Around 551 million years ago, during the Ediacaran
Period, weird animals had taken over the seafloor. In the shallows, the slug-like Kimberella
lazily mowed down bacteria and algae. In the deeps, fronds of Charnia waved in the
current, filter-feeding or absorbing nutrients from the water. But in between these bizarre creatures were
many animals with a more familiar shape. Elongated tubes, flat ribbons, and other “worm-like”
body plans were so varied and abundant that this part of the Ediacaran is sometimes known
as Wormworld. But familiar bodies or not, most of the strange
organisms of the Ediacaran disappeared about 540 million years ago. Lots of different factors probably played
a role in this, but the biggest seems to have been a revolutionary adaptation: the ability
to dig. While digging worms ending the world sounds
like the plot of a certain brillant movie, it really happened. And how it happened is just as odd as the
culprits themselves. This is the story of how the ancient Wormworld
was ended by the actions of its very own worms. Though there are amazingly well-preserved
soft-bodied fossils from a few places in the world, these types of organisms usually don’t
fossilize well. This makes it pretty hard to actually catch
one of those world-ending worms in the act. So, to solve this murder mystery, we’re
looking at the clues they left behind: trackways, burrows, and well-preserved poo. These are collectively called trace fossils,
or more formally, ichnofossils. Like many clues, it is not always clear who
left these traces. And sometimes, several different organisms
can leave almost the same traces, too. So trace fossils are all given unique names
of their own. Trace fossils made by amoeba-like organisms
show up early in Earth’s history, but the first definite animal trace fossils show up
in the Ediacaran. And traces can be very distinct - like Kimberichnus,
a series of centimeter-long scratch marks that form trails of radiating fan shapes. It’s named for the many body fossils of
Kimberella - the suspected trace-maker - that are preserved on or near these traces. Scientists call marks like these a grazing
trail, or traces left as an animal scraped its mouth across the seafloor, munching on
bacteria. Kimberichnus and fossils like it show that
a huge evolutionary innovation had already happened: some animals were able to detect
and move towards food. But these ichnofossils also tell us something
important about what these animals were moving across. The trace marks themselves are very shallow,
and show that the animal was scraping away the top surface of the seafloor to eat. This sounds like a good way to get a mouth
full of mud and nothing else. But there’s actually a modern version – and
if you have a fish tank, you can see it for yourself! Aquatic snails leave similar grazing trails
as they eat algae that grows on hard surfaces. Which means that during the Ediacaran, the
substrate, or seafloor, was very different than it is today. Instead of soft muddy sand, the sediment was
held together so tightly by a mat of bacteria that it formed a hard surface. Scientists call this type of seafloor a matground,
and it was sometimes so thick and sturdy that it even fossilized itself, creating a texture
known as elephant skin. Because the seafloor was so different, the
lifestyles of early animals were different too. Most organisms lived on top of the mat, or
clung to the mat, but didn’t really dig too much below it. And not just because it was hard to dig in. The bacteria that likely made the mat grounds
typically suck oxygen out of the sediment and some produce toxic hydrogen sulfide as
a byproduct. So even if organisms were inclined to dig,
or had the ability to dig, they risked suffocation or being poisoned if they spent too long under
the ground. But life finds a way - or in this case, worms
did. At the very end of the Ediacaran, a new type
of trace fossil shows up. It’s called Treptichnus, and it’s a small,
branching, mostly-horizontal burrow. And it’s only been in the last few years
that scientists have figured out the probable culprit behind these trace fossils – priapulid
worms. Their body fossils are known from at least
the early Cambrian - and they’re still alive today. Sometimes, they wash up on the shore in big
groups – and their pale pink color and soft exterior has earned them the nickname penis
worms. Yes, really..that’s what they went with. See, priapulids have heads that are lined
with tiny barbs. They use these to move, hooking onto the ground
and pulling themselves forward, but these barbs are also used as a type of sensory organ. They poke their barbed heads around as they
look for food, leaving traces that look very, very similar to Treptichnus. And some species of modern priapulid worms
coat themselves in layers of mucus and bacteria, which helps to protect them from toxic environments. This makes them very resistant to hydrogen
sulfide and anoxia! With that fancy mucus coat and barbed pulley
system for digging, they would’ve easily been able to excavate into, and survive underneath,
the matgrounds of the Ediacaran. Which is why it makes sense that Treptichnus
shows up in the Ediacaran, then becomes much more common in the early Cambrian. Priapulids also became more common in the
Cambrian. We’ve found nice, full-body fossils of priapulids
like Ottoia in the Burgess Shale. Some specimens of Ottoia are so well preserved
that we can even see tiny arthropods inside their stomach. That means that Ottoia may have hidden in
its burrow and then attacked prey that came by – like a tiny marine version of the graboids
from Tremors. Now, while death by barbed penis worm is certainly
not an appealing way to go, priapulids themselves didn’t bring about the end of Wormworld
– their burrows did. Burrowing by worms broke up the hard seafloor
surface - like tiny ancient plows. Communities of priapulids and other similar
organisms changed the seafloor from hard matgrounds to the softer sediment found in the Cambrian. This change is called the Cambrian agronomic
revolution, which seems like a very peaceful name for what was actually a devastating environmental
change. And it was devastating because many early
organisms were adapted to life on a solid surface. Organisms like Charnia had survived by fastening
themselves into the mats, and they couldn’t stay upright in the softer ground. Several groups of Ediacaran animals went extinct
completely, and others struggled to adapt. But the surviving worms weren’t finished
yet. Even with all that mixing, the Early Cambrian
sediment still wasn’t as soft as it is today, because most of these burrows were shallow
– just a few centimeters deep, and mostly horizontal. Which makes sense, because there wasn’t
really a reason to dig any deeper. After all, most of the decaying organic matter
was near the surface, so going down further wouldn’t get you any more food. But by the middle Cambrian, a new use for
digging had evolved. This is best seen in the trace fossil Skolithos. Unlike the shallow horizontal burrows of Treptichnus,
Skolithos burrows are vertical and up to a meter deep. Like all trace fossils, there were probably
several types of organisms responsible for making Skolithos. and they might have lived a lot like feather duster worms do today. Feather duster worms are a type of tube worm
that dig down into the substrate and filter feed above it with their enormous feathery
tentacles. They use their burrows not for food, but for
shelter. When scared or threatened, the worms duck
down into their hole to hide. The deeper the burrow, the bigger the worm
that could hide inside – and so digging deeper suddenly became advantageous. These industrious worms were soon excavating
much further than organisms had before, carving out so much sediment that they left behind
a type of rock now known as “pipe rock.” The pipe rocks pushed resources like nitrogen,
oxygen, and organic carbon deeper into the sediment. And bacteria followed, forming colonies inside
the sand itself - but not making more matgrounds. They didn’t even have the chance! Other animals drilled into the substrate after
those bacteria, churning up the sediment, driving nutrients even further down, and so
the cycle continued. This change in how animals used the seafloor
is known as the Cambrian Substrate Revolution, and it created the ocean bottom we see today. The extensive matgrounds of the Ediacaran
retreated to the deepest parts of the ocean, far from intrepid worm excavators, and they’re
still thriving there to this day. But the rest of the ocean floor is full of
holes, burrows, bacteria, and life. And, notably – lots and lots of worms. Today, worms have completely taken over. Nematode worms are one of the most abundant
multicellular organisms in the ocean, and other species of worms have moved onto land
and some even invade our own bodies! So while Treptichnus heralded the end of the
Ediacaran Wormworld, Skolithos created a substrate perfect for the new Wormworld, the one we
still live in today. The Ediacaran Wormworld is dead… Long live the new Wormworld. If you wanna learn more about the boom-times
of the Ediacaran, watch our episode,”The Other Explosion You Should Know About”. And thanks to this month’s Eontologists,
who mean the worm to us….: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison
Ng, and Patrick Seifert! Become an Eonite at patreon.com/eons and you could submit a joke for us to read,
like this one from Lulu Did you know that Frank was starting up a
Dodo appreciation club? I hear it hasn't really gotten off the ground. [laughs] because they can't fly, that's good And as always thank you for joining me in
the Konstantin Haase studio. Subscribe at youtube.com/eons for more journeys
in deep time.
Fucken' love PBS Eons