Unusual coiling patterns, including hyperstrophy (i.e., coiling "up" rather than "down") as well as sinistrality, are more common in the Paleozoic than today.  One possible reason is that the major lineages that became extinct for other reasons happened to be the ones coiling oddly.  At least some rare coiling patterns, such as open coiling, probably were selected against with increasing predation, being easier to attack.  Finally, it is possible that variation in coiling patterns is associated with general genetic variability, and over geologic time greater specialization and lower variability has won out, with dextral orthostrophic happening to be the standard that succeeded.

Today, variations in coiling pattern seem more common in freshwater and terrestrial gastropods.  These habitats have less competition and lack many kinds of predators found in marine habitats.

Among the Pliocene sinistral taxa, the sinistral whelks of the Western Atlantic also survive to the present.  Extinction of Pliocene mollusks from eastern North America has been exceptionally high, whether or not unusual coiling or other variations were present.

Many bivalves can show a similar, but less obvious, reversal by having the hinge teeth reversed.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droigate Spa