Eons

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 seascorpions prowled the oceansas the first plants and arthropodscontinued to slowly conquer the land.

And as far as we could tell, the climate was aperfect 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 duringthe Cambrian period, over 500 million years ago.

And graptolite fossils are not the animal itself.

Instead, theyre flattened tubes with manyopenings that once hosted colonies of tentacledpolyps, 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.

Theyre 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 worldfaster 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 andable to withstand currents.

They became well-adapted to livingin deep ocean environments, too,capturing plankton as theyfloated in the open water.

And their abundance and wide-spread naturehas made these tiny organisms hugely useful,because they allow us to trace some pretty bigchanges in the ocean that foreverchanged our view of the Silurian.

In the 1990's, graptolite researchers startednoticing some strange trends recorded inthe 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 matchedchanges in the rock record - fluctuations betweendeep-water black clay deposits and shallow-water deposits of limestone.

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

And when therewere shallow-water signals, like limestone,graptolites would not only disappear, buttheir forms would go extinct entirely.

These major shifts back and forthsuggested that something very bigwas happening with the ocean.

And itseemed to be happening very quickly,with the environment changing between twodifferent types every 10,000 to 200,000 years.

Scientists began to realize that this patternof 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 typeswere given names - Primo Episodesand Secundo Episodes.

While it'sstill debated what caused them,some scientists think it's relatedto changes in how water circulated.

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

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

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

Without cycling of cold water at the poles, theocean became stratified with different layers thatdidnt mix much.

The bottom layer of the oceanin 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.

Thismeant that any wind or water woulderode the continents pretty quickly.

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

Theres also evidence of massive dust storms during this time, which may have filled the oceans withnutrients, changing productivity andcarbon storage.

This caused waters to loseoxygen, 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 sulfidethat can build up and make the water toxic.

So during warm periods, the oceans losttheir 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 couldnt 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 hugeaccumulations of single species of graptolites.

Scientists imagine that these booms would havemade 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 oncethought 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.