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
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