Zoogenetes harpa is a small land snail; many of them easily fit into a bottle cap. Thomas Say described this species in 1824 based on the shells he had collected a year earlier during his 2nd western expedition under the command of Major Long. That expedition visited Ohio, Indiana, the “Northwest Territory” (Wisconsin and Minnesota), Michigan and New York.
Zoogenetes harpa, found only in the northern hemisphere, has an intriguing distribution pattern. The orange colored patches in the map below show the ranges of this species across 3 continents forming a roughly circular pattern around the North Pole1. How can we explain this pattern in the distribution of an animal that cannot swim or fly? Could a floating bottle cap have carried a few lucky snails across the treacherous Arctic Ocean from one continent to another?
Background map was created at OMC.
Fantasy aside, the only meaningful answer comes from the consideration of the positions of the continents about 190 million years ago during the early Jurassic period. That was when the continents that were later to become North America, Europe and Asia were still connected. Clearly, Zoogenetes harpa first appeared sometime before then, dispersed across the then connected land masses and was subsequently carried on the drifting continents to the places where it lives today.
Map is from Scotese.
What is also intriguing is that the shell morphologies of the populations on North America and Europe don’t appear to have changed since the Jurassic. It would be instructive to compare their genes.
1. The limits of the ranges indicated on the map are approximate, since I don't know the exact boundaries, especially in Asia. I have compiled the distribution data from Pilsbry, 1948; Likharev & Rammel'meier, 1952; Kerney & Cameron, 1979.