Unusual preservation on a Pennsylvanian Coleoid

[BobWill]

I found fragments of a cephalopod at the Lost Creek spillway site near Jacksboro Texas on August 21, 2021. Pennsylvanian, Graham formation, Finis Shale member. It includes a 90mm long section and two short phragmacones. The 8 pieces of the longer section were found adjacent to each other and fit together perfectly but the two smaller pieces were a few cm away and no points of contact could be confirmed.

 

The long section exposes the inside of part of the shell and is encased in a concretion on the outside, not the usual way these are found. The smaller pieces exposed the outside of the shell in the more common way. The long section is broken along the long axis with approximately one half of the shell missing along one side.

 

There are possible fragments of the missing side lying on the interior surface. The smallest phragmocone has a ventral siphuncle suggesting a Bactrites but an intermediate layer with the finger-print pattern is visible on both short pieces and some of the broken pieces lying inside of the long piece, supporting the possibility of it being Shimanskya postremus instead.

 

 

 

At first the absence of apparent cameral deposits on most of the long section suggested it may be the body chamber. Both smaller sections had septa. Evidence of a single cameral deposit at one end on the long piece caused me to think it may be towards the adapical direction and the diameter of the larger phragmacone section was the same size as that end at 29.7mm.

 

The diameter of the smaller phragmacone varied from 20.6mm to 22.3mm. However, the diameter at the opposite end of the larger section was smaller than the end with the possible cameral deposit at 24.9mm. Of course if it were toward the adoral direction it should have had a greater diameter uming the shell continued to increase in size as it grew. The smaller phragmacone was found near the smaller end and the larger phragmacone near the larger end. This makes me doubt that the apparent lack of septa or cameral deposits in the long piece actually confirms that it is the body chamber. It could be that an outer layer is all that is present since that may not have evidence of cameral deposits.

 

Are my assumptions about this specimen correct? If so, is this a common way to see these specimens preserved or is the view of the inner surface fairly unique?

Edited August 23, 2021 by BobWill
spelling

[ClearLake]

Hey Bob, nice specimen and always glad to see interesting stuff turn up at Jacksboro. Can’t help with the I’d, but it is very interesting. 

 

Mike

[BobWill]

  After I posted this I took a closer look at the longer section searching for evidence of septa and found it. Most of the cameral deposits were worn away but by calculating the distance they should be apart I was able to find evidence of 4 deposits with the right spacing. To do this I calculated the cameral ratio on specimens of a similar size from this site (1.3 which fits well in the range of 1 - 1.7 depending on the ontogenetic stage) and worked from that one possible deposit that can also be seen on the photo of the assembled specimen at the far left end. Small portions of the other three appeared where expected, so definitely not a body chamber.

[erose]

Some of the best stuff comes in "kit form."

 

[tombk]

You have a good eye for finding scattered pieces, and a skill at putting together puzzles. It sounds like you know more than I do about what your find may be, so I can't help there. Just wanted to say nice work!

[Wrangellian]

Every time I try to assemble something that I found in pieces, there is one glaring hole left in the middle!