See Part 1 for the history, geology and descriptions of the snails. This post’s goal is to offer explanations for the changes observed. It is useful to keep these 2 pictures open as a reference: geochemistry and sea level and biostratigraphy (2-13 are the Gyraulus snails).
In the kleini layers (Schicht = layer), G. kleini, the common ancestor, is found. The split to steinheimensis has already taken place. The water level was at its lowest, as was the carbon concentration. For us, this layer acts as the basis to which we will compare the others. In the steinheimensis layer, G. kleini and G. steinheimensis are found together. G. steinhemensis‘s population size increases, hinting that its larger size and thicker shell is ecologically advantageous. The water and carbon levels rise.
At the start of the sulcatus layer, G. kleini and G. steinhemensis die out and G. sulcatus dominates, meaning that it managed to adapt to its environment better. An ecological explanation for this would be that its larger size and very thick shell was a successful defence against predators. The rising water levels did in fact cause a fish radiation, which only G. sulcatus survived, since G. kleini and G. steinheimensis were easier to eat. Another explanation would be that the changes in the shell’s construction reflect some kind of physiological imporvement – but this is hard to prove using fossils.
Although G. sulcatus developed relatively early in the lake’s history, it took a long time until it became dominant. It is rare to find it in the steinheimensis layer, so obviously G. kleini and G. steinheimensis were much better adapted for the environment at that time. The question is whether G. sulcatus caused the extinction of G. kleini and G. steinheimensis (ecological dominance because of better physiology), or whether the extinction of G. kleini and G. steinheimensis (due to predators and/or environmental changes) caused the radiation of G. sulcatus.
The trochiformis layer follows on from the sulcatus layer. Here we see the dominance of G. trochiformis and the extinction of G. sulcatus. A similar story happens in the following oxystoma layer: G. trochiformis dies out and G. oxystoma dominates. The question is the same as before. But here, it’s important to note the massive changes happening in the lake. Towards the end of the sulcatus layer, the water level starts to sink, a trend that goes until the middle of the oxystoma layer. The carbon concentration follows the water level. G. trochiformis lived in a time when the water level was sinking and the salinity was rising. That’s what G. oxystoma developed from and opbviously, it was much better adapted to the new environment. As soon as the conditions change and the water level rises again, it dies out. The question is, as always, why and how. Until now, the size and thickness of the shells were somewhat proportional to the water level – as it rose, the shells got bigger and thicker. Is this as a result of the ecology or of the geochemistry?
The next layer is the revertens layer. The transition from oxystoma to revertens is marked by a transgression (a rise in water level) caused by an increase in groundwater flow (influenced by tectonics). This is important for the salinity levels in the lake. In the time span covered by this layer, the water level remained constant and only G. revertens dominated. We don’t have water level data for ther supremus layer, but it’s a fair assumption to say that the lake was drying out. he G. revertens population decreases drastically and G. supremus prevails. Think of our correlation between size and water level from before; here, although G. supremus is relatively large, the water level is sinking.
The side series should also not be forgotten. It first arises in the sulcatus layer, as the water level was at its highest. They die ut at the same time as G. oxystoma, during the regression (decrease in water level). As the water level rises again after the revertens layer though, they make a comeback. There are interesting comparisons to be made between the two series, from a population dynamics perspective: the side series shows a dominance in species number (the three species always exist at the same time), but the main series has very high individual numbers (i.e. one dominant species with a large population). This could be important in explaining the adaptations of the various snails.
The Evolution of the Snails
According to Gorthner (1992), we have to keep 4 factors in mind:
- What were the adaptations for?
- How is the shell constructed?
- Why have the shells changed?
- How have the shells changed?
Obviously, these are all linked but it’s important to differentiate between these planes. We are lucky to have similar recent ecosystems with similar faunas to use as comparisons.
Pollard (1975) noticed that snail shells are stronger in areas of low precipitation – this is an adaptation against dryness. But the Steinheim Basin snail shells do not get stronger, so it follows that there were no drought problems. Another observation is that the shells of limnic gastropods get bigger and thinner in the deeper areas of a lake. In the Steinheim Basin, we see an increase in size but also an increase in thickness. This could be an adaptation, it could be caused by physiological changes or simply be a side-effect of changes in the chemistry of the water.
A general observation from evolution is that endemic species are larger (remember, the Steinheim Lake was a closed system, so the Gyraulus snails were endemic). We see a clear increase in size (G. steinheimensis → G. trochiformis and G. oxystoma → G. supremus). This also agrees with Cope’s Rule, which states that body size increases with evolutionary time. This has several advantage: protection against predators, reproductive success and even a higher life expectancy – but these are not free advantages. Limnic gastropods tend to adopt an r-strategy, since lakes are normally short-lived ecosystems and therefore unstable habitats. Larger snails (such as those of the main series here) adopt a K-strategy. Overall, this leads to an ecosystem where many small species exist, but is dominated by large individuals. Also, with the increase in shell size, the sculpturing changes for purely physiological reasons, leading to the bulges. This improves the respiratory efficiency of the larger species.
That said, the reasons for the changes in the Steinheim Basin snails are not totally clear, and are still being hotly discussed. Originally, Hilgendorf did not propose any explanations, but did later suggest that the changes were a result of water currents: the thickness increased as protection against strong currents or waves. But there is no correlation between wave strength and sculpturing – it is debunked by asking one simple question: why are the shells of deep-water snails sculptured? Still, this was a better proposition than some other laughable theories published, which I will not mention. Instead, we will discuss the theories considered nowadays.
One theory says that the sculpturing is nothing more than a consequence of the shell construction process. Biomineralisation is influenced by available ions and the salinity. This theory is not only supported by geochemitry, but also by recent studies in ancient lakes, where a positive correlation between salinity and sculpturing (and shell thickness) is observed. However, the problem is biological: biomineralisation in molluscs is not fully understood yet; a molluscan shell is not only composed of minerals, but also of highly complex proteins and glycolipids. The physiological and biochemical processes remain unclear.
Another theory (and the one I support) has to do with selection pressure from predators. Three cyprinid species have been found in the Steinheim Basin: Tinca, Barbus and Leuciscus. Recent representatives of these taxa feed partly on gastropods. The increase in size and thickness may have been a protection against getting eaten. The threat comes not only from these fishes: Trochotherium, a weasel-like carnivore found in sediments associated with the lake, has an untypical dentition: not so piercing, but has unusual buttonish teeth. This is ideal for cracking hard-shelled animals, like snails. Of course, they played a minor role compared to the fishes. However, a major blow to the theory is that no positive correlation has been found between predators and shell changes.
Finally, we come to the genetic theory, which states that the changes are not influenced by the ecology, but are just the results of random mutations. This may sound silly, but it is an observable fact that in large snail populations, sculpturing does originate without any special selective pressures. Problematic, also from a philosophical point of view, is that with or without these pressures, the same result come about.
Our data is limited to the snail shells and their changes. We have no information about the soft parts. On top of that, several factors could have, and most probably did, play a role in this evolutionary series. It is especially hard to identify these.
On more philosophical levels, our fossil record is spotty. Then there is the species problem: in palaeontology, species are differentiated based on their morphology. An objective way of doing this is, in most cases, extremely difficult. As an example, should all measured features be treated equally, or should they be weighted according to their relative importance?
On a much more practical level, any sedimentary structures observed at the site could be imprinted on and so give us invalid ecological information that is useless in an argument.
To summarise, even though this was the very first described phylogenetic tree, explanations are still eluding us. In the past 150 years of research, we’ve made a lot of progress. New methods and advances help in understanding the different processes at work here. Nevertheless, there are many open questions remaining – the answers, in my opinion, lie in observing and studying recent ancient lakes.
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