07 April 2006

Paper read this week: Land snails of Beringia

About a week ago I reviewed a book on Beringia, the land connection that existed between Alaska and the northeastern Asia during several geological periods in the past and most recently during the last ice age. This week's paper review is about the land snails of the same area. By coincidence, a few days ago Pascal at Research at a snail's pace mentioned the same paper.

Bernard Lauriol, Yannick Cabana, Jacques Cinq-Mars, Marie-Anne Geurts, F. Wayne Grimm. 2002. Cliff-top eolian deposits and associated molluscan assemblages as indicators of Late Pleistocene and Holocene environments in Beringia. Quaternary International 87:59-79.
The study area was along the Porcupine River about 100 km north of the Arctic Circle and just east of the U.S. (Alaska)-Canada border. The purpose of the study was to examine the usefulness of cliff-top eolian deposits* as indicators of past climate and environments. Samples were taken from several sites, radiocarbon dates of the deposits were determined, pollen analyses were carried out and snail shells were extracted and identified.

The radiocarbon dates obtained for the deposits range from about 130 to 15900 years before present (BP). Twenty-one species of land snails were found in the samples from eolian deposits. In comparison, one sample indicated to be "Modern" yielded only 6 species of land snails. But the paper does not clearly state if those snails actually lived at the site or if their shells were also wind deposits.

The majority of the fossil species belong to Vertigo, a genus of tiny (~2 mm or less) snails. One such species, V. ovata, was represented by one shell out of 640 shells in one sample of 25 kg of sediment. The author who identified the snails, Wayne Grimm, considered this single record "significant northern extensions [sic]" of the range of V. ovata. I would hesitate to make such a broad generalization based on one shell from a wind deposit. Furthermore, this record doesn't extend the present range of the species, because the sample the shell came from was dated to 8200 BP or earlier.

To explain the presence of so many species of snails in wind deposits, the authors suggest that the fauna they recovered could "represent a dynamic combination of endemic species and of new species that were constantly introduced by migratory birds coming from the unglaciated southern regions of the continent, or from Asia". There is quite a bit of evidence indicating that birds indeed disperse mollusks, which I have discussed here and here.

However, when the authors further speculate that "cliff-top eolian deposits, with their rich molluscan colonies, could well have been very attractive sites for the migratory birds particularly around 10,000 BP. The birds would have been able to feed on molluscs, preferably on shells, since live snails may contains [sic] toxic compounds", they seem to be forgetting that the majority of the species they found had shell lengths of about 6.5 mm or less. Birds do eat snails, but I am not aware of any records of birds eating such small snails, although the 2 succineids grow larger and may have been eaten.

Pupilla hebes, one of the tiny fossil species recovered from cliff-top eolian deposits. According to Pilsbry (1948), most records of this species were from high altitude mountains in the U.S.

Considering that most fossil species they found are rather small snails, it is also quite likely that most shells could have been brought by winds from distant locations. Nevertheless, direct evidence for long distance wind transport of live snails or their shells is lacking, as far as I know.

One land snail species whose distribution has been puzzling me is Zoogenetes harpa. It is possible that this snail, whose range extends from northern America, thru northern Asia, all the way to northern Europe, may have used Beringia to migrate between Asia and America. Interestingly, however, no specimens of Z. harpa were found during this study.

*An eolian deposit is soil deposited by the wind.


budak said...

Hi Aydin,

Thanks for dropping by!

You may be interested in a Singapore-based molluscan group (http://www.molluscan.com/) discussing Southeast Asian molluscs.

pascal said...

I have to find time to read this article - their data seems interesting, but some of their conclusions seem a tad far-fetched. I don't think that snail shells have sufficient caloric value to be of interest to birds.

Eolian deposition of snail shells should be something one could demonstrate - and provide evidence for/against in a lab setting. The photomicrographs they show don't display any telltale signs of eolian deposition such as a smooth, frosty appearance - the scultpure is generally in good shape.

My personal Boo Radley right now is Columella columella (alticola). It seems to be a recent addition to the continent and this study at least helps put some dates on some northern locations. Now to find some rocky mountain loess...