Heteromorph Ammonite Lifestyle

May 26, 2005
Before I begin, let me say that I am not a paleontologist nor a biologist, and this thread is just for layman-type discussion, so my ignorance may show. :wink:

Anyway, while gazing at some wierd Heteromorph Ammonites (like Pravitoceras), a friend asked me how could mollusks with such strange shells have possibly lived?

I told him that in some species, it is believed that the animal was a passive, planktonic drifter. He replied that it was unlikely.

Me: "Why?"

Him: "Because all planktonic, passively drifting mollusks have a tendency to lose the shell and become transparent and gelatinous. For example, sea butterflies (pteropods) are shell-less snails. The only way I could see that these shells belonged to planktonic drifters were if these shells are actually vestigial, and that the actual animal dwarfed the shell, like a bubble shell or batwing slug Sagaminopteron."

Me: "So if they weren't drifters, what could they have been?"

The answer eluded us. But then, I recalled that there were certain gastropods that actually anchored themselves to a rock, then continued to grow, adding to their shell. These shells have often been mistaken for serpulid tube-worms, since they no longer confirm to a strict helical structure. These sessile snails were filter feeders.

So my idea was that these ammonites were actually sessile. Their strangely shaped shells would have served to anchor themselves in various soft sediments, while they pursued a sea-cucumber-like lifestyle. They would have an advantage over other sessile sediment-dwellers by retaining some degree of mobilty, perhaps by expanding their tissues to lift themselves up above the mud, like certain single-polyp corals do today.

Of course, I have no idea about the type of sediments that the fossils of these cephalopods were found in. So any thoughts?

I hope that this will be an interesting discussion.
Hi Mimic Octopus, and welcome to TONMO. Thanks for a very interesting first post indeed. Whatever you do, don’t worry about being a layman, being able to speculate is fun! Most of us fossil posting types here are not professionals but people with a hobby and an interest. (Except Kevin, he knows everything! :notworth: )

You have raised a great many exceptionally interesting points. I’m afraid I don’t think there are any definitive answers to heteromorph ammonoid lifestyle yet. However, I do have just the paper for you that is available in its entirety on-line:


Neale Monks and Jeremy R. Young 1998


As far as I understand, the angle of the aperture is key to this question; following floatation studies in some forms of heteromorph, the angle of the aperture is orientated so that the head must have protruded vertically so it could not have made contact with the sea floor (e.g Scaphites). However, to dreadfully oversimplify and paraphrase, Neale Monks has written that if the head was small and could be withdrawn into the shell then the creature could have rocked to the desired angle to allow it to feed on benthic organisms. Therefore a shell with a vertical aperture would not necessarily imply a non-nektobenthic lifestyle.

Taken from Neales’ article and to add to this, it is worth pointing out that:

1) No heteromorph ammonoid has a siphonic notch, indicating that they must have been poor swimmers, or had a very weak siphon at best.
2) Ammonite stomach contents tend to indicate a benthic diet e.g. small crustaceans, crinoids, ostracods. (I’m not sure if this applies to heteromorphs though as I couldn’t find any references to specific heteromorph stomach contents)
3) Heteromorphs tend to be found in clay or marl which would indicate a substrate environment. (Certainly Hamites, and other similar forms, are very common in the clays at Folkestone for example).

Adding this up it does appear that heteromorphs may, as you have indicated, had a lifestyle closely linked to the sea floor or just above it. Certainly the orthocone ammonoid Baculites is thought to have flourished head down on the sea bed with its long straight shell towering over the animal. So yes, you could well be right, perhaps these animals did indeed live a sessile lifestyle.

On another note, I think you could be onto a winner with the gelatinous theory, but not just for heteromorphs, but all ammonoids. As no confirmed soft bodied ammonite creature has ever been found the animal itself was almost certainly very delicate or had some bizarre chemical property that did not lend itself to fossilisation. We have soft-bodied belemnites, vampyromorphs and even octopi but never an ammonite, even from the same strata. Strange.

Hmm...you've got me thinking now...at bedtime too. Hmmm...

Welcome again,

Ya, maybe they filled the shells with gas and floated around above the ocean feeding on pterodactyls and their eggs. :rainbow:

I believe there are some flying ammonites in a Rudy Rucker novel, guarding a hole at the South Pole that leads into the Hollow Earth :cool2:
Ah yes, I have already read that study. And I find it interesting that no heteromorph ammonoid has a siphonic notch, implying small or weak siphons. Hmmm...

On another note, I've recently seen a picture of a fossil coral called Aulophyllum, and the caption read that "the pointed end was buried in sediment on the sea bed, and the soft polyp sat on top of the other end."

Why do I find this interesting? Because Aulophyllum has a horn-shaped or claw-shaped skeleton, almost like a hook.

From this, it doesn't take much to imagine something like Pravitoceras stuck in the sand filter-feeding like a giant tube anemone.

I am not aware of any sessile modern cephalopods, and I have found that I always thought of ammonites as ancient squids/octopods, but then I realized that some of them may have no surviving analogies of their life-styles. They may have experimented with various modes of life not represented by modern head-foots (cleaner Scaphites removing barnacles and crinoids from plesiosaurs?).
Well, an interesting idea but corals don't have gas filled chambers. Even weird ammonites such as Pravitoceras had a chambered shell that would, via the creation of an osmotic gradient in the chambers, have been free of water. With this water replaced by gas via the siphuncle and therefore being relatively bouyant surely the conch would have have floated above the body as it does with Nautilus and flotation studies with 'normal' ammonites?

Unless there was something deeply weird with the heteromorphs, I can see no scenario whereby the conch would not have floated over the body. Given that, I find it hard to imagine a sessile lifestyle with the conch under the body buried in the sand.

Scaphites acting akin to a cleaner fish to a plesiosaur? Hmm... could a cephalopod without a siphon, or a very weak one, be able to manoever to get itself into position in that way? It seems unlikely that they would lie in wait in shoals to attach themselves to a fast moving marine reptile. I can only see them spinning helplessly in the wake of a fast moving plesiosaur.

I'm sure you are right in that ammonites had varied lifestyles, I'm sure they filled many different ecological niches. Maybe I'm wrong though......
what if they had a sucker, like a modern remora, or a slug/snail foot for that matter? idle speculation since we have no soft body.
I've always held that heteromorph ammonites were purpose-bred by Cthulhu's spawn to be harvested for the constructing of jazz instruments. Prehistoric submarine bebop, as it were. :ammonite: :cthulhu: :band:

Okay, that's just silly. Sorry.
Snafflehound said:
what if they had a sucker, like a modern remora, or a slug/snail foot for that matter? idle speculation since we have no soft body.

Well, that is certainly an innovative and original suggestion! As you say we have no soft-body fossils so until a number can be found, who can say for certain.

To me, the problem with this idea is that no other cephalopods, living or extinct, have adopted this mode of lifestyle. Of course heteromorphs may have done so, but as far as I know, that makes it unlikely. Heteromorphs tended to be unstreamlined with peculiar shaped shells probably generating heavy water resistance, I can't see them adopting a lifestyle similar to a highly streamlined and manoeverable remora. Remoras swim and attach themselves with precision swimming; heteromorphs bobbing about with their bulky shells and (possibly) abscence of a siphon would probably have had much greater difficulty. If anything, if they did somehow attach themselves to marine reptiles, I'd expect they would have slowly evolved a more streamlined form.

That's only my opinion, mind!

Shop Amazon

Shop Amazon
Shop Amazon; support TONMO!
Shop Amazon
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.