This a deep water shell - 100m plus. For some reason, which I haven't worked out yet, deepwater shells and fish are red - but red colours can't be seen down there, so why?
Quite why these shells grow all their bits is a mystery. Perhaps those that did don't get eaten so much had longer spines. But the shells themselves are very thick and strong, so maybe the survivors who got spat out, evolved all this.The ones with longer spines live in deeper, calmer waters.