OK gang, this one has me stumped.
This is the last specimen from last year's final trip to the Kiewitz shale, Stoner Limestone, Stanton formation along the Platte River in Cass County Nebraska.
I thought at the time, and until a few hours ago, that this was one of the disc shaped oncolites that have been found at that site and related sites in the area.
I was playing with a recently LED upgraded microscope and found it had cellular structure barely visible. so I decided to do a laborious prep on it with a bicarb blast.
It is in fact nothing I have ever seen before and is very fragile. Frustratingly, when I stabilized the fragile little thing, it made the structure less visible.
So, I had to get creative as I realized too late all my scope cameras and adapters are sitting on my desk at work...
Anyway, I have no clue what this is. the simplest solution is a disc shaped bryozoan, but the more I worked on identifying it, the less convinced it is within my skill level to identify.
So, here we go: