The Mystery of the Eocene’s Lethal Lake


47 million years ago, the world was a hothouse. Lush jungles covered much of the globe, stretching
far from the equator, and they were rife with mammals. This was life in the Eocene Epoch. And around a quiet lake in what’s now Germany,
a typical scene was unfolding in late spring. Crocodiles patrolled the waters. Bats and birds flew overhead. Horses, many of them pregnant, came by for
a drink. And turtles were … having turtle sex. But then, all of these animals died, and quickly,
tumbling down to their final resting place at the bottom of the lake. Then it happened again. And again, probably over thousands of years. In time, the lake filled with corpses. No one would discover this eerie scene until
the 1800’s, when miners began working in exposed deposits of mud, near the town of
Messel, Germany. They were extracting oil from the rock. And along with the oil, they found phenomenal
fossils from those events 47 million years ago. The fossils were so beautifully preserved,
it was as if the rocks had captured snapshots of life in the Eocene. The fossils contained early mammals with their
hair still intact, birds with feathers, and turtles that died right in the middle of mating,
the first known vertebrates found to have fossilized while doing the deed. What a way to go. But what happened to these Eocene animals? And why were their remains so exquisitely
preserved? The answers lie at the bottom of Germany’s
lethal lake. The fossil pit at Messel is one of the world’s
best examples of a Lagerstätte, an area of especially amazing fossil preservation. It may sound like some kind of beer, but Lagerstätte
means “storage place”, and in the lake deposits of Messel, it’s not just bones
that were stored away. Take the holotype of Darwinius, an early lemur-like
primate. Not only are all of its bones present in Messel,
but the fossil also has a full body of hair still intact, proving it was just as cute
and fuzzy as you might have guessed. Or look at the fossils of the early horse
Eurohippus, some of which still have leaves preserved in their guts. Eight of the Eurohippus specimens found at
Messel were pregnant, and their fetuses were preserved as well. Still other fossils reveal black outlines
of animals’ bodies impressed into the rock, showing pudgy bellies, big ears, and sleek
silhouettes in a way that bones alone never could. But it’s not just the level of preservation
that sets Messel apart. Even more interesting are the types of animals
preserved there. Some of the earliest known bat fossils anywhere
come from Messel, preserved as full skeletons with the outlines of wings and ears. These beautiful specimens have shown paleontologists
that bats started looking like bats a very long time ago – and that echolocation developed
much earlier than we once thought. And bats aren’t the only flying animals
that show up. One of the most common animals found in Messel
are birds, complete with feathers that have melanosomes — the cellular structures that
contain pigment — still intact. So far, Messel has turned up 70 species of
birds, and 8 species of bats, which is an incredible abundance of these kinds of animals
that tend not to fossilize very well. But perhaps even stranger than the flying
animals are the turtles. Maybe it doesn’t sound strange to find fossil
turtles in a big lake deposit, but it’s pretty strange to find nine sets of them fossilized
in the middle of mating. So, what happened to the turtles and the horses? How did so many birds and bats fall into a
lake? And how are all of these things preserved
so perfectly, with so much of their bones, feathers, and fur in place? Well, one clue is that, in order for the turtles
to have fossilized while mating, they must have died really quickly. And in order to preserve flying animals like
birds and bats, the event would’ve had to affect not just those in and around the lake,
but also those that lived above it. So, one possible explanation for all of this
is that there was a big bloom of cyanobacteria. We see this happen sometimes in lakes today. Cyanobacteria can grow out of control in warm,
nutrient-rich water, and release chemicals so toxic that any animal that ingests them
dies almost instantly. Blooms of cyanobacteria are also seasonal,
and there’s some indication that many of the fossilized animals at Messel died during
late Spring or early Summer that’s when modern horses tend to be pregnant, and when modern
turtles tend to mate. And these kinds of bacterial blooms could
have killed birds and bats, if they drank the toxic water. This explanation, proposed by German scientists
in 2004, is a good one. But there’s still one big problem: those turtles. After all, these were aquatic turtles. They lived in this lake, and presumably ingested
its water all the time. Cyanobacteria would have killed them long
before they had a chance to mate. And even more troublesome, there’s plenty
of algae preserved in the lake, but there’s no fossil evidence of cyanobacteria. So if it wasn’t cyanobacteria, then what
was it? Well, that relates back to the lake itself,
and why it was there in the first place. Lake Messel formed in a steep-sided volcanic
crater called a maar. We know this because the base of the lake
is full of the broken volcanic rock that typically fills a crater after a major eruption. So, the crater filled with water. But that didn’t mean the volcano was done
erupting. Some scientists think that what happened at
Messel is similar to what happened at another Maar, called Lake Nyos in Cameroon, West Africa
in 1986 — it was an eruption of carbon dioxide. Volcanic gases can be instantly deadly, and
eruptions of carbon dioxide are known in Swahili as mazukus or evil winds – invisible clouds
of gas that asphyxiate anything in their path. A sudden release of CO2 would have choked
out anything in the water, as well as anything by the water, and anything above the water
– from mating turtles, to pregnant horses, to bats, all at the same time. And Mazukus can happen in a couple of different
ways. For example, the magma chamber that made the
crater of Lake Messel could have just released a whole bunch of CO2 all at once, like a big,
nasty volcanic burp. And this can happen if magma comes in contact
with carbon-rich rocks. Or, Messel could have undergone what’s known
as a limnic eruption, where CO2 that was dissolved in the lake water was suddenly released. CO2 dissolves in water, which is normally
not a problem. But in deep, still-water lakes, dissolved
gases can separate into layers, in a process called stratification. Heavy gases, like CO2, will sink to the bottom
of the lake, while lighter gases, like oxygen, will stay close to the top. This is what most likely happened at Lake
Nyos in 1986. A thick layer of dissolved carbon dioxide
built up at the bottom of the lake. And then something shook up the layers of
gas in the water, causing the CO2 to race to the surface and eventually form a Mazuku. The trigger could’ve been an underwater
landslide, or an earthquake, or a large amount of water entering the lake at once, like during
a big flood. We’re still not totally sure what caused
Lake Nyos to erupt, and we definitely don’t know what triggered the lake at Messel. But because the fossils of Messel were preserved
over long periods of time, it seems likely that Mazukus struck there more than once. After each release of carbon dioxide, the
animals in and around the lake died. Then the gas slowly built up again in the
water, until something disturbed it, and the cycle of death happened all over again. But Mazukus would have only killed the animals
of Messel. They didn’t preserve them. The reason the animals were so well preserved
was because of the chemistry of the lake itself. Once the animals died, they sunk into the
still, cold, CO2-rich waters at the bottom of the lake. With so much CO2 and so little oxygen, the
lake bottom wasn’t habitable, which kept scavengers from disturbing the corpses. That’s why so many fossils are preserved
as articulated skeletons. But anaerobic bacteria could thrive in this
environment, and they got to work. Harder tissues like hair, feathers, and bones
were more resistant to the bacteria, but the bacteria could eat away at the soft tissues,
like skin, muscles, and organs. Then, the chemical byproducts of this decay
reacted with dissolved iron in the oxygen-poor waters to create a mineral called siderite. This mineral slowly settled onto the bacteria
and preserved them in place, leaving a layer of siderite around the organs and soft tissues. Then, this layer became filled with oily organic
material, and over time, it created dark shadowy haloes around fleshy tissues like skin and
organs. Finally, all of this material fossilized into
the rocks that those miners found, millions of years later. Small particles of mud and clay drifted down
to cover the remains. As the layers built up, their weight compressed
the fossils and dirt into thin layers called varves. Some layers built up at only a hundredth of
a millimeter a year, an entire year of sediment crammed into a layer that’s thinner than
a page of your favorite novel. And there are a lot of varves in Messel. In fact, there’s a varve for each of the
approximately 1 million to 1 and a half million years of deposition that occurred there about
47 million years ago. These thin pages of oily muck captured not
just the death of the animals, but also how they lived, what they ate, and – quite literally-
who they loved. So it was the truly remarkable combination
of all of these steps – rapid death, followed by bacterial decay, then siderite, and oil,
and the formation of varves – that preserved the beautiful fossils found in the Messel
Pit. And that’s also why Lagerstätten are so
rare. They’re the sum of dozens of just-right
conditions, which don’t happen every day. So when we do find a lagerstatte, we have
to protect it. And at first, the Messel pit was excavated
for one specific type of fossil: the fossilized algae that had long since turned into oil,
permeating the spaces between grains of mud, to form oil shale. But when mining stopped in the 1970’s, the
German government considered turning the pit into a garbage dump. Thankfully, a group stepped in, to keep the
complex and beautiful story of Messel — which began with that peaceful scene 47 million
years ago — from being lost forever, to time and trash. Thanks for joining me today, and thanks to
all of our patrons who help make these videos possible. And we want to thank our first two eontologists,
Duncan Miller and David Rasmussen. Thank you so much for your support! If you’d like to join them, head over to
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how things came to be, then I beseech you to check out our new sister channel from PBS
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and future, of the great experiment that is America. Now, what do you want to know about the history
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