The Steinheim Basin Snail Series [Part 1]

Hilgendorf's original phylogenetic tree
Fig. 1: Hilgendorf’s original phylogenetic tree

While Darwin’s Origin of Species provided plenty of evidence for the concept of descent with modification, there were no phylogenetic trees presented in the text (besides a completely hypothetical one). The first phylogenetic tree depicting fossil organisms came in 1866, in a dissertation by Franz Hilgendorf on the Planorbis snails found in the Steinheim Basin. It was a complete endorsement of Darwin’s theory and controversial; some ignored it completely, while others had a different conception of how the series actually went. The reasons for the changes in the snails was also discussed. Miller (1900) said it was due to hydrothermal vents (based on a misinterpretation of the Steinheim Basin’s origin as volcanic). Klähn (1923) supported the idea of new species migrating into the lake, causing the changes. Until the 1980s, the main idea was that these were not different species but simply ecophenotypes.  Since then, a series of research projects have taken place, including some stunningly bad examples of how to do science (Mensink 1984 measured hundreds of specimens and conducted geochemical experiments on them, with no real purpose). These form the basis for today’s understanding of the snail series, but Hilgendorf’s contribution back in the 1860s should not be forgotten.

Before moving on to describe the snail series itself, a geological introduction is needed.

Geology of the Steinheim Basin

Steinheim am Albuch lies in the middle of a meteor crater called the Steinheim Basin. In the middle Miocene an asteroid penetrated the atmosphere. During it’s entry, a piece of around 100 m diameter broke off and formed the 3.5 km large Steinheim Crater. The main asteroid, which was 10 times larger, struck to the east and formed the 24 km large Nördlinger Ries. While there is no direct evidence of the asteroid and meteorite, typical impact structures are found, including coesite and stishovite (extremely high pressure forms of quartz, formed when the asteroid hits the ground) and shatter cones. The age, from Buchner et al. (2003), is 14.3 ± 0.2 Ma.

A lake soon formed in both impact craters. The Steinheim Lake was, at the beginning, nutrient-poor, with few primary producers. There was a high oxygen concentration in the water as a result. The only water coming into the lake was from the underground and from the rain. There were many water level fluctuations (Fig. 2); these are related to tectonic processes and, to a lesser extent, evaporation.

Fig. 2: The geochemistry and water level of the lake.

These fluctuations affected the ecosystem and geochemistry of the lake. At it’s highest level, the lake was 120 – 160 m deep. At that point, the lake was meromictic: there was no mixing of deep and surface waters, so the deeper parts of the lake were deprived of oxygen. At other times, the lake was very shallow. We don’t know when the lake completely disappeared, but it did survive for several thousand years until it dried out. This makes it an ancient lake, allowing us to compare it with recent ones, like Lake Baikal or LakeOhrid.

Such lakes are excellent for evolution research. They are natural test tubes, where the inputs and outputs can be reliably quantified. The longer the ecosystem survives, the longer evolution can be observed. Recent ancient lakes give us spectacular results (e.g. cichlids in Lake Malawi and Lake Tanganyika). The Steinheim Basin is a similar story, where we can observe the evolution of the endemic Gyraulus snails.

With that, we come to the actual point of this post: the changes in the snails.

The Snail Series

Invertebrates of the Steinheim Basin

As you can see in Fig. 3, there was a large biodeiversity in the Steinheim Lake., but what we’re primarily interested in is the Gyraulus series (2 – 13 in Fig. 3). Although Pseudamnicola, another snail genus, also survived throughout the history of the lake, it is the changes in Gyraulus‘s shell through time that make this lake especially fascinating.

Gyraulus is a planorbid genus. It’s characteristics include a small size, a relatively thin shell and the sculpturing on the shell (periostracal sculpturing). They also have well-defined embryonal stripes, seen on the protoconch (the embryonal shell) as longitudinal stripes. These vary in number depending on the species.

The radiation of the Gyraulus snails in the Steinheim Lake shows two clear trends: an increase in the size and thickness of the shells. This is a trend seen in other snails undergoing radiations in current ancient lakes, allowing direct comparisons.

Before moving on to possible explanations for these changes, let’s look at the individual species.

Changes in the Snails’ Shells

In order to best follow this, keep Figure 1 in mind.

Fig. 4: G. kleini. Scale line: 400 µm

G. kleini was the original Gyraulus species in the lake. The shell has no sculpturing and relatively thin. It looks similar to recent Gyraulus species. It has not undergone any evolutionary changes in the lake yet. The other species come from this one.

G. steinheimensis
Fig. 5: G. steinheimensis, 7x magnification.

G. kleini evolved into 2 species: G. steinheimensis (main series) and G. crescens (side series). G. steinheimensis is similar in shape to G. kleini, but a bit larger and thicker.

G. tenuis

G. tenuis is a transitional form between G. kleini and G. sulcatus. The aperture is not rounded but tetragonal, the shell is becoming flatter, and slight bulges appears on the suture.

Fig. 7: G. sulcatus. Scale line: 1000 µm

G. sulcatus evolved from G. kleini. It is much larger, the aperture is tetragonal and there are four well-defined bulges. The shell’s winding also got translated, as can be seen by the light asymmetry.

Fig. 8: G. trochiformis. Scale line: 100 µm

G. sulcatus then changes to G. trochiformis. The winding’s translation goes further and results in a compact spiral shell, with the bulges integrated into the coils. An important thing to note is the lack of a columella.

Fig. 9: G. oxystoma. 7x magnification.

G. oxystoma is the next step. The shell becomes smaller and thinner; it starts to look more like the original G. kleini. This trend goes on in the next species.

Fig. 10: G. revertens. Scale line: 100 µm

The aptly-named G. revertens goes back to the same form as G. kleini: small, with a thin shell. Their forms and sizes are almost identical. The difference lies in the comarginal sculpturing (the stripes in Fig. 9), which is not found in G. kleini. It is still disputed whether G. revertens a side branch or part of the main series is.

Fig. 11: G. supremus. 7x magnification.

The development of G. supremus cn be compared to that of G. sulcatus/G. tenuis. It’s the same tendency, with less defined edges and bulges.

That was the main series, a perfect example of anagenetic evolution. There is also a side series that split off from G. kleini, and it followed different trends.

Fig. 12: G. crescens. Scale line: 200 µm.

G. crescens evolved from G. kleini. The only differences between them is the comarginal sculpturing.

Fig. 13: G. costatus major. Scale line: 200 µm.

The comarginal sculpturing becomes much better defined in the next step, G. costatus. Fig. 13 shows exceptionally well-built ribs around the coil. The shell itself, however, remains small.

Fig. 14: G. denudatus, progressively unrolling. Scale line: 200 µm.

The final step is G. denudatus. This shows a progressive uncoiling of the shell and scalarid-formation. With rising scalaridity [this is not a real word] the ribs become less well defined and more distanced from each other, until eventually there is a complete loss of sculpturing.

A scalarid is a snail shell whereby the winding goes along the longitudinal axis. Recent scalarids are viewed as ecophenotypes. The physiological basis of scalarid development is not known, but a constant observation is that they form in shallowing waters. There are no real evolutionary advantages to be gleaned: scalarid shells are fragile. G. denudatus counts as a different species, since its progression can be followed from G. costatus. That such a ‘disadvantageous’ form can survive means that there were less selection pressures in the ecosystem. By comparing to modern scalarids, we can tell that they formed during a phase in which the lake was drying out (the high salinity would lead to the extinction of G. trochiformis) – and it is with G. denudatus that the lake completely evaporated.

Look at part 2 for the explanations for these changes!


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