Two nautiloids from the Kjørven formation

[PaleoOrdo]

Last autom I found two nautiloids in Hadeland, Oslo-field, in another a little older formation than the other nautiloids I have found in the area, but also Katian, Upper Ordovicium; Kjørven formation (a. 447-448 my). I waited until now to present it, because I used some time to identify the right formation. The first one I thought first that it is an orthocerid nautiloid, but it seems (if I am right) to be a discosorid, because it has bullettes and possibly thick connecting rings and relativ broad or oval shaped siphuncle near the ventral margin of the conch. Its shape, although a mature part seems missing (and the apex is not well preserved), is slightly endogastric curved (seen clearly by the overall shape of the sequence of the connecting rings). I have marked where is the apex with arrows in pict. 3. The aperture with the living chamber should be in the opposite direction, because the bullettes are "hanging" down in the direction of the apex (the juvenile part of the conch). 

Pic. 1

Pic. 2:

Pic. 3:

Pic.4 - the other side of the rock, shows that is is almost flat, similar to an actinocerid, on that side too, maybe because of erosion?:

Pic. 5 - aclose up on the connecting rings:

Pic. 6-7 - show the the siphuncle (see arrows):

Pic. 8 - cross-section:

It is not clear everywhere that the connecting rings here are thick, but in some parts it is so, as shown in the next picture 9. It often happens with discosorids that only the outer layer of the connecting ring are preserved. According to Flower and Teichert “a striking peculiarity” for discosorids is “the unusual thickness of the connecting rings”, composed of a spherulitic–prismatic outer layer and an inner and thinner organic fibrous layer. I wonder if these two layers are seen in my speciemen? In the below picture I mark the two layers indicated by arrows by "I" for the inner layer (which often is not preserved) and "O" for the outer layer.

Pic. 9 - close up of layered connecting rings:

 

 

Edited July 11, 2021 by PaleoOrdo

[PaleoOrdo]

The second is a very small one, about 1cm long, with only 3 chambers, and I think it is an orthocerid nautiloid. It is also some strange forms near it.