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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 that 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.
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So there’s another fossil that we mentioned here that we’ve actually done an episode on.
Check out “The Most Useful Fossils in the World”, to learn more about conodonts.
And thanks to this month’s primo Eontologists!
Jake Hart, Raphael Haase, Annie & Eric Higgins, John Davison Ng,
Melanie Lam Carnevale, Addie, Tony Dai, and JuanM.
By becoming an Eonite at patreon.com/eons you can
get fun perks like submitting a joke for me to read!
Here’s one from The Bergens.
How do we know there were hipster dinosaurs?
Because of all the fossil records they left behind!
Thought that was gonna go a different direction.
Fossil rec-
You know, like...
Yeah, they were here first, or like, they knew about Earth before you, or you wouldn't know...
And as always thanks for joining me in the Adam Lowe studio.
Subscribe at youtube.com/eons for more strolls in the Silurian.
Trying to not move so much so my skulls can stay facing [forward]...
I normally go so fast, I'm going to indulge the slowness for my kitty earrings today.
Maybe I should get clip-ons next time... but they're so cute, ok...
The free-floating graptolites, the free-floating graptolites
even change shape, becoming more cone-like...
They're now my, like, little comfort toys, this is fine.
Is the bloopers just gonna be me looking up into space
trying to fix these cats?
That's fine, it's worth it.