Re: Ammonite Aptychi

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As Neale says, Noel Morris is right about the reduction and obsolescence of 
gastropod opercula, another example is strombid gastropods, who use their 
reduced and claw-like operculum as an anchor to to haul themselves along 
with, it is quite useless for any other purpose.  Of course many gastropds 
have dispensed with the opeculum altogether, not to mention the shell 
itself!

Of course the hydrodynamics of many ammonite shells clearly indicate their 
unsuitability as fast-moving active predators, more likely they filled many 
of the detritivore/scavanger/molluscivore/vermivore niches now occupied by 
neogastropods, whose proliferation is chiefly post-Cretaceous.  

Dismissing the jaw-hypothesis may be premature if based on comparison with 
extant cephalopods.  Ammonites were a pretty highly-derived group with no 
modern representatives.  Nautiloids ammonoids and coleoids diverged from 
one-another during the middle Palaeozoic, which certainly leaves plenty 
of time for some pretty whacky and divergent jaw structures to 
proliferate.  Still, the apparent lack of wear facets on aptychi makes this 
all a tad perplexing ...

A key to understanding the purpose of ammonite aptychi may lie in looking 
for some additional common features amongst the ammonite species which 
possess them.  As I am entirely unfamiliar with the ammonite literature, I 
do not know how much is already known about the various species which 
possessed these structures.

Some possibilities that suggest themselves are that they provided armour 
plating for certain of the ammonite's vulnerable body parts, or may even 
have served as reinforcements to brood pouches, analogous (to an extent) 
with the shell secreted by _Argonauta_?   Most bizarrely, were they perhaps 
a secondary sexual characteristic of the male ammonite, serving a similar 
purpose to the gaudy plumage of some bird species, in attracting a mate?
Many ammonites were certainly strongly sexually dimorphic.  

They seem excessively robust for these puposes, though, when contrasted 
with the rather flimsy (but mechanically strong) ammonite shells.  Better 
knowledge (from existing literature/new studies) of the size/age/gender of 
aptychus-bearing ammonites may serve to confirm or (more probably) deny the 
above speculation.


Apologies if all the above is old-hat to all you "aptychologists" out there.

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