字幕表 動画を再生する 英語字幕をプリント In 2011, paleontologists working on the Spanish island of Minorca announced the discovery of some very odd fossils. They were the bones of a rabbit, but it wasn't like any rabbit you've ever seen. For one thing, this bunny was a giant, up to six times heavier than your average cottontail. It also had short hindlimbs, compared to its forelimbs, and feet that were tipped with claws. And this rabbit almost certainly couldn't hop. It had a stiff spine and splayed toes - very different from the flexible spines and tightly packed toes of living rabbits. The scientists named this huge bunny Nuralagus rex -- “the Rabbit King of Minorca” -- and they determined that it ruled its island kingdom during the Pliocene Epoch, from about 5 million to 3 million years ago. Now, we've talked before about insular gigantism, where small animals that become isolated on islands evolve into larger forms due to a lack of predators. And that seems to be what allowed Nuralagus rex to get so big. But how did the normal-sized ancestor of Nuralagus make it onto a Mediterranean island in the first place? Well, it looks like the answer to this biological mystery is actually wrapped up in an even older geological mystery. Since the 1800s, scientists have known that the layers under the floor of the Mediterranean Sea weren't just made up of the usual sediments, like mud and sand. Instead, they're full of salt crystals -- lots and lots of salt -- forming mega-deposits so large that they're sometimes called the Mediterranean Salt Giant. And salt deposits like these are typically found in places where bodies of water have dried up. So the existence of this Salt Giant suggests that, at one point in history, the Mediterranean Sea must have evaporated. But how could a body of water as big as the Mediterranean just...disappear? It would take decades and more than 1,000 research studies to even start to figure out the cause -- or causes -- of one of the greatest vanishing acts in Earth's history. Today, ocean water flows into the Mediterranean Sea from the Atlantic through a narrow passage between Europe and Africa, called the Strait of Gibraltar. And that's the main source of water for the Sea. Some freshwater in the form of rainfall and rivers also flows into it, but that's not enough to keep the Sea filled up without the water from the Atlantic, because it has very high rates of evaporation. So, to the geologists who were trying to explain the existence of the Salt Giant, it looked like the main water source to the Mediterranean had somehow been turned off, like turning off a faucet in a bathtub. They called this event the Messinian Salinity Crisis, or MSC. But it wasn't easy to figure out how that water source got turned off. And researchers have been arguing about it since the 1970s. How did it happen? How long did it take? In the end, they came up with three main hypotheses to answer these questions and explain how the salt giant got there. First, some scientists thought there was a global cooling event at the beginning of the crisis, in the Late Miocene Epoch around 6 million years ago. If the whole world cooled off into an ice age, then lots of water would've been taken out of the ocean and frozen in glaciers, reducing the water flow into the Mediterranean Sea from both the Atlantic and the rivers. An event of this magnitude would have to have been global, not local. However, this idea was proved wrong pretty quickly. Soon after the Salt Giant was discovered, researchers started studying oxygen isotopes and other geochemical data from sediment and ice cores around the world. Their data showed that the rest of the Earth wasn't abnormally hot, cold, or dry during the crisis. Plus, they found that some of the salt was deposited before any changes in sea level. So there didn't seem to be a cooling event that was big enough to turn off the flow of water. That ruled out the first hypothesis. The second idea was that tectonic events had somehow blocked the Mediterranean off from the Atlantic, and cut off the water flow. Some researchers thought that shifting ocean crusts slowly blocked off the waterway between the Atlantic and the Mediterranean. As the water left behind in the deep basin evaporated, it became saltier and saltier, depositing layers of salt as it dried. And this explanation was *almost* right. Other scientists thought that it might have been a combination of shifting crusts and climate change that made the Mediterranean dry up. This was the third hypothesis. According to this model, the crust under the Strait of Gibraltar rose up over time, reducing the flow of water from the Atlantic. Then, because of changes in regional climate -- like periods of less rain and higher temperatures -- the amount of freshwater that made it into the Mediterranean varied. So by this thinking, the MSC didn't happen all at once. Instead, water levels started to drop after the strait closed, and then fluctuated according to changes in the climate. And this hypothesis ended up being … pretty much right! Or at least close to it. As geologists began collecting evidence to test these hypotheses, they eventually found that the MSC was indeed caused by changes in Earth's crust, but those shifts actually happened repeatedly, not just in one fell swoop. For example, in sediments near the Nile, geologists found evidence of repeated erosion events, not just a single big erosion. This meant that the water level dropped, created a new shoreline for a bit, and then dropped again a few more times. Some researchers estimate that there were 16 climate cycles just in the beginning of the MSC. And these changes also correlated pretty closely with what we know about climate cycles. During periods of decreasing sea level, the position and angle of the Earth changed with respect to the Sun, so there were periods of lower solar energy, and others of higher solar energy, which increased evaporation rates in the Mediterranean. At the same time, an actively folding and uplifting tectonic belt caused water input to decrease. Researchers were able to use chemical, and even magnetic, signatures in the sediments laid down during this time to estimate how long the MSC lasted. And the data suggest that it went on for over 600,000 years, with the very driest period occurring about 5.6 million years ago! At the height of the MSC, external water sources were completely cut off, and most of the water left behind in the Mediterranean basin was evaporating. Geologists think the water level dropped by a few hundred meters -- the length of multiple American football fields. And the water that was left was supersaturated, so the salt continued to precipitate out at the bottom of the Sea. The longer there was salty water that could precipitate out salt, the thicker the final deposits became. And the salt giant is possibly up to 3 kilometers thick, which means that the sea was extremely salty for the hundreds of thousands of years during the MSC. And this, of course, had enormous effects on living things in and around the Mediterranean. Back then, life in the sea was dramatically different. Today the Mediterranean Sea is home to thousands of marine species and is famous for its crystal blue water. But when the sea almost dried up completely, it became uninhabitable. Almost nothing could live there. Most of the animals and plants that lived in the Mediterranean before the MSC either migrated away or died because the water was too salty and too shallow. Some marine paleontologists think that no true marine organism was able to survive, and the evidence for that is pretty good. For one thing, the sediments that were deposited during this period weren't disturbed, which suggests that there were no burrowing creatures living there. And even now, there are hardly any deep sea animals that are unique to the Mediterranean, because all of them died during the MSC. Some shelled creatures, like gastropods, might have survived, though they weren't exactly thriving. Sediment cores showed that small populations may have eked out a living in isolated pockets. But the disappearance of the sea wasn't all bad news for living things; the lower water levels were also an opportunity for some creatures to flourish. The distribution of fossils of now-extinct megafauna suggests that there was once a land bridge between the mainland and several Mediterranean islands, like Sardinia and Corsica, that has now disappeared. This allowed hippos, elephants, and other megafauna from Africa to walk and swim across the Mediterranean. And we know that it wasn't just megafauna that took advantage of the crisis to move around. This seems to be when the ancestors of our giant friend, Nuralagus, left the European mainland for Minorca. Paleontologists have found other giant fauna, like dormice and hamsters, on islands that haven't been connected to each other since the MSC, like Malta and Sicily. This wasn't the first time that animals made a long journey to lands around the sea -- it was just one of several migration events during the late Miocene. The remnants of these migrations are found in the fossilized remains of the hippos and elephants that lived there. But when the water rose back up at the end of the MSC and land bridges disappeared, the populations were isolated from each other, and from northern Africa. That isolation led to small versions of large animals, or insular dwarfism, and large versions of small animals, or insular gigantism, on islands around the Mediterranean. Which solves the mystery of how Nuralagus got to Minorca and why it ended up being so big. The Mediterranean Sea is back again, of course, so obviously the water returned at some point. Models of the ocean crust suggest that the Strait of Gibraltar opened up as the tectonic plates shifted again and sediments eroded, lowering the barrier between the Atlantic and Mediterranean, letting water flow through the Strait, and into the basin. But just like the arguments about how the MSC started, there was a lot of debate about how it ended, too. At first, some scientists thought that it was replenished by a giant waterfall cascading into the Mediterranean, with water pouring so fast that it filled up in only a few months. They even found sediment deposits that suggested a rapid flooding event occurred. And while that sounds amazing, it looks like it's not true --- at least, not the waterfall part. More recently, other geologists using seismic data discovered that the slope between the Atlantic and the Mediterranean wasn't steep enough for there to have been a waterfall. Instead, what re-filled the sea was probably more like a river. The basin did fill up quickly, though; recent estimates say it only took around 2 years to end the MSC, during an event sometimes called the Zanclean Flood. But that doesn't mean this story is over. The plates of the Earth are always shifting, and if the perfect storm of plate movement and climate change repeated itself, it could conceivably happen again. Scientists are still finding more evidence of the MSC, from those fossilized mini-elephants, to Nuralagus, to salt crystals found on land and under the Mediterranean Sea. And while some events leave obvious marks on the planet's surface, like mountains and craters, there are others that you have to look harder to find traces of. Even though the MSC shaped life in and around the Mediterranean for hundreds of thousands of years, its fingerprints lie mostly out of sight, buried beneath the waves. But the fossils of those dwarf elephants and giant rabbits let us see its effects, and help remind us of that moment in geological time when the Mediterranean Sea disappeared. Hey! Love binging PBS shows? Well the PBS Video App is your home for amazing educational shows ranging from science films from NOVA to concerts on Austin City Limits to documentaries from Ken Burns and Frontline, all with no ads or algorithms. Oh and Eons. We're there too! If you live in the United States, check out the link in our description to start watching anytime on nearly any device. Now, giant bunny kicks to this month's Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, Sean Dennis, Hollis, and Steve! Pledge your support and become an Eonite at patreon.com/eons! And as always, thank you for joining me in the Konstantin Haase Studio. Be sure to subscribe at youtube.com/eons.