After viewing the links with all the pictures, I can't help but doubt whether that is what actually happened to my chicks egg. The outer "egg" had no shell and there is fatty tissue inside of the soft tissue that encased the tiny egg. Very strange indeed.
No Marans here, but I agree it was dark colored even for any of my girls. At first I thought it was an acorn and wondered how on earth THAT could have happened....duh it's impossible! This 'egg', (I think) came from a Delaware. Here's a pic of what I usually get for eggs from four RIR, two Black Sexlinks, one Barred Rock and two Delaware:
The thing is, my girls are one year old next month and have been laying regularly for several months now. It isn't like these are their first eggs.
One of my Delawares got really sick nearly to the point of death a few months ago. After spending time indoors in recovery, she just now is looking really healthy and fluffy. In fact she's looking awesome now. She had been down to skin and bones and I really wasn't sure she would fully recover especially since most of the other hens rejected her upon returning to the coop. She is doing better than ever, but her history makes me wonder if she just never laid until now and this happened, or if she has other problems (if it is her at all). Out of nine hens, I have only been getting 4-8 eggs a day. Never 9, which in such young birds seems odd to me, even with winter upon us. They all get the same layer pellets, mealie worms once in a while for treats, free range outings an hour before sundown (I'm in the City), apple cider vinegar water, plenty of free-choice oyster shells, DE, two hours of extra light in the mornings, and then greens, lentils, yogurt and garlic from the kitchen.