one small chunk of the supercontinent Gondawana, and

  • was located a bit farther north than it is now.

  • But by about 100 million years ago, most of the landmass that would become Antarctica

  • had migrated to the bottom of the world.

  • By the early Eocene, the western part of Antarctica had just split from the tip of South America,

  • but the eastern part was still mostly linked to Australia.

  • And right around this time, the world was going through a dramatic heat spike.

  • This event is known as the Paleocene-Eocene Thermal Maximum, and we did a whole episode

  • about it, because the theories about what caused it -- and what made it stop -- are

  • really complex, fascinating, and little scary

  • During this period, the global average temperature increased by 5 to 8 degrees Celsius in 220,000

  • years or less!

  • And as the world's climate changed, so did its flora and fauna.

  • Tropical trees like palms, as well as ferns and tree-ferns, were able to spread onto every

  • continent, including Antarctica.

  • And mind you, Antarctica is a really big place; like...the entire country of Australia can

  • easily fit inside its boundaries!

  • So given its size, it was able to support many different ecosystems in the Eocene.

  • Farther inland, and at higher elevations, sporomorphs and leaf impressions have been

  • found from plants that are normally found in temperate rainforests, like southern beech

  • trees.

  • It's also been suggested that some areas

  • even experienced monsoons, getting more than 60% of their annual rainfall in the summer.

  • And of course, plants didn't have the whole continent to themselves.

  • On Seymour Island, off the Antarctic Peninsula, paleontologists have recovered brood balls

  • of ancient dung beetles.

  • Those are balls of dung that female beetles lay their eggs in.

  • So if these beetles were rolling dung balls around, where did that poop come from?

  • Well, some of it came from ancient marsupials!

  • Fragmentary remains and isolated teeth tell us that a number of these little mammals lived

  • in Western Antarctica.

  • Judging by their teeth, it seems that some of them belonged to the same order of marsupials

  • as the modern colocolo opossum, a small and adorable insect-eater that's native to South

  • America.

  • Another Antarctic marsupial was Antarctodolops.

  • First described in 1984, this opossum-like critter was the first terrestrial mammal ever

  • discovered in the continent's fossil record.

  • Its ancestors most likely came over from South America.

  • Other residents of Eocene Antarctica probably came from South America as well.

  • For example, a single contentious toe bone suggests that xenarthrans--the group of mammals

  • that includes modern-day sloths--might have lived in Antarctica.

  • Xenarthrans originally evolved in South America, as did the forerunners of a hoofed herbivore

  • that's been found in western Antarctic, called Notiolofos,.

  • The teeth of this creature tell us that it was a browser, stripping twigs off if tree

  • branches and maybe eating the occasional sapling.

  • Not many specimens have been found, but we do know there were at least two species of

  • Notiolofos in Antarctica.

  • Judging by the sizes of their teeth, the bigger of these ungulates weighed up to 230 kilograms

  • while its smaller cousin was about one-fourth that size.

  • And the fact that these two species had such different sizes means that they might have

  • both been specialists, eating different types of plants to avoid direct competition with

  • each other.

  • Another big hoofed mammal known from Eocene deposits in West Antarctica is Antarctodon,

  • orAntarctic Tooth.”

  • Scientists think it was a kind of astrapothere, an unusual group of extinct and mostly South

  • American herbivores.

  • The only Antarctodon fossils that have turned up so far are teeth.

  • But more complete skeletons of other astrapotheres show that these animals looked kind of like

  • tapirs.

  • Some species had self-sharpening canine teeth and ate a combination of soft plants and hard

  • nuts.

  • Others may have been semiaquatic, like modern-day hippos.

  • And paleontologists think Antarctodon was yet another animal whose ancestors crossed

  • into Antarctica from South America.

  • So, these and the other animals that shared their prehistoric habitat are extremely important

  • to paleontologists.

  • Because, Antarctica's fossil record isn't as comprehensive as those on other continents,

  • and many of the bones that we do find are isolated or fragmentary.

  • Still, the coexistence of all these Eocene creatures tells us that Antarctica was home

  • to a variety of land mammals.

  • But why isn't that the case anymore?

  • What happened to Green Antarctica?

  • Well, while Antarctica's land mammals were still kicking around, some pretty big changes

  • loomed on the horizon.

  • Scientists are still working out the timeline

  • of events, but they think that, by about 56 million years ago Antarctica and South America

  • had pulled away from each other.

  • Then by about 40 million years ago, Antarctica and Australia had become separated by an emerging

  • seaway.

  • This expanse of water--which still exists today--is sometimes called the Tasmanian Gateway.

  • And at some point, another seaway formed, the Drake Passage, off the tip of the Antarctic

  • Peninsula, sometime between 36 million and 23 million years ago.

  • So as time wore on, Antarctica went from being a land bridge between South America and Australia

  • to being an isolated continent.

  • The stage was set for a dominant new force in the Southern Ocean: The Antarctic Circumpolar

  • Current, or ACC.

  • This current still swirls around Antarctica, and hands down, it is the most powerful current

  • on earth.

  • Its volume is 1000 times bigger than the Amazon River, and it chugs along at the breakneck

  • speed of 40 centimeters per second in some locations.

  • Propelled by winds and unimpeded by land, the swirling current blocks warmer waters

  • farther north, keeping them away from the mainland.

  • It also dredges cold water from the ocean floor to the surface.

  • And those two factors work together, creating a chilling effect on Antarctica.

  • Climatologists think that the ACC is between 41 and 23 million years old.

  • But there's not a lot of agreement about how the formation of this current actually

  • affected the drop in temperatures -- and the rise in glaciation -- on ancient Antarctica.

  • What we do know is that the late Eocene and early Oligocene was a time of global cooling.

  • At high latitudes in both hemispheres, temperatures dropped by about 15 degrees Celsius.

  • Around the world, atmospheric carbon dioxide was decreasing, possibly because large quantities

  • of it were being absorbed by marine plankton or buried in ocean floor sediments.

  • This may have contributed to the worldwide cooling trend.

  • And the formation of the ACC could've forced temperatures in Antarctica to drop even further.

  • Regardless, we know that from about 36.5 million years onward, glaciers became more widespread

  • across the continent.

  • As ice blanketed Antarctica's surface, many plant communities suffered.

  • A study of fossil plant samples from the Cross Valley formation in the Antarctic Peninsula

  • found that its plant diversity dropped by 47 percent between the late Paleocene and

  • middle Eocene.

  • Slowly, warmth-loving trees and ferns found themselves replaced by temperate forests.

  • These were dominated by Southern Beech trees, which we know had been living on the continent

  • since the late Cretaceous Period, based on fossilized leaf impressions and sporomorphs.

  • And even their days were numbered.

  • Their sporomorphs tell us that there were southern beech trees on Antarctica as recently

  • as 2.5 million years ago.

  • But today, it's a treeless continent, a polar desert whose remaining plants mostly

  • consist of hardy mosses, grasses, lichens and algae.

  • Clearly, Antarctica's biodiversity took a hit after the Eocene.

  • And yet, life continued to flourish on its two former neighbors.

  • After they split with Antarctica, South America and Australia were both totally isolated from

  • the rest of the world for millions of years.

  • And those two continents had something special in common: Marsupials.

  • New World opossums originated in South America before some of them migrated north into Central

  • and North America.

  • Meanwhile, Australia is world-famous for its charismatic marsupials, including kangaroos,

  • wallabies and the now-extinct Thylacine.

  • And DNA evidence suggests that the common ancestor of today's marsupials lived in

  • South America about 70 to 80 million years ago.

  • So, from there, marsupials spread through Antarctica and into Australia back when those

  • three continents were still connected.

  • And as evidence of this journey, they left behind the remains of marsupials like Antarctodolops--relatives

  • of the mammals that Australia is famous for today.

  • So even though Antarctica has lost its big land animals, it was once a forested pathway

  • for life.

  • Which is why, even today, our world retains the ecological fingerprints of a time when

  • Antarctica was green.

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It's the world's most remote and isolated continent.

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南極が緑だった頃 (When Antarctica Was Green)

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    彭成豪 に公開 2021 年 01 月 14 日
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