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  • 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 beingpretty 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.

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  • Now, giant bunny kicks to this month's Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng,

  • Sean Dennis, Hollis, and Steve!

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In 2011, paleontologists working on the Spanish island of Minorca announced the discovery

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That Time the Mediterranean Sea Disappeared

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    joey joey に公開 2021 年 05 月 01 日
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