What is it????

[Paleome]

I just received a new shipment of matrix/prep material from the Wheeler Formation of Utah this afternoon.  What should be looking at me from within the box is THIS!  It is huge, about 3 inches wide by 5 inches long, very smooth and flat, with a well-defined margin to it.  I don't yet know if I have the other side to it, yet.  

 

First, I thought it might be the remains of a jelly, but I don't see any structures within, like concentric rings or radiating lines. 

 

Then, I thought maybe it is a cephalon shield or carapace of a very large phyllocarid, like Branchiocaris.  I am still feverishly researching every type of Phyllocarid it might be.  There is only a very

faint area of changing width of slightly darker material, wrapping around the inside of the circumference of its margin/edge. 

 

Did I finally get lucky?

 

 

[connorp]

I think it's just mineral staining.

[Paleome]

At the very least, you are correct.  I have looked at many varieties of phyllocarid fossil photos, and I would be much more convinced of a carapace/shield if the shape of this thing were a little different, namely flat along one lengthwise side.  Perhaps I shall dig around from the other side. It could be a geological thing, like a lenticular deposit of lighter-colored matrix.

[Rockwood]

Using an "if it walks like duck" parallel, I'd say this is a concretion.  

[Ludwigia]

I would say that this is a concretion split through the middle.

[Rockwood]

Bisected concretion. 

[Paleome]

Thank you all for your inputs.