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,