424 million years ago, a small, unassuming  organism known as a graptolite drifted   through the sea, unaware of how important  paleontologists would one day find it.

Though only a few centimeters long, these  graptolites would help unlock much  bigger secrets of the Silurian period.

For a long time, paleontologists thought the Silurian was a time of relative stability, with   life recovering and flourishing after the mass  extinctions at the end of the Ordovician period.

Fish, cephalopods, and sea scorpions  prowled the oceans as the first plants   and arthropods continued  to slowly conquer the land.

And as far as we could tell, the climate was  a perfect match for the growth of oceanic life:   a mildly warmer climate that  provided a stable and secure planet.

But that picture of the Silurian  could not have been more wrong.

What graptolites tell us is a story  of incredible changes in the ocean, of periods where the oceans became poisonous  and suffocating before eventually clearing   up again.

They unlock extinctions and  recoveries that scientists didn't see.

And, most of all, they show us how unpredictable  the Silurian period really could be.

Graptolites started from humble beginnings.

They first show up in the fossil record during the  Cambrian period, over 500 million years ago.

And graptolite fossils are  not the animal itself.

Instead, they’re flattened tubes with  many openings that once hosted colonies   of tentacled polyps, kind of  like how coral lives today.

Now while graptolites are  probably extinct today,   we can get an idea of how they lived from  their extant relatives, the pterobranchs.

They’re similar to graptolites and may  be close cousins or still-living members   of the group depending on  which scientist you ask.

Both are filter-feeders, with early  graptolite colonies using their   tentacles to snag plankton out of the water  from their location fused to the seafloor.

But towards the end of the  Cambrian, graptolites gained   one big evolutionary advantage that living  pterobranchs lack: the ability to move.

Around 485 million years ago, graptolites  lost the fused holdfast that pinned them   to the seafloor and began moving with the current… Which is when these graptolites  start to show up everywhere.

These drifting graptolites spread around the   world faster than the species  attached to the seafloor, carried much further than any organism tied  to the ocean floor could hope to travel.

They might also have been capable of  moving themselves, creating momentum  through the filter-feeding  movement of their tentacles.

The free-floating graptolites even changed  shape, becoming more cone-like and more   symmetrical, making them stable  and able to withstand currents.

They became well-adapted to  living in deep ocean environments,   too, capturing plankton as  they floated in the open water.

And their abundance and wide-spread  nature has made these tiny organisms   hugely useful, because they allow us to trace some pretty big changes in the ocean that  forever changed our view of the Silurian.

In the 1990's, graptolite researchers  started noticing some strange trends recorded in the Silurian rocks of the Northern   Hemisphere, including the Nordic  countries and Central Europe.

There would be sudden periods where groups  of graptolites would become extinct,   only to be replaced by a species or two of  graptolites that moved in from somewhere else.

Slowly, the researchers realized  that these changes in graptolites   matched changes in the rock record - fluctuations  between deep-water black clay deposits and   shallow-water deposits of limestone.

Where the rock showed signs of a  deep ocean, there were graptolites.

And when there were shallow-water signals, like  limestone, graptolites would not only disappear,   but their forms would go extinct entirely.

These major shifts back and forth suggested that  something very big was happening with the ocean.

And it seemed to be happening very  quickly, with the environment changing between two different types  every 10,000 to 200,000 years.

Scientists began to realize that  this pattern of change was also   replicated in other small, specialized  marine creatures of the Silurian, like   the eel-like conodonts and clam-like brachiopods.

These ocean environment types were given  names - Primo Episodes and Secundo Episodes.

While it's still debated what caused them, some   scientists think it's related to  changes in how water circulated.

We think that Primo episodes  probably had oceans that looked   a lot like ours, with cold water at the  poles and warm water near the equator.

The cold water would sink, causing  circulation in the deep ocean   that kept oxygen distributed  throughout the water column.

But during the Secundo episodes, something  changed that pumping system.

Without cycling of cold water at the poles, the ocean became stratified with  different layers that didn’t mix much.

The bottom layer of the ocean in particular  became anoxic - it lost its oxygen.

These events happened at least  15 times throughout the Silurian, and we're still not actually sure why.

It's possible that changes in the Earth's orbit around the sun caused them, much like the  changes we see recorded in the Pleistocene epoch.

But the Silurian was also very different from the  Earth today in a number of other ways.

For one, there were few to  no plants living on land.

This meant that any wind or water  would erode the continents pretty quickly.

Plus, the supercontinent Gondwana was  located near the south pole, which meant that if there were glaciers, they  would accelerate that weathering.

There’s also evidence of massive  dust storms during this time, which may have filled the oceans with nutrients,  changing productivity and carbon storage.

This caused waters to lose oxygen,  especially at the bottom of the ocean.

Normally we assume oxygen is a good thing,   but not all organisms like to   live in high-oxygen environments.

This is especially true for certain bacteria, some   of which produce hydrogen sulfide that  can build up and make the water toxic.

So during warm periods, the oceans lost their oxygen and gained hydrogen sulfide,   which made them much harder to live in.

Unless, of course, you were  adapted to survive these things, the way that the free-floating graptolites were!

They flourished in deep water, seemingly  indifferent to low-oxygen conditions.

Unlike many other animals, graptolites appear to have thrived in  warmer Secundo climates with vast seas.

But their ranges shrank and shifted  during cooler, oxygenated Primo climates, which killed off graptolites that  couldn’t adapt to a drop in sea level.

Their place would eventually  be taken over by other species of graptolites moving in from deeper  waters once the sea levels rose again.

But it became harder and harder  each time this cycle happened.

By killing off graptolites that couldn't  survive in shallower, oxygen-rich water, only deep-water free-floating  varieties were able to survive.

This left a strongly limited gene  pool, one that may not have had   time to recover or become ecologically  diverse before the climate shifted again.

In fact, immediately after  these extinction events,  the fossil record of the Silurian shows the  same trend in graptolites, over and over again.

Initially, a species from the open  ocean moves into an off-shore area.

With no competition from other graptolites, the  new residents experience a population boom - so   much so that the fossil record shows huge accumulations of  single species of graptolites.

Scientists imagine that these booms would  have made graptolites the equivalent   of krill during the Silurian.

But all things must come to an end,  and nothing about this scenario   would have prepared graptolites  for the climate change to come.

After all, the extinctions killed the predators  of graptolites, too - and that meant that there  was very little evolutionary pressure to evolve  into different species or use different tactics.

So when the waters began to cool again,  these new populations of graptolites were   no more likely to survive the change  than the species they had replaced.

This is why these events wiped out such large  percentages of graptolites in particular.

While other animals were certainly impacted,   graptolites show huge population changes  at each shift between primo and secundo.

These massive ocean and climate swings  caused graptolites to almost completely   go extinct in the middle of the Silurian - 95%  of the species across the globe disappeared.

And this happened again and again,  on smaller to larger scales.

The Lau or Kozlowski Event in the  late Silurian wiped out 70% of the   graptolite species, and also  nearly 23% of all marine life.

Considering the consequences of these  extinctions, it seems unbelievable   that we once thought the Silurian was so peaceful.

Today we know that the Silurian  was a time of incredible change,   a trend now seen in chemistry, rocks,  and many different types of fossils.

But if it weren't for the detailed record  of the tiny graptolites and other animals   like them, we might never have seen the  extinctions hidden beneath our noses.