I have some research to do :) I guess it's time to buy a book!
I have a few 'nodules' which are vague ammonite shaped, with a central short 'stalk' and an cracked eggshell like surface. Do you know if the eggshell-like surface is part of the preserved ammonite or just the nodule crust ?
It is likely just the nodule crust. If you can see impressions of ribs or some flakes of shell material it could be coming off the fossil inside. We get alot of septarian nodules around here, they are nodules with calcite crystals growing in a star pattern radiating from the center of the nodule. They form a cracked egg looking surface on the outside of the nodule, all the museums around here have them brought in by people thinking they are dinosaur eggs! usually any fossil inside is destroyed by the growth of the calcite crystals.
This is a Jurassic (Bajocian) Stemmatoceras (looks alot like some of those you found) the inner whorls have been destoyed by the calcite growth. The flat area in the center is the surface of a vein of calcite.
It is hard to really tell from a photo, but it looks like the shell covering the body chamber, with part of the umbilicus preserved on the right side. It looks like the phrgmocone is crushed or missing. Can you tell if any of the phragmocone is buried in the nodule or is the part we can see all there is?
That is not what I pictured before. Septarian Nodules
these have been polished or carved, most are turned into bookends, and the description isnt exactly how I would put it, but it will pass.
that looks akin to a pair of Cardioceras from Pakefield that I'm rebuilding,
the shell on mine appears like original material, dull, brittle, fragile & a complete nightmare to work with but I've dilute PVA & the patience of the proverbial.
Nice pics over at the other forum, Rob. The sectioned example is really nice. Good to see some familiar names over there too, eh, AndyS and Spartacus! Good to see the specimen ID'd as Cadoceras by Andy too. It really was a very globular ammonite.
Top stuff, thanks again for posting the images and details.