Sunday, January 21, 2007

Where the fossils are

Today I had a rare opportunity to see something that gets most palaentologists excited. First, a little introduction. If you're not very familiar with finding fossils, you'll first need to know that fossils are found in sedimentary rock: the type of rock that is formed by deposited sediments (ie. sand, mud, or chemical precipitates). However, as any palaeontologist or amateur fossil collector will recount, you can search through vast amounts of sedimentary rock without ever finding a fossil. One can sift through tons of rock in some places and not find a single scrap of bone, or shell, or leaf of plant. On the other hand, there are places where one cannot take two steps without walking on fossils.

Fossil preservation can be a very selective thing. Some environments are more conducive to finding fossils than others. This week, I am in Wales where I had the oppotunity to visit some Early Devonian fossil sites (about 410 million years old) that are worked by a local amateur palaeontologist. At one of his sites, I pointed out some geological structure that explains the high quality of the material collected there, and the promise for more fossils. If you're out looking for fossils, this is where you want to look.

Take a look at the image below. It shows a sequence of sedimentary rock layers and shows a classic type of structure known as a channel form. Notice the two different rock types. The upper rock is a coarse material, with heavy bedding. It's base is tapered to the left forming what's normally called a "lense" or a "lenticular bed". Below it is a noticably different-textured rock. It's heavily cracked and broken up. It is mudstone.

Here's the same image with some guides.

In the mudstone below the massively bedded (typically coarse-grained, but not greatly in this case) is where the fossils are. This is one of the best types of sequences for finding fossils and, in large part, is where articulated fossil animals are to be found. It should be no surprise then, that this friend of mine has actually recovered quite a few articulated fossils from there. He became quite excited when I remarked that this is the ideal type of sedimentary sequence in which to find articulated fossils. So, let's hope, some exciting discoveries will come from this site.

Why do fossils preserve so well in these sequences? What are they? These deposits form in a river channel, and the image below shows quite nicely the lenticular shape of the channel.

What you can see is that there is deposition of sediments in one direction that partly causes the channel to migrate (concomitant erosion of the opposite bank is the other cause). In such settings, bodies of animals are buried very rapidly. Moreover, they are quite prone to flooding and the rapid deposition of sediments (that is often why the bedding above is massive, as it was filled in rapidly, rather than in progressive layering).

This is where the fossils are.

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