Thursday, October 27, 2005

Quals, vol. 1

This semester, I passed my qualifying exam. More of a formality than any real test, but it was still a lot of work. The format in the sed/paleo department at Madison consists of eight essay questions with an hour (sans notes) to answer the question. I'm going to try and put some of those questions and my answers up here in vain hopes of getting some comments from other paleontologists.

Anyway, here's the first in what may or may not be a series:

Why do some ancient lake deposits have diverse fauna that evolved through time, whereas in other deposits of similar duration the fauna appear less diverse and relatively static?

To fully answer this question would be a complete Ph.D. in itself. However, there are many factors that could play a role in the disparity between different lacustrine faunas. In a broad sense, there is a large body of evolutionary thought that holds the more stable an environment is, the more diverse the corresponding flora and fauna will be. And conversely, the less stable the environment, the less diverse the corresponding flora and fauna.

As an example, Lake Tanganyika contains more than 300 species of Cichlid fish, while nearby lake Malawi does not support nearly that many. So why are there so many cichlid species within Lake Tanganyika? The answer lies partially within its stability. It is a large, hydrologically open lake. Tectonic processes have kept it more or less open for the past several million years. Lake Malawi appears to have been more varied in its hydrology – and perhaps water chemistry.

An analogy would be the fauna found on tidal flats versus the fauna in carbonate reefs. Organisms that are tolerant of environmental extremes often dominate the tidal flats. The dramatic fluctuations in water level on a daily basis and the seasonal fluctuations of temperature and tide strength favor organisms that are generalists. These generalists can occupy several niches, and the intense competition for scarce resources keeps overall diversity low. The highly productive carbonate reefs, however, do not experience the seasonal extremes, nor the heavy tides associated with the flats. Instead, these environments are relatively stable, allowing organisms to exploit very specific niches. This specialization can reduce competition for varying niches, allowing a greater diversity of specialists within the same ecospace as compared to the tidal flats.

Another part of the explanation lies in the organisms themselves. Some organisms appear more prone to speciation than others. For instance, there have been relatively few species of lake sturgeon, despite it being a very long-lived group. Cichlids, by comparison, haven’t been around as long, but are an extremely diverse group. There is a group of cichlid fish that do nothing but feed on the scales of other cichlids. Within this scale-eating group of cichlids, there is one species with jaws that curve off to the side so that it can better grab scales off the body of a nearby fish. There is a separate species whose jaws curve the opposite direction, which allows them to more easily grab scales off the other side.

However, this “diversity” can be misleading. What the above example describes is phenotypic diversity – different physical forms, exploiting specific resources. There is another kind of diversity that can remain hidden from the observer: genetic diversity. Another cichlid example involves a group where the male grows very large and protects a harem of smaller females. The females hide in the empty shells of gastropods. Males will try to steal females from other males by taking the snail shells (complete with female resident). Larger males will have larger harems and produce more offspring. However, there is an alternate breeding strategy employed by some males of this species. Instead of being very large, they instead are rather small, and resemble the females. These males sneak into the harems of the larger males and mate with the females, right under the proverbial nose of the bigger males – thus making more of the small males. These are the same species of fish, with presumably low genetic diversity within the group but a high phenotypic (physical) diversity within the group.

Another example comes from the Green River Formation. Goniobasis tenera exhibits a great deal of variation in the decoration on its shells. Some are very subtle, with relatively flat whorls, and little sculpture. Others have very rounded whorls and prominent bumps and ridges on its shell. These are, however, the same species – and it is very likely they are the same species because nearly all pleurocerid gastropods (the family that includes Goniobasis) show this type of phenotypic plasticity. Goniobasis dominates the gastropod fauna within the Luman Tongue (or member, if you prefer) and several beds of the Laney Member. Viviparous sp. fossils are common in the Tipton Member, and one might be tempted to interpret the Luman and Laney units as being a longer-lived and more stable lake environment. However, the actual genetic diversity may very well be higher in Viviparous sp. from the Tipton than in Goniobasis tenera from the Luman or Laney.

Another reason might have to do with taphonomy. A lake with a relatively low sedimentation rate might have a high diversity of fossils within a lithologic unit, yet the fossils within that unit might not represent the actual diversity at any given time in the history of the lake. While a lake with a high sedimentation rate might only appear to have a low diversity: the abundant organisms separated by the relatively high volume of sediment dumped into the lake. These time-averaged deposits can make a low-diversity deposit seem abundant, and an abundant environment seem almost empty.

There are also biases in preservation – some organisms might be quite abundant, but contain relatively few hard parts to be fossilized. Shelled organisms dominate the described faunal character of the Cambrian, but how much of that is a bias in preservation. There is also a bias in perception (which partly plays into the genotypic versus phenotypic argument): much of paleontology seems vertebro-centric. Vertebrate fauna can appear at first glance to be more diverse than invertebrate ones, but most often the reverse is the case.

While it is likely that stable environments do often contain a more diverse fauna than unstable ones (even though the environments may account for a similar length of time), the actual diversity needs to be carefully considered. How much of the diversity is a bias, whether in preservation or the observer, and how much of that diversity is actual? For long-lived lakes, we can study modern diversity both on the phenotypic and genetic levels. For fossil lakes, such as Lake Gosiute, we are often left with searching for modern analogs and cannot separate the sedimentology from the paleontology. Only the combination of the two may provide a complete answer.

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