March 22, 2010

Reconstructing full-glacial Europe

When Charles Lyell first used the term Pleistocene in 1839 to replace the "long and awkward" use of Newer Pliocene (preceded by, of course, the Older Pliocene), it was done in the interest of streamlining the terminology, but the split of Newer and Older Pliocene was based on the fossil evidence of the time. Lyell designated the split of the Pliocene by recognizing a higher percentage of extant snails and other mollusks found in strata from this Newer Pliocene or, now, Pleistocene period. About 70 percent of the mollusks from this period were extant animals, compared with 50 - 70 percent from the Older Pliocene.

Today, snails are still used as an important index for reconstructing paleoenvironments. Michal Horsák and researchers from the Czech Republic, Russia, Poland and Slovakia published a study a couple of weeks back in the Journal of Biogeography that attempts to determine the ecology of full-glacial Europe using index species like snails and trees to identify possible refugia - modern environmental/ecological analogs of the coldest periods in Europe tens of thousands of years ago based on fossil evidence and modern assemblages.

Recently, evidence has pointed to the Russian Altai and Sayan Mountains as modern analogs. The climate matches, cool and relatively dry. Surface pollen spectra in these areas are very similar to samples from full-glacial central Europe. The ranges are also home to similar and in some cases, identical biota. Mammal assemblages are largely comparable. So-called sibling species of trees from full-glacial Europe have been found in the area, including the Siberian pine, Siberian larch and Round-leaved dwarf birch(related to or subspecies of the Swiss pine, European larch and arctic dwarf birch respectively).

Photo by Ondřej Žváček

The most important biotic reference is the snails found in the Altai Mountains for several reasons, as the authors note:

First, snail shells can be easily identified to species, whereas plant pollen is often identifiable only to genus or family. Second, each shell represents one individual, which enables an accurate estimation of population density. Third, unlike pollen or vertebrate remains, snail fossils are usually deposited in places where they lived, thus enabling a fine spatial resolution of the resulting palaeoenvironmental reconstruction. Fourth, snail fossils are well preserved in conditions that are usually poor in other fossils, especially in dry and calcium-rich sediments.

Samples were taken at 118 sites in a 300 km transect in southern Siberia, 10 x 10 m2 plots and the sites were all characterized according to environmental conditions. The researchers used 13 designations, and instead of reinterpreting or just glossing them over, I'd rather just quote in detail since they were able succinctly describe each environment. These categories are not only important in interpreting the results, they give a good impression of what the environment is like in this portion of the Altai Mountains (n signifies the number of sampled sites).

Taiga (n = 11): mesic to wet coniferous forest with a species-poor herb layer and a well developed moss layer, usually dominated by Abies sibirica, Larix sibirica, Picea obovata or Pinus sibirica.

Hemiboreal forest (n = 22): dry to mesic coniferous or deciduous forest with a species-rich herb layer and a sparse moss layer, usually dominated by Betula pendula, Larix sibirica or Pinus sylvestris.

Wooded fen (n = 2): wet woodland with accumulation of base-rich organic sediment, dominated by Picea obovata.

Treeless fen (n = 4): open base-rich fen with sedges and mosses.

Acidic mire (n = 7): bogs and mineral-poor fens.

Alluvial scrub (n = 3): riverine woody vegetation with willows (Salix spp.).

Tall-forb vegetation (n = 5): tall herbaceous vegetation in wet places, usually along mountain streams.

Alpine grassland (n = 3): short herbaceous vegetation above the timberline.

Shrubby tundra (n = 8): low-shrub vegetation above the timberline, dominated mainly by Betula rotundifolia (= B. nana s.l.), in places also by Dryas oxyodonta.

Steppe (n = 36): both short-grass steppe and tall-grass (meadow) steppe, in places with low shrubs such as Caragana or Spiraea.

Saline grassland (n = 4): grassland on soils with elevated salt concentration, usually in shallow depressions in steppe landscapes.

Meadow (n = 8): mesic grassland used for regular or occasional hay-making.

Scree (n = 5): treeless talus slopes.

As you can see, steppe and Hemiboreal forest tend to dominate the landscape, dry, cold areas very similar to what is thought to have been the conditions of central Europe during Pleistocene glacial periods. The strange thing, generally speaking, is that snails are poikilothermic animals, meaning that their body temperature is susceptible to changes in ambient temperature, placing them under risk during sudden cold snaps. The authors suspect that these relict snails have not colonized warmer areas because of increased predatory pressures, which seems to line up with a particular ecological circumstance: homeothermic predators have higher metabolic requirements and might not be able to live in certain areas where these poikilotherms thrive.

Most of the index species of snails in this study were found in wooded or shrubby areas, wet microclimates protected by tree canopy or shrub cover. These wooded areas tended to be dominated by Larix sibirica and Picea obovata, the sibling species of the European L. decidua and P. abies, evidence that the vast steppes of full-glacial Europe may not have been as devoid of trees as previously thought.

So the reconstruction proceeds, trying to refine the conditions of ancient ecosystems and biomes under particular climatic or geologic pressures. It's a fascinating process, assembling such seemingly disparate evidences and mechanisms reaching across an interdisciplinary expanse to make relatively small but essential contributions like this one.

ResearchBlogging.orgHorsák, M., Chytrý, M., Pokryszko, B., Danihelka, J., Ermakov, N., Hájek, M., Hájková, P., Kintrová, K., Kočí, M., Kubešová, S., Lustyk, P., Otýpková, Z., Pelánková, B., & Valachovič, M. (2010). Habitats of relict terrestrial snails in southern Siberia: lessons for the reconstruction of palaeoenvironments of full-glacial Europe Journal of Biogeography DOI: 10.1111/j.1365-2699.2010.02280.x

Laporte, L F. (1990). Establishment of a geologic framework for paleoanthropology. Boulder, CO: Geological Society of America.

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