>The suggestion was made that most of the
>lefties were the same species, with a good deal of variation, but some
>European cone spp were also sinistral, so that is not a total
>explanation.
 
The only consistently sinistral cones I know of are those in the Pliocene
of the southeastern U.S., though as noted sinistral mutants are known from
the modern Mediterranean cone and possibly others.  There are a few usually
sinistral turrids as well.
 
>A more pertinant question might be why the vast majority
>of present-day spp  in nearly all families are dextral- is there some
>advantage to this state?   If so, why are so many land-snails sinistral,
>and why has the Triphoriidae family been so successful, if
>left-handedness confers some sort of disadvantage?
 
No relative advantage to one handedness is known.
 
>Further, is there
>more diversity in "handedness" in other families in the fossil record,
>and not just Conidae?
 
Not especially, although the Paleozoic macluritids and euomphalids were
frequently hyperstrophic, a rare condition in modern marine snails.  There
is some question whether they are true snails, though.  The early to mid
Pliocene (when sinistral Busycon and Conus appear) is a time of high
variability in other mollusks in the eastern U.S, too (wrinkles in
Glycymeris americana and Mercenaria "tridacnoides", etc.) but no other
sinistral snails appear at that time that I know of.
 
>Does natural selection seem to be playing the
>predominant role here, or are we simply living in a period where
>dextrality just happens to predominate? (or is there any way of
>gathering evidence to support either circumstance, or is it likely to to
>remain one of the many things we are unlikely to ever know??)
 
A consistent coiling direction would be associated with greater
developmental rigidity.  Being constant is probably more significant than
which direction-perhaps right happened to predominate early on.  Once one
direction is prevalent, it may be favored because of disadvantages of being
in the minority.  At least in some species with variable handedness, it is
difficult for opposite-coiling individuals to mate.
 
David Campbell
 
"Old Seashells"
 
Department of Geosciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA
 
919-962-0685
FAX 919-966-4519
 
"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug