Here is some input into this interesting discussion from Iceland:

The molluscan land fauna of Iceland was introduced, mostly from Europe, in
the last 10 000 years. Iceland was swept clean by the Pleistocene glacier
so this is certain. Most of the species are fairly cosmopolitan and are in
Iceland at their northern limit of distribution. Land bridges did not
exist, so how did they make it across the ocean?  Several ways come to
mind:
1. Brought by humans. Iceland was colonized from the mainland in the
nineth century, bringing lifestock and crops.
2. Birds. Many of the species are migratory, and may bring eggs on their
feet.
3. Driftwood, mainly from Siberia, is very common on Icelandic shores.

It seems all these methods would also work for European species introduced
into Eastern North America.
An interesting question: Why have so few American species spread to
Europe? Physa acuta, Zonitoides nitidus, Z. arboreus, more?

Best wishes,
Pall Einarsson

>> How did it get here? One theory has it that there was late Tertiary and
>> Pleistocene uplift of the Arctic lands and a land bridge existed from
>> Scotland to Iceland to Greenland to Labrador. Such a connection might
>> also
>> explain the European-Eastern North American distribution of other
>> landsnails, e.g., Zoogenetes harpa, certain Euconulus spp., Pupilla
>> muscorum, certain Vallonia spp., etc.
>
> Unfortunately such landbridges are relics of preplate tectonic
> thinking, and are extremely unlikely. I would consider migrations
> across the Bering strait, which has been closed at times, with Siberia
> and Alaska in contact.
>
> --
> Regards
> Andrew Grebneff
> Dunedin
> New Zealand
> Fossil preparator
> Mollusc, Toyota & VW van fan
>
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