Ordovician or Silurian Fossil ID - Help!

[Crinoid Carrier]

Recently I had found a fossil on a Southern Lake Michigan beach (South Wisconsin / North Illinois) and was extremely confused as to what it could be. 
 

Most of the agatized fossils here are of tabular / rugose coral, crinoids, or stromatoporoids.

 

What confuses me is the symmetrical cloud like shape; it does not seem to be a trilobite…

Is it some sort of a sponge? or a slug?

It is about 3 inches in length.

 

Thanks

 

 

Edited May 25, 2022 by Crinoid Carrier

[abyssunder]

Maybe it could be a very worn cephalopod fragment with a visible siphuncle?

[Crinoid Carrier]

Yeah … maybe it’s some sort of weird cephalopod. 
 

The only thing that remains to puzzle me is the petal like pattern that runs down the sides.

 

Forgot to post a side view.

 

Thanks Again!

 

Edited May 25, 2022 by Crinoid Carrier

[SilurianSalamander]

Fascinating. Chiton? The mollusk 

[Coco]

Hi,

 

I don’t think it’s chiton. They have eight plates and they’re flat, not in volume. The plates of a chiton are thin, it’s the only thing left of them when they are decomposed.

 

https://www.alamyimages.fr/cote-est-des-coquilles-marines-la-description-des-mollusques-de-la-cote-avec-de-nombreux-vivant-sous-mer-du-maine-au-texas-inclus-en-particulier-en-floride-fig-29-plaques-de-chiton-premier-quatrieme-et-huitieme-de-vannes-un-chiton-montrant-les-lamines-1-1-de-l-insertion-n-n-des-encoches-s-l-a-l-sutural-lajnlnae-x2-commander-famille-lepidopleuridae-lepidochiton-polyplacophora-genre-gris-1821-jolie-lepidochiton-chitons-alba-linne-chiton-blanc-petite-coquille-au-microscope-la-surface-couverte-graniilated-magnifiquement-avec-la-couleur-bleu-noir-qui-s-efface-facilement-soupapes-legerement-carenee-une-faible-ridge-image178490624.html

 

Coco

[Tales From the Shale]

It appears to me like the heavily eroded phragmocone (shell) of some nautiloid cephlapod.

[Crinoid Carrier]

On 6/6/2022 at 10:58 AM, SilurianSalamander said:

Fascinating. Chiton? The mollusk 

 Wow Yes! It is a Chiton with 8 segments. 

[Coco]

Are you really sure ? Chitons have 8 plates, of course, but they are so thin that they break from nothing. Do we see the thickness of the shell on the photo that shows the hole ?

 

Coco

[Fossildude19]

Chitons date back to the Devonian, but I don't think that is what you have here.

Looks more like a siphuncle from an orthocone cephalopod.

 

@FossilDAWG

[Crinoid Carrier]

4 hours ago, Coco said:

Are you really sure ? Chitons have 8 plates, of course, but they are so thin that they break from nothing. Do we see the thickness of the shell on the photo that shows the hole ?

 

Coco

Here are some closer pictures of the hole in question.

The difficult part is that the fossil is severely agatized. There are spherical almost grape agate like structures inside the hole due to the crystallization.

 

As for the dating of the fossil, I am quite sure it is Ordovician or Silurian and not Devonian. Please see the picture of other fossils found at the same site (Illinois-Wisconsin border near Lake Michigan).

 

The main reason for this train of thought is the Fusispira Shell (also severely agatized) right below the fossil in question.

 

Thanks again! Any help is much appreciated in ID’ing this headache.
 

 

[FossilDAWG]

I am almost 100% certain this is a siphuncle of an actinocerid nautilod.

 

Don

[Coco]

More research is needed, but it seems to me that the Chitons appeared in the Devonian period, so they did not exist in Ordovician or Silurian.

 

Coco