I find all sorts of fossils that are in a situation that I can’t describe, then someone tells me the word for it. Steinkern is one of those. A Steinkern is a fossil consisting of mud or sediment formed by entering the hollow portion of a natural object. A bivalve shell can become one easily. A great example of the end result of this process can be found on my Composita post. Below, a photo of a steinkern from a Brachiopod is displayed.
The fossil above formed when sediment entered the hollow shell of the deceased Composita. I often thought that perhaps pockets of air could indeed persist after hundreds of millions of years, but it appears that isn’t the case. Rock itself is mostly permeable. Water will carry all sorts of minerals into these hollow spaces over time.
Cephalopods preserved as Steinkerns
Often, a large number of the cephalopods I find are preserved as a steinkern of the original creature. The fossilized shell portions often stick securely to the outside limestone matrix, and the shell and the inner steinkern separate easier than the shell and the limestone. The septal walls remain embedded in the steinkern and make for interesting-looking fossils.