Before I begin, let me say that I am not a paleontologist nor a biologist, and this thread is just for layman-type discussion, so my ignorance may show. Anyway, while gazing at some wierd Heteromorph Ammonites (like Pravitoceras), a friend asked me how could mollusks with such strange shells have possibly lived? I told him that in some species, it is believed that the animal was a passive, planktonic drifter. He replied that it was unlikely. Me: "Why?" Him: "Because all planktonic, passively drifting mollusks have a tendency to lose the shell and become transparent and gelatinous. For example, sea butterflies (pteropods) are shell-less snails. The only way I could see that these shells belonged to planktonic drifters were if these shells are actually vestigial, and that the actual animal dwarfed the shell, like a bubble shell or batwing slug Sagaminopteron." Me: "So if they weren't drifters, what could they have been?" The answer eluded us. But then, I recalled that there were certain gastropods that actually anchored themselves to a rock, then continued to grow, adding to their shell. These shells have often been mistaken for serpulid tube-worms, since they no longer confirm to a strict helical structure. These sessile snails were filter feeders. So my idea was that these ammonites were actually sessile. Their strangely shaped shells would have served to anchor themselves in various soft sediments, while they pursued a sea-cucumber-like lifestyle. They would have an advantage over other sessile sediment-dwellers by retaining some degree of mobilty, perhaps by expanding their tissues to lift themselves up above the mud, like certain single-polyp corals do today. Of course, I have no idea about the type of sediments that the fossils of these cephalopods were found in. So any thoughts? I hope that this will be an interesting discussion.