A strange geometric fossil hides in the sedimentary rocks of the Alps.
It's built of interconnected, hardened tubes that weave together into honeycomb
shapes. They spread and radiate into larger hexagons of different sizes,
from those only a little bigger than a quarter, to some that are over 10 centimeters across.
The pattern looks almost like the bottom of a precisely woven basket, or a trimmed
down net - which is probably why this fossil is named Paleodictyon, which means "ancient net."
The raised surfaces and regular, repeating geometry are so intriguing that even
Leonardo DaVinci found them worth studying, drawing them in one of his many notebooks.
These fossils are found exclusively in the sort of
fine-grained rocks that were once mud on the bottom of an ancient seafloor.
But while we know where they're from, their maker has remained… a real puzzle.
Are they burrows? Traces of an ancient feeding strategy? Or maybe some sort of…
hexagon-shaped organism whose body was preserved in mud?
That's how odd they are - we don't even know if this is
actually a trace fossil or the organism itself.
What’s weirder is that we’ve now found modern specimens at the bottom of our oceans,
chilling out over 3000 meters under the sea near hydrothermal vents….
and we still don't quite know what they are.
But DNA samples and specimens pulled up to the surface have at least given us
some new ideas about what might be behind the modern mystery of this ancient fossil.
In 1976 scientists were exploring the deep oceans of the mid-Atlantic ridge,
dragging a camera through the water looking for the unexpected.
You might know these explorations for their weird ghostly crabs and black smokers,
but that's not all that's at the bottom of the sea.
Researchers also found thousands of hexagon-shaped grids of regularly-spaced,
very small holes. They looked like someone was giving an allergy test to the bottom of the ocean.
The research team initially described them as an "invertebrate of uncertain identity,"
and it wasn't for another few years that the hexagon spacing of the holes
was connected to the hexagon fossil Paleodictyon.
That link was eventually confirmed with the help of an innovative piece of scientific equipment:
a squirt gun.
Usually these deep sea vehicles have with little water pumps meant for collecting specimens.
But some enterprising scientists modified that pump into a tiny jet,
which the vehicle carried down to the ocean floor.
Then, they attached the jet to a big mechanical arm and basically sprayed
the area with holes until all the loose, soft mud flew off, revealing the hardened,
hexagonal tubes we see in fossilized Paleodictyon.
Squirt guns for science!
As a result, we now know that Paleodictyon is not only still alive, but that it is /everywhere/.
Since the 1970s we've found modern specimens in every ocean other than the Arctic!
In some places, they pepper the ocean floor. And at the same time as we've been finding modern
specimens, scientists have found more and more in the fossil record. Turns out that Paleodictyon
has been around for over 500 million years! Throughout that time period, they've mostly
been deep sea specialists, living quiet lives in the dark depths of our oceans.
And the modern specimens give us a clue as to why they prefer the deep sea.
They seem to be the best preserved and most common where sediment builds up very
slowly - as slowly as 1 centimeter over 650 years.
It looks like Paleodictyon doesn't like places where mud gets piled up on top of it.
Which is funny, because the way they’re preserved throughout time is in rocks called turbidites,
also known as underwater landslides. So we know that Paleodictyon has been
hanging around with few changes for hundreds of millions of years. But that brings us back
to the big question: what IS this thing? With all these living and fossil specimens,
surely the identity of Paleodictyon would be easy to crack… except, not so much.
Since the fossil was initially described in the
1850s there have been a few different ideas about what it was created by.
While worms are responsible for lots of similar fossilized tunnels,
these hexagons have such sharp angles that it actually couldn't have been made by a burrowing
organism – cutting those precise corners at regular intervals would be nearly impossible.
Instead, Paleodictyon appears to have grown in place. And there are animals
that do grow with repeating fractal patterns in their bodies - animals like glass sponges.
These beautiful, complex sponges build their skeletons out of, well,
glass! Ok, silica, but it's the same idea.
Their skeletons are built with regular hexagon shapes - and like the modern Paleodictyon,
some glass sponges live their lives dug under the sediment. They push up
little "spouts" above the mud surface to help filter water through their bodies.
And those aren’t just for breathing - sponges are filter feeders,
so they consume whatever comes along in the water.
But that's only one hypothesis. Another option is the deep-sea
dwelling organism known as a xenophyophore.
Xenophyophores are… weird. They're a type of protist, meaning they’re
related to things like algae and amoebas. But despite being 10 centimeters or larger across,
they are actually… a single celled organism.
Admittedly, they have a lot of nuclei in their single cell, but it's still.. odd.
They live on the ocean floor, with some species of xenophyophores
living entirely underneath the sediment with little spouts like Paleodictyon,
though their bodies don't form as regular geometric shapes.
And while glass sponges are filter feeders, xenophyophores are more complicated.
We think, anyhow. It's kind of hard to tell what an organism
like a xenophyophore eats - it's not as if you can watch them chew.
But we've found evidence that some xenophyophores absorb organic material from the water,
and others seem to eat mostly bacteria… which some people have suggested they
might actually grow on their own poops… Which they never poop out, by the way.
They accumulate poop in their bodies, which feeds the bacteria,
which feeds them, which produces more poop which… it's a living, I guess.
They also build little shells of sediment,
much like the hardened tubes of Paleodictyon. And they excrete
barium crystals! There's actually a whole SciShow episode about them, if you want to know more.
So we have two potential culprits,
both of which live strange lives at the bottom of the sea and match the
profile. But when scientists went ahead and tried to get DNA evidence they found very little.
Specifically, they found NOTHING
living inside the modern Paleodictyon. They had apparently been abandoned – nobody was home.
And as far as lingering DNA evidence goes? Well,
they did find some protist DNA… but it was from other species, not xenophyophores,
and it was on the outside of the tubes, not the inside. But there wasn't any sponge DNA either.
They also didn't find any of the barium crystals that xenophyophores produce.
But their efforts weren't entirely a bust. See, these modern Paleodictyon
have something that fossil ones rarely have preserved: those little spouts,
which formed the holes that the scientists spotted on the surface in the first place.
And on closer examination, researchers have found that the pipes aren't of even heights.
The ones in the middle are slightly taller than those
on the sides - which means that when a current passes over the hexagons,
water gets forced down into the pipes more than it otherwise would.
This would help refresh the water inside Paleodictyon's tubes,
and it would force nutrients and any too-slow swimmers down into the fossil. Which would make
the hexagon net a literal net – good at trapping animals for consumption.
Now, to some scientists,
that makes it seem like Paleodictyon should be a sponge – that's how they eat things!
But other researchers have pointed out that the repeating fractal pattern of Paleodictyon is
actually typical of organisms that have a different lifestyle…
Specifically, of organisms that need a perfectly regular pattern to get exactly the right amount
of water and oxygen flowing in a system – as if, perhaps, they were doing some sort of
hydroponics thing to farm their poop bacteria, the way we think some xenophyophores might do.
And sponges usually leave their skeletons behind – they don't make
hard sediment tubes the way xenophyophores do.
Ultimately, both explanations seem plausible, but unproven.
And until we find a real living organism in one of these things, the debate will continue to rage on.
Paleodictyon is a really strange case.
It's a bizarre fossil to start off with, but the fact that it remains unidentified
really speaks to how much science is still out there, waiting to be discovered.
And it's definitely not the only mystery lurking at the bottom of the ocean.
As one scientist pointed out, the only reason we found Paleodictyon at
all was because it’s a sequence of geometrically spaced repeating holes. That kind of stands out.
Many other similarly strange trace fossils might have modern versions,
but we don't find them because they’re made by organisms much more secretive and less showy.
If Paleodictyon only had a single spout, we might never have known it was there at all.
Which is one of the amazing things about science. Who knows what the next dive will bring us?
Not Megalodon, that's for sure. But, maybe someday,
the solution to this puzzle… and probably lots of other new mysteries, too.
On the latest episode of the Eons podcast, we try to track down what exactly happened
to the lost Homo erectus fossils of World War II. Check it out now wherever you get your podcasts!
Be sure to check out our episode, “The Story of the Dino Stampede”, to learn more about trace
fossils and the challenges of interpreting them. And we’ve gotta thank this month’s /net/work
of Eontologists! – You know, like Paleodictyon means “ancient net”...
Become an Eonite at patreon.com/eons and you can get fun perks like submitting a
joke for us to read, like this one from The Lizard of Odd
Which dinosaur makes the best pet for a pirate? An AAAARRRRchaeopteryx!
And as always thank you for joining me in the Adam Lowe studio. Subscribe at youtube.com/eons
for more fantastic fossils!