I’m sure you’ve noticed that animals like octopuses, squids,
and nautiluses are generally squishy creatures...
So it might surprise you to hear that this group, the cephalopods, actually has a pretty good fossil
record -- with one major exception that, for about 30 years, was a scientific mystery.
Most of the fossil record of these animals is made up of their few hard parts.
The hooks on their arms and their beak-like mouths are made of chitin,
just like the exoskeleton of insects.
And their shells, which can be internal or external,
are either chitin or calcium carbonate, so they tend to fossilize pretty well.
But there are also some ancient cephalopods with truly beautiful soft tissue preservation.
The places where we find these remarkable fossils are lagerstatte: sites of exceptional
preservation, which are famous for the number and quality of their fossils.
But in these fossil sites that seem to have it all,
what isn’t preserved can be just as intriguing as what is.
Because we’ve found lots of the remains of extinct squid-like creatures called belemnites,
as well as the soft tissues of ancient octopuses.
BUT we’ve never found the fossil of any actual squids.
And they should be there.
The squid lineage is as old as the belemnites’, and we do find fossilised hard parts from
their closest relatives - and that means squids must have been around then, too.
So if squids were swimming around in the same oceans as their closest cousins,
where did all the squids go?
No, seriously. Where are all the fossil squids?
The story starts in 1986, when two paleontologists had a realization that would lead to a major
shake up of the family tree of cephalopods.
Today, we often split the living cephalopods into two groups. There are the coleoids,
which have soft bodies, and either a very reduced internal shell or none at all:
these include the
the octopuses, squids, and cuttlefish.
And yes, it’s octopuses! Not octopi! Because octopus is a Greek word, not Latin.
And the preferred plural of squid is squids, when you’re talking about multiple species.
Now. What was I saying?
Oh! In addition to the coleoids, there are also the nautiloids,
which have a hard outer shell. The only living members of this group are the nautiluses.
Now throughout the 19th century, most fossil coleoids were assigned to the squid family,
based on the shape of their internal shell, which was thought to be exclusive to this group.
But what those two paleontologists in the 80s realized was that many cephalopod fossils that had been
assigned to the true squids didn’t have the defining trait of squids.
A squid is only a squid if it has eight arms and two tentacles.
Yes so I know what you’re thinking; This means Squidward is not a true squid.
And what’s the difference between an arm and a tentacle, you ask?
I had to ask, too: Turns out arms tend to have suckers along the whole length, whereas
tentacles only have them close to the end.
So all those fossils that were classified in
the 1800s were moved out of the squid group and into a new group: the Vampyromorpha.
This group was redefined to include the vampire squids -- which are also not true
squids -- and octopuses, based on their shared anatomical features, like having eight limbs.
There's still some debate about this grouping, but what does seem to be the case is that
not a single definitive cephalopod fossil with
a squid-like set of limbs - eight arms and two tentacles - has been found to date.
What makes this even weirder is that we have fossils and even
some soft tissue fossils of belemnites, the ancestors and relatives of squids.
And data from molecular clock estimates suggest that true squids have been
around since the Late Jurassic Period. So, why aren’t there any fossils of squids?
It wasn’t until 30 years after that reclassification in the 80s that we
got an answer - and it didn’t come from the fossil record.
In 2017, a study investigated what happens to squids and octopuses after they die.
This research came from the subfield of paleontology called taphonomy, which is
the study of what happens after things die, like burial or decay, and how they become fossilized.
Taphonomists often don’t just study fossils; they also perform experiments,
like trying to replicate the conditions of fossilization as closely as possible...
Or watching something decay in real time to see what happens
and what it might end up looking like in the fossil record.
These kinds of experiments were the key to solving the case of the missing squids.
For this experiment, dead squid and octopuses were monitored as they decayed naturally in a lab.
Which sounds pretty...unpleasant. And extremely stinky. And, how is that a job? That people have?
We actually looked into it and it turns out
the watching dead things decay job was given to graduate students
because of course they were
In any case, the results shed some light on the mystery.
The octopuses decayed the way the researchers expected them
to - their bodies stayed more or less intact.
But the squids virtually fell apart, and, by the end of two weeks, they were just piles of mush.
And in addition to that, something fishy was going on with their pH.
When we find the soft tissues of other cephalopods in the fossil record,
they’ve generally been phosphatised.
This means that, during the fossilization process,
their tissues have been preserved as calcium phosphate.
And it’s been calculated that, in order for this type of fossilization to happen,
the pH of the remains must fall below 6.38 on the pH scale, becoming acidic.
While the pH of the rotting octopuses fell below that threshold,
the squids remained well above it -- which means that it would be impossible for them to fossilize.
Mystery solved!… kinda.
I still want to know
why? What makes the pH of rotting squids different from that of rotting octopuses?
Well, the researchers had an idea about that, too. It all comes down to lifestyle.
Octopuses spend most of their time on the seafloor, while squids tend
to swim around in the water column.
This means that they have different
buoyancy requirements. And if squids are more buoyant, then they don’t have to spend
as much energy keeping themselves afloat.
And one adaptation that some squids have for this is
retaining ammonia, which would usually be excreted as waste in other animals.
This is because ammonia is a little lighter than water, so it gives them the buoyancy they need -
but they need a lot of it. In fact, over 50 to 60 percent of a squid's body mass is ammonia fluid!
But since octopuses generally live on the seafloor, they don’t have this adaptation.
And in addition to being lighter than water, ammonia also has another trait
that makes it important in this story: a high pH level, which makes it a base.
In those decay experiments, the high concentration of ammonia actually
prevented the pH of the specimens from falling below the threshold needed for fossilization.
So if ancient squids had the same adaptation for buoyancy, that would explain why
we just don’t find fossil squids.
Mystery solved, for real this time!
And, in solving this mystery, paleontologists might also have figured out why there aren’t
very many soft tissue fossils of another group of extinct cephalopods, the ammonites.
Ammonites are more closely related to the coleoids than to the nautiloids,
but they had an external shell, which we’ve found many fossils of.
And the general lack of soft tissue fossils from this group suggests that they might’ve
had enough ammonia in their tissues to disrupt fossilization, too.
But then!, paleontologists got really lucky.
In January 2021, a fossil of the soft tissues of an ammonite, without its shell, was described.
And the fossil showed us pretty much the animal’s whole soft body,
minus its arms, coiled up with many of its organs preserved.
The researchers think that the absence of a shell could
be due to this ammonite having been preyed upon by something,
but whatever was trying to eat it dropped it’s prize after it got it out of the shell.
This rare find gives us hope that somewhere out there, there’s a soft-bodied squid fossil,
just waiting to be found. And I for one very much want to see it.
For now, though, we’re closing the case on the mystery of the missing squids,
thanks to some very gross detective work.
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like this one from Andrew, ok here we go,
Why do Tyrannosaurs hunt Triceratopses?
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Ugh that hurts
And as always thanks for joining me in the Konstantin Haase studio.
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