A "badly preserved silicified **echinoid**", according to an expert (via Jamie).
Nevertheless, a great toy specimen to [explore photogrammetry!](http://mielke-bio.info/falk/posts/22.photogrammetry)
Photos and reconstruction: Falk Mielke (using Agisoft Metashape),
decimated to 500k faces for upload.
The little paper shows 5mm squares as size reference.
Fossil found and donated by Katrin (Dankeschön!)