Internal structure ammonite

[Nikodeimos]

Dear All,

 

Another ID request for you, though only of part of a fossil. I recently acquired a Macrocephalites madagascariensis, originally found in Anwil, Switzerland. It has some sort of internal structure in the shell (see attached photos), but I'm not entirely sure what it is and searching for ammonite anatomy on the internet hasn't given me the answers I've been looking for either. Would any of you happen to know what it is? Many thanks in advance!

 

Nick

 

 

[Tidgy's Dad]

The big hole in the bottom of the picture is the siphuncle.

The rest of it is the form of the chamber wall of the ammonite that separates it from the previous chamber and provides attachment points for the soft tissues of the living organism.

It is a very beautiful specimen.

Edited December 18, 2023 by Tidgy's Dad

[Ludwigia]

Adam describes it well, or to put it another way, these structures are also called the septal walls or septum, which separate the various chambers which make up the phragmocone, or inner buoyancy apparatus of the ammonite. This website page gives a good description of ammonite morphology.

[Nikodeimos]

I get it now! For some reason I thought the chambers would be rather empty, but this certainly make sense. The part where the septum comes into contact with the outer shell is where the sutures are then, hence their unusual pattern?

[Ludwigia]

1 hour ago, Nikodeimos said:

I get it now! For some reason I thought the chambers would be rather empty, but this certainly make sense. The part where the septum comes into contact with the outer shell is where the sutures are then, hence their unusual pattern?

The chambers are often filled with sediment, although sometimes they are empty or crystalized. And you are correct in your assumption about the sutures.

[BobWill]

On 12/18/2023 at 1:45 PM, Tidgy's Dad said:

provides attachment points for the soft tissues of the living organism.

 

This is confusing to me. I always thought the septum was simply the surface formed by the secretions from the part of the mantle at the rear of the body. I thought that when the animal moved into a newly formed section of shell formed at the aperture it pulled away from the previous septum and settled into place just beyond it to begin secreting material to form the next one. At that point it is in contact with the inner surface of the shell but not attached to anything.

[Tidgy's Dad]

2 minutes ago, BobWill said:

This is confusing to me. I always thought the septum was simply the surface formed by the secretions from the part of the mantle at the rear of the body. I thought that when the animal moved into a newly formed section of shell formed at the aperture it pulled away from the previous septum and settled into place just beyond it to begin secreting material to form the next one. At that point it is in contact with the inner surface of the shell but not attached to anything.

I'm no expert.

But I think that you are mostly correct, except that the animal must have been "glued" to the latest chamber wall to some extent or it would just fall out.

Obviously, this could be unglued when the animal moved to a larger chamber.

[BobWill]

29 minutes ago, Tidgy's Dad said:

the animal must have been "glued" to the latest chamber wall 

If that were the case then what would hold it in place as it begins building the new septum? If it needed anchoring then perhaps some temporary adhesion to the inside of the shell along the next eventual suture line may have been enough. I would guess this has been studied with extant nautiloids but I'm no expert either.

[Ludwigia]

5 hours ago, BobWill said:

If that were the case then what would hold it in place as it begins building the new septum? If it needed anchoring then perhaps some temporary adhesion to the inside of the shell along the next eventual suture line may have been enough. I would guess this has been studied with extant nautiloids but I'm no expert either.

 

Maybe this paper on ammonite muscles could help answer the question.

[BobWill]

Thanks for the link @Ludwigia. This makes more sense than attaching to something that starts out not-even-there and is very thin for much of the time in contact. I thought someone would have probably researched this since other mollusks left the scars too but I've never seen scars or even looked for them. 

[Nikodeimos]

On 12/20/2023 at 10:21 AM, Ludwigia said:

 

Maybe this paper on ammonite muscles could help answer the question.

 

Very fascinating! Thanks for sharing.