Odd Broken Bits from Jacksboro

[BobWill]

I have found several of these at the Pennsylvanian Sub-period site in Jacksboro Texas. I've always had a good idea of what they were and never thought others might have different views until I brought up the question. If you know or have ideas post a reply and lets get a consensus of opinions before I say what I think.

 

I didn't post this in “fossil ID” because I think I already know and I also want to discuss why they appear the way they do so give your opinion of that too. I recently found some that suggest an answer to that too. They may have already been discussed or even written about so if you know of a paper or old topic on them please post a link. I couldn't think of a good term to use for a search. Scale is mm.

 

[DPS Ammonite]

Internal molds of sections of

coiled cephalopods. Every more of a guess: Tainoceras or Metacoceras?

 

For help, look for recent posts about Jacksboro.

[BobWill]

13 hours ago, DPS Ammonite said:

Internal molds of sections of

coiled cephalopods. Every more of a guess: Tainoceras or Metacoceras?

Thanks for the reply John. No one seems to disagree with you and neither do I. Someone suggested it's a piece of this gastropod (maybe Euomphalus sp.)

 

 

or even a worm tube but I have reasons to discount both of those.

 

So, the next question is which part of a cephalopod, where are the sutures and why is the rest missing ? I'll post more clues to help tell which cephalopod it might be later.

[DPS Ammonite]

20 minutes ago, BobWill said:

Thanks for the reply John. No one seems to disagree with you and neither do I. Someone suggested it's a piece of this gastropod (maybe Euomphalus sp.)

 

 

or even a worm tube but I have reasons to discount both of those.

 

So, the next question is which part of a cephalopod, where are the sutures and why is the rest missing ? I'll post more clues to help tell which cephalopod it might be later.

I would think that your fossils are the living chambers that lack sutures. The living chambers were then filled with sediment. The rest of the cephalopod which was carbonate dissolved/eroded away. If the chambers were fossilized unfilled with sediment or filled with carbonate, the whole thing could have disappeared without a trace. 

 

An alternate hypothesis is that an unfilled cephalopod dissolved away leaving a mold that was later filled with sediment creating a natural cast that lacked all traces of sutures. 

[Innocentx]

20 minutes ago, DPS Ammonite said:

an unfilled cephalopod dissolved away leaving a mold that was later filled with sediment creating a natural cast that lacked all traces of sutures. 

I have one such. No sign of sutures but the projections/bumps (I don't know the terminology) on the ribs are evident on the internal cast.

[BobWill]

15 minutes ago, Innocentx said:

I have one such. No sign of sutures but the projections/bumps (I don't know the terminology) on the ribs are evident on the internal cast.

Yours may be a different genus if they have tubercles but you would expect any ornamentation to appear on the inside too on a cephalopod. I believe the ones without sutures are simply body chambers.

 

What strikes me about these is how the only part left is often just the umbilical margin which is broken away very cleanly and in a similar way on many that I find. I agree about the body chamber but why are they eroded away leaving pieces of this shape? Here I have glued together three pieces found near each other that fit perfectly. Notice the evenly broken surface in the second photo.

 

 

 

Does anyone know why they have this form without the rest of the original shape preserved?

[Innocentx]

 

Perhaps the suggestion of Euomphalus sp. is correct. It seems too skinny for coiled cephalopod once you put the pieces together.

[Innocentx]

The squareness at this end also seems more like gastropod.

[BobWill]

3 hours ago, Innocentx said:

 

Perhaps the suggestion of Euomphalus sp. is correct. It seems too skinny for coiled cephalopod once you put the pieces together.

I ruled out that gastropod by taking some measurements. If you devised a ratio of whorl cross-sectional area to radius, the number for the gastropod would be at least double the number for these pieces. I always thought they looked more like just the very edge of a cephalopod broken off along the umbilical margin, not the whole whorl section. That became a good possibility when I started to find pieces that extended a little farther across the shell like this

 

 

Notice the two little grooves at the top left where it hasn't completely broken off like the others. Yet!  Then I found more that extended even farther like this

 

 

and this

 

and then I found this

 

 

which also had the last septum on one end

 

 

which looks a lot like this smaller version with part of a previous whorl.

 

Bingo! It's a Goniatite! Maybe one of the ones John mentioned, maybe Vidrioceras uddeni which we also find at Jacksboro.

 

That just left the question of why they were broken off into the narrow strips so often so I looked through a box of samples and found this piece with a hole in one end

 

which extended all the way through to the other end which also had a septum

 

 

Then I saw this one also with a hole

 

 

and this slot on the other end which also has a hole not visible in the picture.

 

Here's another view of the slot

 

 

 

The two holes almost line up with each other and everything south of the slot looks a lot like the original pieces!.

Could this be some kind of predation? I can't think of why something would bore along the edge like that but what else could form the holes and could they weaken the pieces enough to cause them to break away easily. The one with the slot also has a crack forming seen here with the crack pointing towards the slot.

 

 

It seems to me like the crack could eventually break away leaving a piece just like the smaller ones. Can anyone explain this and do you think I'm on the right track with my reasoning?

 

 

 

 

 

 

 

 

 

[Innocentx]

18 hours ago, BobWill said:

which also had the last septum on one end

 

Definitely septum.

[DPS Ammonite]

19 hours ago, BobWill said:

 

 

 

It seems to me like the crack could eventually break away leaving a piece just like the smaller ones. Can anyone explain this and do you think I'm on the right track with my reasoning?

 

 

 

The crack in the goniatite looks like a shrinkage crack in mud caused by drying. 

 

Trying to figure out why the goniatites broke the way they did might not be easy. Sometimes you need to say that this is the way the goniatite crumbles.

 

 

 

 

 

[BobWill]

25 minutes ago, Innocentx said:

Definitely septum.

i don't know which goniatite it is but it looks a lot like this which I think is Vidreoceras uddeni. Notice the umbilical margins at the red arrows that resemble the small pieces that break off and the whorl profile at the broken body chamber at the blue arrow.

 

 

4 minutes ago, DPS Ammonite said:

The crack in the goniatite looks like a shrinkage crack in mud caused by drying. 

 

Trying to figure out why the goniatites broke the way they did might not be easy. Sometimes you need to say that is the way the goniatite crumbles.

 

 

 

Okay now I need to go find a crumbled cookie

I never thought of shrinkage. That makes sense but what about the groove which appears to have a rough texture?

This just seems like the kind of thing a bored (or desperate) researcher would have taken up by now. Maybe there's a paper out there about it but I don't know what key words would find it.