Occasional paper: When Armor Met Lips

by Doug Muir on March 16, 2024

So about five hundred million years ago, give or take, there was this little creature called Plectronoceras.  It was about 2 cm long — just under an inch — and it had a conical shell with a bunch of tentacles sticking out.  It was a cephalopod, an early member of the group that includes octopuses and squid.  And it was an /armored/ cephalopod, with most of its soft body protected by that hard little shell.

Let’s pause here and rewind:  this was five hundred million years ago.  That’s the late Cambrian, if you’re a geology nerd.  It’s before the dinosaurs.  It’s before sharks or cockroaches or ferns.  This is *old*.  Complex life had barely gotten started.  Life in general was pretty much confined to the oceans.  But there were no fish yet — just invertebrates.   Half a billion years, yeah?  Long, long time.

And a lot of the stuff swimming around was weirdly alien.  Again, if you’re a geology nerd, you know about stuff like Opabinia, Anomalocaris, or Hallucigenia.  If you don’t, then let’s just say that you wouldn’t have recognized much from those ancient seas.  Not just “no fish”.  There were no clams or lobsters, no starfish or barnacles or crabs or anemones, no coral or kelp.  The world was new.  Those things hadn’t evolved yet.

But almost from the beginning, there was this thing: shell, plus tentacles.

If you have even the slightest interest in the ancient world, you’ve probably heard of ammonites.  The usual narrative goes something like this:

“Ammonites were armored cephalopods, relatives of squids and octopi (1), but with a hard external shell.  They lived in the oceans for hundreds of millions of years.  Then they all died out in the mass extinction at the end of the Cretaceous, the same extinction that wiped out the dinosaurs.  Today there are only chambered nautiluses, which look somewhat like ammonites but are a different sort of creature.”

Human-size ammonites swam the Atlantic Ocean 80 million years ago | Live  Science

And this is true as far as it goes, but it misses a couple of key points.

First, ammonites were just one sort of armored cephalopod.  That design seems to have evolved independently several times.  And… sure, think about it.  It’s actually a great design.  It combines the two best features of being a mollusk: a hard protective shell, and grasping manipulative tentacles.  Modern mollusks have to choose one or the other.  But hey, por que no los dos?

Armored cephalopods were free-swimming, or rather free-floating: they filled their shells with gas, so they floated along at zero buoyancy.(2)  They were mostly slow, but could put on sudden short bursts of jet-propelled speed, just enough to catch prey or avoid danger.  They didn’t “rule the seas” or anything like that, but for hundreds of millions of years they were around pretty much everywhere, in every marine ecosystem from the poles to the equator.   

If you want to buy a fossil today? and you want something that looks cool but is not too expensive?  A fossil ammonite is your go-to.  Some years back my kid found one, pretty intact, just lying in a field.  That’s how ubiquitous they were.  Because it was a great design, yeah?  Like the cockroach or the shark:  basic, efficient, central.

Ammonoidea - Wikipedia

Second thing, the chambered nautilus.  It’s not an ammonite, no, but it’s like an ammonite — it’s another armored cephalopod, a spiral snail-like shell plus a bunch of tentacles.(3)  But where earlier armored cephalopods were everywhere, the nautilus is restricted to a small-ish patch of ocean in the western Pacific, from northern Australia through Indonesia up to Vietnam or so.  And while ammonites floated boldly everywhere, the nautilus is a shy creature that lives fairly deep down, mostly in the twilight zone 100 to 500 meters deep.

Ammonites - British Geological Survey

(Years ago, I used to scuba dive in that area.  My dive instructor told me that a nautilus never comes above 100 meters depth unless it’s seriously ill or injured.  Meanwhile, a recreational diver should never go below 40 meters depth — nitrogen narcosis, oxygen poisoning, the bends, just don’t. “So if you’re diving and you see a nautilus,” he said, “at least one of you is in big trouble.”)

Okay, so: why? Why is the nautilus restricted to this small area, when older armored cephalopods roamed worldwide? Is the nautilus just… worse?  An inferior design, or something?

No.  Here’s where things get mysterious and interesting.  After the asteroid hit, the ammonites all died out.  But chambered nautiluses survived, and they thrived!  They weren’t as ubiquitous as the ammonites, but for about 30 million years they were found all over the world.  Dozens of species, all sorts of different habitats.  The “shell, but also tentacles” design was still solid.

But then around 30 million years ago — halfway through the Age of Mammals, give or take — something happened.  The nautiloids started disappearing.  Fewer species, less diversity.  Bit by bit they shrank back into their current small range.

What happened halfway through the Age of Mammals?  Well, here’s one clue: the nautiloids’ long retreat showed a pattern.  It wasn’t everywhere and all at once.  They disappeared first in the northern arctic regions; then in the Antarctic; then in temperate zones; finally across most of the tropics except that one small patch.  This pattern suggested a culprit: a warm-blooded predator that evolved in the Arctic and then spread around the world.

But… the armored cephalopod design had been around forever.  They’d been living with predators for *half a billion years*.  Sharks.  Primitive armored fish.  Not-so-primitive modern fish.  In the age of dinosaurs, they had to deal with ichthyosaurs, plesiosaurs, and mosasaurs.  Back in the Paleozoic, they were hunted by eight-foot-long giant sea scorpions.  Way back in the Cambrian, they had to live with the anomalocariids.  In the early Age of Mammals, there were primitive whales and sea-going crocodiles.  The armored cephalopod design took them all in stride and kept going.  

So what happened?

— The authors of this paper here think that they’ve found the answer.

To understand, we have to zoom in on exactly what happens when something tries to eat an armored cephalopod.  Threatened, the beast withdraws its tentacles into its shell and clams up. Okay, so now either you’re big enough to crunch through the shell, or you’re not.  We know that some large predators did crunch through, because we’ve found fragments of ammonite shell fossilized in the stomachs of plesiosaurs and ancient sharks and such.

But if you do that, now you have a stomach full of tasty mollusk meat… mixed with unpleasantly sharp and indigestible shell chunks.  An option if you’re hungry enough, but hardly ideal.  Shark, ichthyosaurs, marine crocodiles: it’s not that they couldn’t eat armored cephalopods.  It’s just that it usually wasn’t worth it.  Predators have to do a cost-benefit analysis, yeah?  And for half a billion years, armored cephalopods were in the black.

But then something evolved with a feeding apparatus that could separate the meat from the shell.  And suddenly, armored cephalopods were in the red.

The paper says: it was seals.

Are leopard seals dangerous towards humans? - Quora

Seals — pinnipeds, if you’re fancy — aren’t usually that large, and aren’t usually apex predators.  (Predators, absolutely yes.  Apex, no.)  (4)  So what do seals have that mosasaurs and sharks and sea scorpions didn’t have?

Lips.  Seals have lips.  Lips, cheeks, and a muscular tongue.

A seal can grab a shelled prey, and puncture the shell with sharp strong teeth — and then just *schlorp* out the tasty meat inside.  The technical term for this is “suction feeding”.  Pinnipeds generally are good at it, and some are so good that they prefer to eat shelly prey — clams, crabs, mussels, whatever — and don’t eat much else.

Stuff that lives on or near the ocean floor — clams, crabs, lobsters, and such — could evolve various defensive and avoidance strategies.  But free-floating armored cephalopods?  All they had was a quick burst of speed, and that wouldn’t help much against a hot-blooded predator that could maintain high speeds much longer.

And everything else matches, too.  Pinnipeds evolved about thirty million years ago.  They showed up first in the colder parts of the northern hemisphere, then in the Antarctic, then in temperate zones.  Even today, although there are some tropical species, they’re mostly cold-to-cool water creatures. 

And — here’s the smoking gun — the one part of the globe they still haven’t colonized?  The southwestern Pacific, from northern Australia across Indonesia.  The modern range of nautiloids is exactly the one part of the ocean that those sucking, slurping seals haven’t reached yet.

And that’s all: just a little story about evolution.  “Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot will be shot.”




Doug Muir 03.16.24 at 3:49 pm

(1)  If you’re inclined to be pedantic about the plural of “octopus”, pause and ask yourself this question:  “Am I a sophomore in college?”  If yes, please go right ahead!  If no… dude.

(2) They could actually control their buoyancy, and in a very cool way, but this post is really long enough already.

(3)  If you’re inclined to be pedantic about the nautilus’ limbs and say that /actually/ they are “arms” and not “tentacles” because tentacles have suckers on them, then (a) congratulations on remembering that long-ago biology class, and (b) see Footnote 1, above.

(4) Yes I am aware of walruses and elephant seals.  Most seals are medium-sized animals, human size or smaller.  As for being an apex predator, seals are a very popular food item for the larger sharks, and even leopard seals get bodied by orcas.


oldster 03.16.24 at 4:20 pm

So, you’re saying the nautiloids did okay until the pinnipeds sealed their fate?


oldster 03.16.24 at 5:11 pm

And als0 — congratulations on finding a photo of a seal saying,
“yeah, it was me. And I’m not saying sorry. What are you going to do about it?”


Cervantes 03.16.24 at 5:35 pm

There seems to be an obvious logical flaw here. Cephalopods without shells, that are just as tasty, are doing just fine, and there are a lot more critters that eat them than just seals.


Kenny Easwaran 03.16.24 at 5:59 pm

I love this!


Doug Muir 03.16.24 at 7:02 pm

@4, what flaw? Cephalopods without shells are either much faster and more maneuverable (squid) or mostly hang out on or near the ocean floor (octopi) where, if threatened, they can suddenly squeeze their amorphous bodies into the tiniest of shelters. Or they’re cuttlefish, who do both. Also, most non-armored cephalopods can change color, allowing some very sophisticated camouflage; can’t do that with armor on.

(There are some interesting edge cases. The vampire squid is not a squid, but rather a rare octopus that hangs out in open water instead of crawling along the bottom. It survives because (1) it mostly lurks in deep, low-oxygen waters with almost no predators, and (2) it has an astonishing threat display.)

Doug M.


Michael Allen 03.16.24 at 7:37 pm

“Threatened, the beast withdraws its tentacles into its shell and clams up.”

I see what you did there. Well played.


Michael Allen 03.16.24 at 8:55 pm

Lots of videos about the vampire not really a squid on YouTube. I had no idea.


Alan White 03.16.24 at 11:36 pm

Another lovely post in both content and expression. The thesis makes sense to me. Now to go on amazon for ammonites. . .


Matt 03.17.24 at 12:55 am

the plural of “octopus”,

It’s octopuseses, right?


Phil 03.17.24 at 11:11 am

Strong Wikipedia opening:

“The vampire squid (Vampyroteuthis infernalis, lit. ‘vampire squid from hell’)”

The vampire squid, or, to give it its technical name, the vampire squid from hell…


Cervantes 03.17.24 at 3:43 pm

I dunno about that Doug, what’s the diet of Physeter Catodon?


Spongebob 03.17.24 at 7:10 pm

Does it gaze at the sun with its wandering eye?


CO 03.17.24 at 7:54 pm

You’ve gotten “in the red” and “in the black” backwards. Thanks for this article!


drs 03.17.24 at 11:24 pm

“Cephalopods without shells, that are just as tasty, are doing just fine, and there are a lot more critters that eat them than just seals.”

A key difference is their reproduction. Checking wikipedia, the chambered nautilus takes 5 years to reach maturity, which is longer than most octopi even live: “Unlike most other octopus species, whose lifespans normally span only one year, the giant Pacific octopus has a lifespan of three to five years”. Even the giant squid “are thought to reach sexual maturity at about three years old”.

Why are nautiloids so slow? According to my notes from Squid Empire, they have big eggs and thick shells. Ammonoids evolved to have lots of eggs, thinner shells with complex sutures for strength, and faster growth. Probably why they evolved more diversity, but maybe also part of why they didn’t pass the K-T boundary, while nautiloids might have survived a dearth of plankton better.

At any rate, they are slow. Shell-less cephs can better survive predation via massive spawning and rapid growth, in addition to their other advantages Doug described.


Bernard 03.18.24 at 3:30 am

Finally I can say “seals suck” and no one can disagree with me.


Evan L Howell 03.18.24 at 8:03 am

a, armored cephalopod 7
then what are we doing to seals??


oldster 03.18.24 at 1:39 pm

CO @14 —

“You’ve gotten “in the red” and “in the black” backwards. Thanks for this article!”

“Predators have to do a cost-benefit analysis, yeah? And for half a billion years, armored cephalopods were in the black.
But then something evolved with a feeding apparatus that could separate the meat from the shell. And suddenly, armored cephalopods were in the red.”

It seems backwards if you view it from the perspective of the wrong agent.
For the predator, the transaction went from unprofitable (red) to profitable (black). But for the mollusks, their business prospects went from black to red. I suspect that Doug just unconsciously switched his perspective from the predators to the prey.


Cervantes 03.18.24 at 1:59 pm

“Ammonoids evolved to have lots of eggs, thinner shells with complex sutures for strength, and faster growth.” Well okay, but if that’s really the story nautiloids could have evolved to mature and reproduce faster — fish hunted by humans did that in a few generations. Also, seals have to mate, give birth, and rest on land so they are limited to coastal areas. Predation by seals has no effect on pelagic species.


Doug Muir 03.18.24 at 6:14 pm

“Also, seals have to mate, give birth, and rest on land so they are limited to coastal areas.”

— That’s a hard “incorrect”. Seals don’t need to rest on land. They do still come ashore to give birth and nurse young pups, true enough. But that’s only for a few weeks per year. The rest of the time, they’re completely free of land.

It’s not at all unusual for seals to travel thousands of miles from their rookeries. Some species, it’s true, like to hug the shore — monk seals, harbor seals — and others, like the walrus, don’t like to leave the continental shelf. But elephant seals, leopard seals, northern fur seals, crab-eating seals and a bunch of others are absolutely long-distance predators. Like, northern elephant seals? Breed on the coast of California… and have been recorded as far west as Hawaii, ~3000 miles away.

TLDR: many, perhaps most species of seals will cheerfully turn up in mid-ocean, far from land, if there’s anything there for them to eat.

Doug M.


John Q 03.19.24 at 12:31 am

Another great post! I vote for “octopus” as both singular and plural.


J-D 03.19.24 at 2:07 am

I vote for “octopus” as both singular and plural.

My reaction is that of Thomas Huxley encountering Darwinism: how extremely stupid [of me] not to have thought of that.


Jonathan Hallam 03.19.24 at 11:42 am

This rather raises the question of why nothing else evolved lips earlier? Why do we not see lipped sharks or lipped seagoing alligators… or is this an issue that soft tissue doesn’t fossilize well – that they did exist but we just can’t tell from current evidence? But if they did exist, why don’t they still exist? Did they hunt their tentacle-shelled prey to extinction and the die out themselves? Puzzling.


Doug Muir 03.19.24 at 5:59 pm

“This rather raises the question of why nothing else evolved lips earlier?”

There are some non-mammals that have lips. Bony fish, in particular, have a number of lippy species. But AFAIK, mammals are the only animals to develop the full package of fleshy muscular lips, large thick cheeks that cover much or most of the mouth even when the mouth is open, and muscular but highly flexible tongues. So, a few shark species have thin lips, but shark tongues are usually thin little strips of cartilage and no shark has much in the way of cheeks.

Put another way, mammals seem to be the only animals that are able to schlorp.

Consider this: pull the corners of your mouth back hard, exposing your teeth while fixing your lips in place. Now try to imagine drinking through a straw. Not great, right? Even if you press your lips together, it doesn’t work very well, because your constricted lips can’t close airtight.

So: crocodiles, sharks, ichthyosaurs: these animals all had formidable jaws and teeth. But not one of them could drink through a straw, because they couldn’t use lips and cheeks to form… well, a seal.

Doug M.


Edward Gregson 03.19.24 at 11:07 pm

If the disappearance of nautiloids coincides that well with the appearance of seals, then seals it probably was, but I’m not sure I buy the explanation. Mosasaurs were basically monitor lizards with flippers, and like monitor lizards they probably had (or could have developed) pretty long, muscular prehensile tongues. There was even a mosasaur that specialized in just hard-shelled prey (https://en.wikipedia.org/wiki/Globidens).

If you really want to be pedantic, the tentacles aren’t really arms or tentacles in the manner of octopuses and squids; they’re thinner, less muscled appendages called cirri and the nautilus has 60 to 90 of them. They don’t have suckers, but the prey-catching ones are sticky.


tom 03.20.24 at 2:30 am

The whole article & comment thread was for the sole purpose of getting to that comment, Doug! Well played sir!


Doug Muir 03.20.24 at 7:07 am

“There was even a mosasaur that specialized in just hard-shelled prey”

Sure. The term for predators that specialize in crunching up shells is “durophagous”, and durophagy has evolved multiple times. There were durophagous ichthyosaurs. There were durophagous sharks… and there still are today! Google “zebra shark jaws” for something interesting.

But there’s a slightly counterintuitive principle at work here: specialized predators don’t usually hunt prey species to extinction. There are various reasons for that, but a big one is that if prey numbers start to decline from predation, the numbers of the specialized predator will decline even faster. So predators of this sort tend to evolve into sustainable coexistence, often showing adaptations such as slowed growth, lower metabolisms, and much lower rates of reproduction.

[I just oversimplified some really complicated stuff there, so any biologists still reading this, don’t @me.]

The nightmare scenario for a prey species isn’t a specialized predator. It’s an efficient generalist predator that can hunt you relentlessly, right into extinction, because when you’re gone they can switch right back to eating something else.

Doug M.


KT2 03.21.24 at 7:19 am

“It’s an efficient generalist predator that can hunt you relentlessly, right into extinction, because when you’re gone they can switch right back to eating something else.”

Generalist predator?
“The possibility that empathy resides in parts of the brain so ancient that we share them with rats should give pause to anyone comparing politicians with those poor, underestimated creatures.”
 —Frans de Waal



KT2 03.21.24 at 7:25 am

Oops – meant to give weight to octopuses.
Tim Sommers in “The Theory of Mind That Says Artificial Intelligence is Possible” said …”What about octopuses? (That’s right, “octopuses” not “octopi.”)”


J-D 03.22.24 at 12:09 am

The featured article on Wikipedia today is about pinnipeds. It doesn’t mention nautiloids specifically, but it does mention that cephalopods in general (along with fish) are a usual prey of pinnipeds and also that walruses are (it says) unique in being suction feeders, using their tongues (it doesn’t mention lips) to extract the meat of bivalves from their shells.


Doug Muir 03.22.24 at 6:52 am

Walruses aren’t unique at all. They’re just unusually good at it.

TBF, they’re /really/ good at it. Walruses in captivity will suck hard on anything suckable, sometimes doing damage to their enclosures. A walrus can literally suck the paint off the walls.

Doug M.


engels 03.22.24 at 11:48 pm

the plural of “octopus”

Hexadecapus (if there are two) etc

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