Tiny Aptychus or Bivalve Steinkern?

[Heteromorph]

Yesterday I hunted an Upper Santonian Austin Chalk site in Ellis county between downpours of rain. This was my first time at the site. I was looking around a pile of rocks with some boulders mixed in and found this on one of the boulders. It is tiny, whatever it is, about 1 cm wide. Now I must admit that I am more familiar with the fauna of the Upper Coniacian Austin Chalk but to me this looks more like half of the aptychus of an ammonite than a bivalve. Since an aptychus was made of calcite I believe that it would be preserved in chalk, though the actual fossilized material is gone and this is just the steinkern of what ever it was. 

 

Here is the best picture that I have of it. Sorry that the quality is poor. I took it while I was at the site and I can’t get a better picture right now. Hopfully this will be sufficient.

[Innocentx]

I think it may be orbiculoidia, a type of brachiopod.  I believe it's an inarticulate brach, ventral valve.

[Tidgy's Dad]

I think it's an aptychus, nice one too! 

Good find! 

The Orbiculoidea died out in the Permian, long before the chalk was laid down. 

[Innocentx]

I've never found one of these.  Learn something every day!  Thanks for this information, @Tidgy's Dad

[Tidgy's Dad]

Just now, Innocentx said:

I've never found one of these.  Learn something every day!  Thanks for this information, @Tidgy's Dad

Pleasure. 

I pretty much get something wrong and learn something new here every day! 

[Miocene_Mason]

I think it’s an aptychus as well, similar to one I have. With the amount of ammonites in the Austin chalk, all those aptychi have to go somewhere!

[KimTexan]

I have tried hunting the upper Austin Chalk numerous times. I have never seen anything but clams. This looks like a clam to me, but I’ve never hunted the Santonian. 

Maybe @Foshunter, @BobWill Or @Uncle Siphuncle I know between the 3 of them they’re either familiar with Aptypcus, the Austin Chalk or Santonian. I would think one of them should have thoughts on it.

[Ludwigia]

Although it's easy to think that this is an aptychus, I would be more inclined to guess bivalve. The form is extremely round and flat and in my observation, cretaceous aptychae tend to be mostly more oval (the shape of most ammonite mouth apertures) and more concave/convex than this sample...but then again, I might be wrong. Just an educated guess.

[BobWill]

16 hours ago, Ludwigia said:

Although it's easy to think that this is an aptychus, I would be more inclined to guess bivalve. The form is extremely round and flat and in my observation, cretaceous aptychae tend to be mostly more oval (the shape of most ammonite mouth apertures) and more concave/convex than this sample...but then again, I might be wrong. Just an educated guess.

This is what I was thinking at first too but it occurred to me that I don't know of any  ammonites from here with round apertures since it would need to need to have very evolute coiling. Even though I haven't seen any from here for comparison I can see this as an aptychus with the other one tilted off to the side to pass around the area where the whorl would overlap the previous whorl. I also see a common feature from pictures I've seen where the growth lines converge toward one end compared to the other end. Another reason to doubt it's a bivalve is my inability to think of one that would look like that. Thirdly I believe I would really like it to be an aptychus;) Does that count as diagostic?

 

edit: I forgot to mention that it is unlikely to be a steinkern since they were not hollow. The two halves would have been side-by-side.

 

[Heteromorph]

Bad news: I am an incompetent chiseler and I broke this into multiple pieces while trying to extract it from the boulder today. DANG!!! Good news: I took more detailed pictures of it before I started hammering and I was able to retrieve part of it. I am considering going back again and digging through the rock chip pile I created while chiseling to look for the other pieces of this, though I already looked through it quite throughly. It is probably a lost cause. 

 

Thanks everyone for the help! I agree that to me this looks more like an aptychus rather than a bivalve. @BobWill, you are right about it not being a steinkern. What I was trying to say, though I used incorrect terminology, is that this is the somewhat convex inner impression of the aptychus with most of the calcite absent. When I went back to the site today I realized that it is not convex but instead the somewhat concave outer impression of the aptychus.

 

Here are more pictures. 

 

P2. Straight on view. This was taken around 5:00 this evening and the sun was at my back. 

 

P3. Slightly angled view allowing light on the specimen.

 

P4. Slightly more angled view than P3.

 

P5. Very angled view, more than P4.

[RyanDye]

On 2/23/2018 at 8:57 PM, Tidgy's Dad said:

Pleasure. 

I pretty much get something wrong and learn something new here every day! 

Isn't that the truth... 

[Ludwigia]

Perhaps @Uncle Siphuncle can give us his opinion.

[Uncle Siphuncle]

Looks more like an aptychus to me than any Austin bivalve I’ve seen.  But my only self found aptychi were from the Jurassic of Germany and looked a bit different than this one.