I went out and looked at several of my Autumn ferns, the only thing I found that even looked remotely close were some old dead fiddleheads.
So I chopped off the bottom part but the top just looked like a black blob.
So I am thinking yours still had some green so they were be at a different stage, but still as black and rotted as mine were they still looked like fiddleheads up close.
In the photo with you holding them in your hand I can see one of the objects near the center that looks like it has the remnant of a stem? Could they be fiddleheads? What was the center of the fern like? Did it look slimy or rotten? Did it look green and healthy?