字幕表 動画を再生する 英語字幕をプリント 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, or “Antarctic 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. 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