Might be plausible. I’ll have to look it up at one point, maybe there’s some research on this. I think it may be hard to guess why because we don’t have many swimming animals with shells. I don’t know if snails may offer some answers but they are maybe to different in lifestyle.
Probably too different, as snails are benthic, while ammonites where probably nektonic.
Iirc the shells being longer is something about allowing gases in the shell to compress or expand as needed to control bouyancy. I would imagine there is a sac of gas, and the ammonites would siphon water in or out as needed to compress or decompress the gas.
Edit: just looked it up on Wikipedia, it appears the heteromorph ammonites are thought to have maybe been planktonic or benthic.
Definitions for non-nerds:
Benthic means living on or near the sea floor.
Nektonic means free swimming
Planktonic means going with the current as plankton. I should note plankton aren’t all tiny, some are visibile to the naked eye. All it means is unable to propel themselves against current.
It is certainly a challenge to keep a large animal with such a thick shell afloat. However that would just explain the immense size of the shells. Their shape is just extremely weird and sometimes I’d even expect it to be detrimental to their ability to navigate the open water. If they were planktonic it would not be as problematic I guess but I still don’t see the functional advantage. Maybe mimicry? But of what?
They look a lot like the calcareous shells of some polychaetes but they have a sedentary lifestyle attached to rock or other substrates which is not what we’d expect for Ammonites.
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