Nipponites - The Ultimate Weird Ammonite?

Just a quick thought before I go to bed (no time to look up anything detailed today). I may be talking completely over-simplified cobblers, but here goes anyway.....

Perhaps each different form of 'heteromorph' nautiloid was particularly specialised and occupied it's own narrow ecological niche. The decline of the nautiloids in their more exotic varieties during the Devonian and Early Carboniferous certainly seems to coincide with the rise of the early ammonoids, i.e the earliest goniatites, bactritids, and some of the other obscure groups. Maybe these new forms 'squeezed' out the nautiloid specialists and occupied some of their niches, forcing the remaining spiral-shaped nautiloids into increasingly deeper waters where they were not in direct competition.

I know a few heteromorph ammonoid forms suddenly appeared in the Triassic, but by time of the heyday of the ammonoid heteromorphs the nautiloids were confined to deeper waters; there was no such competition preventing experimentation amongst the ammonoids as they were not fighting for the same space. In an increasingly hot and tropical environment, this allowed them, specifically the Ancyloceratina, to explore their maximum developmental potential. By then the coleoids were in ascendancy but these appeared to have lived very different lifestyles and were not in direct competition.

I'm sure the real answer lies somewhere in the genomes and mutation potential within DNA, though. (Must ask Richard Dawkins to explain).

As an aside, here is a fantasic depiction of the Ordovician nautiloid Estonioceras for you.
 

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You know, I'm starting to get used to this thumbnail thing. I quite like it!
 
Well, we can not forget that all ammonoids (even this weird heteromorphs) have a siphuncle (not a single one seemed to have lost it), so i find it reasonable to assume that there hydrostatic apparatus was intact. Calculations have proven that the shell volume for Nipponites would have been sufficient to keep this animal in the water column, if the hydrostatic apparatus was functional. A long time a benthonic lifestyle was assumed for Nipponites, because of it's abberant shell form. The shell forms of Nipponites is actually rather regular (in comparison with Vermetids, which do not have siphuncle, and are cemented/attached to the substratum) and a Japanese paleontologist named Okamoto calculated/modeled (if the hydrostatic apparatus was functional = neutrally buoyant) that this ammonite changed his coiling to keep his aperture directed upwards (he changed his coiling as his aperture was directed at a certain boundary angle).
People who are interested in more can download freely these articles from the Paleontology PDF archive:
http://palaeontology.palass-pubs.org/pdf/Vol 31/Pages 35-52.pdf
http://palaeontology.palass-pubs.org/pdf/Vol 31/Pages 281-294.pdf

Some German physicist claims Nipponites and all other ammonoids to be benthic, but this is based on false assumptations on ammonoid growth and his own calculations of negatively buoyancy of ammonoids, which were later proven wrong by other scientists (all other calculations are close to neutral buoyancy). Nautilus is by the way also slightly negatively buoyant and it would be very hard to be on the seafloor with a negatively buoyant shell (must have been a drag on the seafloor).
It is quite clear, that with such a shell form (no horizontal or vertical streamlining), it couldn't have been more than a planktic drifter/floater hanging somewhere in the water column.
 
:welcome: to TONMO, and thanks for that interesting information!
 

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