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And in the terrestrial sphere:

The terrestrial sphere probably, for most of this algal time, even after the earth cooled off, looked very desert like with occasional glaciers. If one poked around inside the moist areas on the planet, you might have found that some of the individuals who were parts of these communities of stromatolites, might be living in the interstices between sand grains in moist environments. We might call that a land plant. It certainly is a photosynthetic organism growing on land but yet, it is not very interesting. But, I am biased. I like big things. If there's phycologist, a student of algae, in the audience, they are sharpening their knives for me now.

But, in terms of influencing the terrestrial environments, in terms of supporting terrestrial animals, these algae are not going to be greatly significant. Here is a diagram of Earth time the Cambrian/Precambrian boundary is the time of the appearance of metazoa, and there is much time (expanding many meters below the slide into the ground) that encompasses the Precambrian period of time. The boundary falls at 544 million years. However, it is not until the middle Silurian that we see serious evidence for land plants. So, it took plants a long time to come out and start to colonize land.

Now it is true that there is a potential prior to the Silurian for some kind of fossil record that we might be missing. For example, and in particular, it is possible that lichens were around prior to the middle Silurian about 400 millions years ago. Lichens are a combination of the unicellular algae with a fungus. It is a commensual relationship. However, they do not have a strong likelihood of fossilization. We know that fungi and algae have been around for about two billion years. So, it is possible that at some time between two billion years ago in the Precambrian and the Silurian, there were terrestrial communities of lichen life. We don't have any fossil record of them. It is more likely that the earliest fossils that we see are showing us something like a carpet of moss growing in a moist situation (In this case I photographed it on a rock).

The early communities of land plants that diversified in the Silurian and into the early Devonian probably were largely restricted to moist, low lying, areas because the plants were still tied to water for life and reproduction. The mountains in the background were probably virtually desert-like. Vegetation is limited to moist areas, and is at most a meter high and in many cases, probably half a meter or only a few centimeters high. It depends on your perspective. If you are an individual bug crawling around in this,  it is going to look like a three dimensional community. From the perspective of us as terrestrial vertebrates of fair size, this looks like a two dimensional community. However, this new community does represent a source of food and shelter growing adjacent to water. The water is well populated with invertebrates and vertebrates. What is going to come out of the water and make use of this new set of resources?


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