>Organisms that are active enough to have detectable preferences
>often show a preference for one side or another in doing things,
>similar to humans being left or right handed.  Almost all of the
>carbon-based molecules in organisms show a distinct "handedness".

Dextral and sinistral isomers, mirror-image molecules. Identical
chemical properties and chemical formulae. Our bodies can process
dextral isomers but not sinistral isomers of the same compounds.
Sugar? Sinistrose, anyone?

>  However, mollusks are relatively unusual in having many kinds that
>are normally distinctly asymmetric.  Many forminifera have
>snail-like coiled shells; these can be sinstral or dextral, too.

Foram handedness is temperature-controlled, with no genetic
component, and is unrelated to gastropod coiling. Indeed, the
organisms (currently in kingdom Protists, so they are not animals or
"creatures") themselves are basically "amorphous" blobs of protoplasm
& organelles. I wonder how they manage to secrete such regular and
beautiful shells?

>Bivalves with hinge teeth typically have different teeth in the left
>and the right, and there are rare individuals with the teeth swapped
>around, analagous to sinistrality in snails, except that some groups
>of teeth may swap while the others are normal.

I have heard of this, but never seen one, nor good images. Is the
entire hinge transposed, or is it that the hiunges are deformed so as
to somewhat resemble transposed teeth?

>This includes the coiling direction of the operculum.  Thus, a
>fossil snail that preserves the operculum can be easily identified
>as sinistral or dextral.  Without the operculum, known close living
>relatives, or distinctive shell features (such as a siphonal canal),
>it is difficult to tell whether a specimen is orthostrophic,
>hyperstrophic, dextral, or sinistral.  This particularly arises for
>certain Paleozoic to early Mesozoic extinct snails (or maybe not
>even true snails!).

Referring to Macluritoidea and Euomphaloidea? I think the presence of
an operculum is  evidence, if not proof, that these are gastropods.
Even if those opercs are different to any occuring today, if these
are the baseline gastropods, you might expect them to be different
(they are calcareous, hardly surprisingly). These shells have a
labral sinus; such an item is expected to be oriented away from the
substrate, and if the specimen is so oriented, Macluritoidea end up
with the apparent "spire" downward; as this is usually rather flat,
it makes a great stable base for the shell to rest upon, especially
in the muddy sediments these shells so often inhabited (or should I
say "habited"?). Such sinuses in gastropods are normally posterior,
and this would indicate (if not prove) that macluritoids are, if
gastropods, hyperstrophic. Of course this requires that the animals
be torted...

Well, heck, somebody PLEASE make me a working time-machine... I WANT
to go and collect living macluritoids, hippuritoids and ammonoids...
--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut

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