ID help - The Thing in the Backyard

[RSO]

Re: "The Thing." The subject item was found in an old flower bed in our backyard a few years ago. Provenance is unknown. It is likely part of a collection of someone in Pass Christian, MS, at the time of Hurricane Katrina. Enormous damage from 23'+ storm surge. Entire structures including a large were obliterated. That said, can anyone identify the object in the attached images? Measurements are 6" x 8" x 3.5." I've consulted with two potters who have said that this is not man-made. I appreciate any help you can offer.  

 

[Bradley Flynn]

Looks like a very weathered gastropod to me, but I would expect the columella "internal spiral" to be intact or at least partially intact. This is just a guess based on its shape. Hopefully someone else can help with a solid ID. 

[Bradley Flynn]

Actually looking at it more closely now it looks as if it's a ceramic man made gastropod, the color bands seem odd for an organic life form. Also the inside looks to smooth and there is no natural spiral going into the tip on the inside and no trace of a columella at all. So it could be a broken ceramic ornament. My mind could be playing tricks on me though 

[JohnJ]

This looks more like a gypsum based evaporite sometimes used in aquariums or as decor. 

[abyssunder]

Hard to say what you have there. The specimen is geological and biological eroded (bioerosion in internal side), so the entire structure might be obsolate.

Somehow, and it's just my opinion, it could be remnant of a rudist, e.g. Lapeirousia or similar. I'm not sure.

........

Titanosarcolites is another guess. 

[Ludwigia]

I'd also go along with gypsum, as JohnJ is suggesting.