Please help me identify this artifact within an ammonite fossil...

[dbarshlomo]

@Auspex

Thank you for the interesting input!

I tried to be as careful, meticulous and patient as I can when I removed the surrounding matrix from that part (well, at least for an amateur I guess), so I am curious if you’re sure the inner whorl is missing?

I’ll add a few additional images that show, as far as my understanding goes, the ribs of the inner whorl. They are not very well preserved but I think they are still visible.

Am I wrong? Doesn’t it mean this must be the outer surface of the inner whorl..?

[Auspex]

I guess the inner whorl is more intact than I thought. I find it odd that there is such an offset, roughly half the width of the successive whorl.
Could it be a heteromorph, with wildly offset whorls?

[dbarshlomo]

@Auspex

I’m not very familiar with the professional jargon and terminology, so I’ll use an image I found to ask my question:

When you mention the offset in width from one whorl to the next, do you refer to “a” or “b”?

 

I don’t think this is a Heteromorph Ammonite, it seems to coil along a single plane.

[Auspex]

You see, though, how depressed the inner whorl is in your fossil, so it is either offset, or missing.

[dbarshlomo]

@Auspex

I do. But I admit I don’t see the depression level as extreme as you seem to.

In the attached image I compare 2 ammonites from the same site, and while the depression is larger in the fossil we discuss, it’s not that wildly different from the other’s.

 

Having said that, I think you’re probably right and that the inner whorl is missing or partially missing. It also makes better sense when regarding the siphuncle’s location.

I have a question, does the siphuncle always go along the symmetry plane of the ammonite?

 

Anyway, the only mystery left for me is what are these rib-like features in the middle? They just seem too perfectly organized, in their locations, sizes and orientations to be just a coincidence, don’t they?

[Auspex]

It seems like much of the face of the inner whorl has been lost:

[BobWill]

This is something I've seen before. Sometimes the siphuncle seems to get dislodged from it's original position in the process of preservation or possibly predation. The siphuncle is directly against the inside of the ventral keel and if the keel is worn off enough it can even be seen along a portion of an outer whorl.

[Mary888]

Thank you, Auspex~^^*

I think so, too.

[Al Dente]

I think this is a worm tube. Here’s a paper describing similar ones. https://www.lymeregismuseum.co.uk/related-article/epifaunal-worm-tubes-lower-lias-ammonites-discussion/

 

 

 

 

[Al Dente]

Here’s another paper and example from the publication.

 

 

[BobWill]

On 4/25/2018 at 12:57 PM, dbarshlomo said:

@Auspex

Thank you for the interesting input!

I tried to be as careful, meticulous and patient as I can when I removed the surrounding matrix from that part (well, at least for an amateur I guess), so I am curious if you’re sure the inner whorl is missing?

I’ll add a few additional images that show, as far as my understanding goes, the ribs of the inner whorl. They are not very well preserved but I think they are still visible.

Am I wrong? Doesn’t it mean this must be the outer surface of the inner whorl..?

 

 

 

I don't see where this question from 2018 was ever answered when this thread came back to life. BTW, Thanks @Al Dente for the serpulid papers.

 

For one thing, when @Auspex says the inner whorl I think he means the inner-most whorl or protoconch. They are thin-shelled and often missing. The whorl between that and the outermost one is certainly well- preserved here. It's a little hard to tell for sure from the rough surface but I think I see sutures in one photo. If so, this is a steinkern. For some mollusks like many gastropods and bivalves that would indeed mean that the outer ornamentation would be lost but not so for most nautiloids. For bivalves I assume it is because the layers are thinker from being built up on top of one another and for gastropods I can't really say why, but nautiloids whatever ornamentation is on the outside surface, ribs, tubercles, nodes, keels, horns, etc. they also appear on an internal mold. Sometimes there can be replaced shell material on the same specimen in some areas with sutures showing in another spot so you can see how thin the  shell is and compare the ornamentation for the inner and outer surface.

 

I'm so glad to hear about the possibility of worms appearing along the umbilical seam. I have some specimens where the siphuncle ID is obvious for various reasons so that should not be discounted but now I know to make sure before I make assumptions. The examples I have show the siphuncles to be black (possible phosphatized) with an outer shell of their own and often broken apart, even sometimes dislodged and laying along the surface of the shell.

Edited May 3, 2021 by BobWill