Strange Composita subtilita brachiopod

[0lderthandirt]

I've found several Compositas lophophore fossils on my place here in northern Arizona but only one like this:

 

[Peat Burns]

6 hours ago, Nimravis said:

Here are the results that I thought would happen by leaving them over night- destroyed 1 ok and 2 very nice Compositas from Olgesby, Illinois.

 

I didn't get to try mine last night.  I think I'm gonna try a localized treatment with a stronger acid for a shorter time.

[Nimravis]

1 hour ago, Peat Burns said:

I didn't get to try mine last night.  I think I'm gonna try a localized treatment with a stronger acid for a shorter time.

I figured that since I had a number of specimens I would go all out 

 

Good luck

[Peat Burns]

1 minute ago, Nimravis said:

I figured that since I had a number of specimens I would go all out 

 

Good luck

Did you see any evidence of brachidium? Maybe here?

 

[Peat Burns]

Bingo!  Brachidia!  

 

I subjected these to a supervised immersion in HCl, rinsing in water from time to time and checking under the dissecting scope, and then re-immersing until I could see the brachidium.  Because the minerals preserving the interior are translucent, I did not allow the acid to eat too far into the fossil.  Once the brachidium was visible, I stopped the acid, soaked in water, dried, and then applied clear fingernail polish to enhance the translucence (largely lost when dry).  So far I have had the best luck with the small Composita from Oglesby.  I will continue to work with the larger ones.  I did find that, to a certain extent, you can "candle" them much like one does for chicken eggs - at least to see if the interior is calcite (=good) or lithified sediment (=bad).  The first successful one was the best.  You can see the brachidium spiral down to the lateral margin of the commissure.  Here is a stacked image (Scale in mm):

 

Here is the second successful one.  You can see the brachidium on the right side.

 

 

My first brachidia.  Thanks @connorp for the heads-up.

 

@Nimravis

 

 

[connorp]

@Peat Burns Yup, I've had better success with the smaller ones. The larger ones seem less likely to be infilled with calcite. I will post pics in a bit of the one I posted before after a while longer in vinegar. I like your method better of spot treatments with stronger acid. I'm not a big fan of how the shells look after complete submersion in vinegar, but that's really the only option outside a lab.

[Nimravis]

Nice job Tony @Peat Burns

[Nimravis]

5 hours ago, Peat Burns said:

Did you see any evidence of brachidium? Maybe here?

No- there is nothing there.