I apologize for errors in grammar, spelling and such - this was essentially an extemporaneous writing exercise.
What are the major problems and limitations involved with using late Quaternary terrestrial gastropods as environmental proxies? Conversely, how are terrestrial gastropods assemblages superior to vertebrate and pollen records with respect to reconstructing late Quaternary environments?
Gastropods can make excellent paleoenvironmental indicators, but there are several overlying problems that need to be either addressed, or at least acknowledged in order to accurately interpret any environmental signal contained in gastropod deposits.
Taphonomy is a very real problem. It affects nearly every other aspect of terrestrial gastropod paleontology as well. The processes that occur to a shell deposit can erase any real effect of environment and introduce false ones. Preferential preservation can remove the trace of some species of gastropods, leaving others behind. One could misinterpret the abundance of the surviving shells as the ones that were most abundant, but in actuality, another species with a shell more susceptible to destruction may have been more abundant. This is a problem with some terrestrial gastropods as their shells are usually thinner than their lacustrine or marine relatives and some species of terrestrial gastropod produce shells that contain almost no calcium carbonate. In some cases, this may be a problem that we never know about – since it is often impossible to ‘know’ for sure that all the species living in a particular environment are present in a fossil assemblage. In fact a safe assumption is that there will always be species not represented in the fossil record. However, it is probably safe to assume that the fossil assemblage bears more than just a superficial similarity to the actual diversity.
A concentrated deposit of gastropod shells may reflect an actual abundance at one particular time, or they may be winnowed from more dilute deposits, producing a time-averaged concentration of shells. This would have the effect of making a particular species appear more abundant that it might have been, and an overall assemblage seem more diverse than it was. A common problem with reconstructing past environments is to depict a little bit of everything from a site living contemporaneously with each other. African mammals are often depicted as roaming the landscape in densely populated, diverse herds. However, any trip to the African savannah shows that the actual density is far less – one can travel for miles without seeing much of wildlife, a far cry from the wildebeast and rhino behind every corner that is often conjured up by descriptions of the region.
Alluvial terraces often contain terrestrial gastropod fossils, not necessarily because the gastropods were living near the stream, but they were washed down off of the high valley ridges and into a more mixed deposit. A high rate of deposition could make what is a diverse assembage of gastropods look sparse because there will be a greater volume of sediment included with the sample.
While most gastropods die from dehydration or starvation, predation can affect the relative diversity of a gastropod deposit. Is the deposit a shell midden, where the shrew, or bird deposited its preferred prey species? Or, is this deposit missing an important species of gastropod because they were more likely to be preyed upon? Larger gastropods especially seem to bear the signs of predation, but at the same time their shells are more likely to survive after the soft organism itself is eaten. It is easier to just eat the entire snail, shell and all for small gastropods, while larger snails can be fished out of their shell without destroying the whole shell.
Often, gastropods are used as environmental proxies because of the foods they eat. However, it is more likely gastropods are temperature and precipitation dependent, and will find foodstuffs in that preferred climatic range, rather than foodstuffs in a different climatic setting.
A demonstrated problem for aquatic gastropods is ecophenotypy. Some species of gastropods exhibit different shell structure dependent upon the substrate or water energy present. There is some evidence to indicate that some species of terrestrial gastropod are also ecophenotypic – if demonstrated, ecophenotypy can be extremely useful for interpreting a snail fossil’s environment. However, it can also lead to false interpretations such as overestimated diversity.
Stable isotopes are often used as climate proxies. Variations in the ∂13C values of carbonates in shells has been used to determine shifts in C3 versus C4 grasses and therefore shifts in precipitation. However, there are kinetic fractionation effects from the shell material and any environmental stable isotope signal can be overwritten by the physiology of the snail.
So, with all the problems associated with gastropod fossils, why study them at all? One of the strengths of gastropods is they are very sensitive to changes in their environment. Unlike a tree, which will remain in an area and produce pollen long after the conditions are no longer ideal for it, snails have a very limited residence time. And, unlike vertebrates, which migrate easily, snails can’t run away from unfavorable environmental conditions. Gastropods do have the ability to survive short-term fluctuations in the environment – some species can remain dormant for several years, thus outlasting long droughts. Large-scale changes in regional climate, however, will affect the distribution and makeup of gastropod assemblages.
Gastropods can be found in tremendous numbers. While the total number of mammoth skeletons in North America is quite small, a single liter of sediment can contain hundreds of thousands of gastropod shells. The sheer volume provides a more statistically significant population size from which to base environmental interpretations.
Processing material for quaternary fossils requires a varying degree of field effort and lab work. Charismatic megafauna take considerable time and effort to excavate. Microvertebrate skeletons, while easier to collect, disassociate into a tedious piles of skeletal elements that must be sorted and identified in order to measure abundance. Pollen work involves long lab preparation with hazardous chemicals and many hours under high-powered microscopes. Pollen can rarely be identified beyond genus, and some can’t be identified beyond the family level.
Gastropod deposits are often easily collected in bags or buckets of unlithified sediment that is easily dried and washed through a series of screens. A simple, low-powered binocular microscope and a sable brush are all that is really needed for identification and sorting. Most gastropods are identifiable to species-level based solely on the shell.
Most Pleistocene gastropod species are extant today, and it is reasonable to assume that the modern climatic preferences of gastropods were similar to those of their Pleistocene relatives. Mapping the biogeography of terrestiral gastropods has the potential to produce a higher-resolution climatic interpretation of the environment.
Terrestrial gastropod biogeography can also show paths of recolonization – directions from which populations expanded their range. This has important implications for population and conservation genetics especially as it relates to extremely rare species, such as Discus macclintocki. They also provide an interesting study in evolutionary biology, as D. macclintocki is an extremely isolated species while a close relative, D. chronkheitei is widely distributed, even occurring west of the Rocky Mountains. Steven J. Gould used the tropical genus Partula to study the mechanisms of speciation. It is not inconceivable that the changes in distribution among species of Discus can provide information on the diversity of North American gastropods.
Finally, terrestrial gastropods are the single most threatened group of land animals. Understanding how changes in ancient climate affected gastropods, can help us anticipate future their future changes. They also have the potential to quantify the extent to which the present day climate is changing by providing a comparison to the variability of the early and mid-holocene.