Thoughts on ammonite aptychi

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Dear All,

This has nothing to do with the K/T boundary!

There seem to be lots of fun ideas about ammonite aptychi floating around,
such as:
(1) Crushing plates, articulating against the 'hinge' between the two
plates, for dealing with shelled invertebrates.
(2) Calcified plates similar to the lateral wings of the lower beak of
cuttlefish. NB! Why not the upper beak, and why so heavy. Dosidicus gigas,
the giant Humboldt current squid, can bite through steel cable, but has a
plain ole' chitin beak. What the hell were ammonites eating!
(3) Opercula: an oldie but goodie. They fit quite well for SOME species,
but certainly not all. But when preserved, the aptychi are often situated
well back from the aperture of the shell. Muscular contraction after death?
Given that the muscle scars analogous to Nautilus' head retractors are
situated much further back (by the last septum) maybe this is not
unreasonable.
(4) Weights. For holding the shell steady like ballast in a ship. I don't
personally think this one even deserves consideration...if nautiloids could
master siphonal/cameral deposits, and all shelled cephalopods can adjust
the flooding of the camerae, why have solid lumps of stone for ballast?
Then again, perhaps having mobile weights allows quicker attitude control,
so the aperture can be tipped up or down...?
(5) As a scoop, like a cow-catcher on the front of a steam-locomotive. Nice
idea, but I cannot sea how ammonites would swim forwards powerfully or
efficiently enough to make this a viable lifestyle. The rostrum gets in the
way for most of them, and as for the heteromorphs like Scaphites, the
entire coiled phragmocone would get in the way.
(6) Sexual ornament. Pretty untestable this one, but not impossible. Both
boy and girl ammonites seem to have aptychi, though.

If you have gotten this far you are now an ammonite aptychi expert...

So lets have your votes for aptychi function of the month.

Thanks,

Neale

Neale Monks, Department of Palaeontology,
Natural History Museum, London, SW7 5BD
Internet: N.Monks@nhm.ac.uk
Telephone: 0171-938-9007

"...now Nature is having the last laugh. The freaky stuff is turning out to
be the mathematics of the natural world"

from 'Arcadia', by Tom Stoppard