Among the numerous bone and mollusk shell fragments scattered throughout our yard is this old, thick clam shell. I have no recollection of where it came from and no idea what species it may have once represented.
The other day when I picked it up to move it elsewhere, I found these 3 slugs, Deroceras reticulatum, safely nestled on the underside of it.
The last common ancestor of gastropods and bivalves must have lived sometime in the Cambrian, more than 500 million years ago*. That makes them very distant relatives indeed. Nevertheless, I had no intention of interfering with their family union of sorts. I returned the clam shell with its inhabitants to its spot.
*Here is a brief intro to mollusk evolution.