soft body ammonite fossils -- dogs that didn't bark?

Neale Monks

Jun 29, 2003

Mention of the Hollingsworth fossil was interesting, I saw the specimen once or twice while helping one of the collectors try to image and interpret the thing. It is very strange, and at best it seems to have something in the body chamber but I wouldn't say for certain it was an ammonite body.

What's much more interesting is that in the places soft body squids turn up (e.g. Solenhofen limestone, Christian Malford clays, etc.) ammmonite shells can also be found, but no soft body ammonites. I wonder if this is telling us something? Perhaps ammonites had glutinous bodies that didn't preserve? Or maybe they weren't strongly attached to the shell? Maybe they could even leave the shell if they wanted to and used it for egg laying or something? All speculation, of course, but curious.



I'd wondered if the soft-body tissues in the Hollingworth Sigaloceras mightn't have proved illusory. As you probably know, a heart was claimed to have been imaged in the chest-cavity of a Cretaceous dinosaur some years ago, a claim that was met with no little skepticism. Mr. Hollingworth and his co-authors at Cardiff couldn't be blamed for deciding not to make any definitive statements about their "ghost of an ammonite past."

Still, it would be nice to see the digital images of Sigaloceras.

Your observation about the paucity of ammonite soft-bodies in places where fossilized squid turn up is a striking one. Looking at the Proteroctopus fossil that came out of Voulte, I half-wondered if it might have been an ammonite evacuated from its shell.

Thanks for the information, Neale.

A few of us here have had a mini-campaign ongoing over the last few months to try to obtain an image of the Hollingworth ammonite, but to avail, unfortunately. It is interesting that you hint that the results are very much open to interpretation, so presumably there is no obvious soft-body morphology after all. Relying on a few chance remarks we have managed to obtain on the internet, I think that we had hoped that Dr Hollingworth was sitting on a fossil Rosetta Stone, it seems that was not the case afterall!

Do you think the structures you observed could just possibly be the buccal mass or the stomach contents? Just a thought?


Re the egg chamber (as a possible use for the shell), an interesting parallel with Argonauta, but it's only the female in this instance that carries the shell (and you've got both [supposed] male and female ammonites, micro- and macroconchs).

It is a pity that the females of other similar pelagic octopod genera, like Ocythoe, Tremoctopus, and Haliphron have lost their shell (if they ever had one) - in fact that goes for the males also. It is also a crying shame that the fossil record is defficient when it comes to further variants on this 'paper nautilus' plan.

Phil, Kevin, did you not talk about some adductor-type muscle scars in the body chamber of the ammonite shell? Neale, can you shed further light on this. Muscle attachment scars would indicate that the animals (at least some of them) were attached to the shell.

I'm still not convinced that the shell was external on all ammonites/nautiloids. There's plenty of scope there for this having been internalised on some representatives.

Best go get a coffee
Perhaps ammonoids were just a little more tasty than calamari, and got eaten before they reached the sea floor.

I have often wondered if octopods were nude ammonoids? (there was a paper on this, but I can't locate it now [strike 2]). The way octopods have an inclination for coiling their arms, could it be in their genes? I can just envision an octopus floating in it's shell with all it's arms coiled, perhaps the same color and shape as the shell. Which one would a predator choose? Or is this just a case of the tail wagging the dog (or the leg wagging the octopus)?


I've got digital images of the specimen Jason Hilton brought into the NHM, but I'm obviously not able to use them without his permission. But what you basically have is an opaque "shell" section to the spiral and then a clearer, rather like amber, "body chamber" final half whorl. Inside this is a dark inclusion, with no particular shape, obviously different to the matrix filling the living chamber, but not octopus or squid like in any way. There was a picture of it in the New Scientist magazine some years ago. Under UV light the shape of the thing is a bit more obvious, pointy at the back end, and spreading out into threads at the front.

It could be the body, shrunk away from the inside of the shell after partial decay but for some reason this decay was arrested and the fossil formed around it. Perhaps it is the whole animal, or maybe just the residue of the tougher parts like the muscles, it isn't obvious to me. But I don't know enough about this fossil or fossilization in general to be an authority either way as to what this fossil shows us. It's sad though that pictures of the thing haven't been distributed more widely.

I don't really buy into the notion ammonites routinely used their shells as egg cases. They are just too diverse for this, and some (like the heteromorph ones) are obviously adapted to some quite bizarre lifestyles way different to the regular ammonite one. Having said that I don't see a real problem to ammonites abandoning their shells, and maybe even secondarily evolving a mechanism to make the pseud-shell egg case.

Muscle scars on ammonites are well documents and reasonably well understood. Compared to nautiluses the muscle scars are relatively small. I'm pretty sure they didn't ventilate the mantle cavity in the exact same way as nautiluses, and probably didn't swim that way either. Looking at the long, narrow body chambers most have I think it more probable they used the retractor muscles primarily defensively, to withdraw into the shell like a snail. If they swam at all it was using a coleoid-like jet system. In fact in many ways I think the clue is to compare ammonites with marine gastropods -- perhaps it is not a coincidence that neogastropods diversified the same as ammonites declined. I find it much easier to visualise ammonites as benthic predators like, say, cone shells or whelks than anything else. Heteromorphs may have been different in some cases at least, probably plankton feeders in the case of things like Turrilites and Baculites.

The muscle scars do at least indicate the ammonites were attached to the shell generally speaking. But there is a big difference to being loosely attached and being able to extend in and out (like a snail) and being welded in like the modern nautilus.


Neale Monks said:
If they swam at all it was using a coleoid-like jet system. In fact in many ways I think the clue is to compare ammonites with marine gastropods -- perhaps it is not a coincidence that neogastropods diversified the same as ammonites declined.


Am I right in thinking that some of the earlier Ordovician nautiloids have a notch cut into the aperture of the shell that may have held the hypernome? I can't remember exactly where I read this, but if so, and given that the ammonoids evolved from these earlier creatures, would be safe to assume they had a similar, or derived, propulsion system? Are there equivalent notches in any of the ammonoids, I wonder?

There was some talk on this site a while ago about the possibility of some ammonoid shells being purely internal, as with Spirula. We were just kicking around a few ideas but I found this rather bizarre and interesting specimen the other day whilst searching around the internet that made me think again:

An unusual ammonite from the Yorkshire coast

This specimen appears to have suffered damage from an attack and grown back asymetrically. If the shell was external, as I'm sure was the case, I wonder how the shell could have grown back? I've always thought that the shell was 'dead' material with the only living section the siphuncle running its' length. The only way that I can imagine the shell undergoing repair is for it to have been surrounded by the mantle at the point of damage.

I suppose it would be interesting to see if the shell of Spirula has any external scarring from muscle attachments and looking to see if there is any equivalent scarring in the ammonoids. The clincher for me that shells were external is that photo of the ammonite in your book with the pigmentation surviving (Wow!), but perhaps, just possibly, this was not so in all species.

Just thinking out loud here!

Thanks again.

I'll take a crack at that one Phil,
Looks to me like the body chamber was broken back on one side where the ribs were close together, but the critter had almost matured and was starting to make mature modifications to it's shell (stronger and more distant ribs) so when it started to repair the damaged portion it repaired it with stronger and more distant ribs.
Of course this could only happen if the damage did'nt destroy the shell producing part of the mantle.
By the way, what is the shell producing part of the mantle? Is it the whole mantle or just the front (and rear, to form new septa)?
Thanks Kevin. Your explanation makes perfect sense as always!

As for your question, I'm afraid I don't know.....perhaps Neale can help.