Jan
04
2009

More Lake Weiss Trilobites

I had a chance to find more trilobite fossils this weekend, and unlike before they were not just impressions, but the whole critter.
a trilobite
The picture below shows a very small fossil that was already loose from wave action.
a trilobite
Looking at the local rocks and their layering suggests an explanation for the spotty distribution of fossils. When the mudstone was laid down in the Cambrian the area was a wide tidal mud flat. Much like the area around Tybee Island
Tybee beach(but not as sandy and NO trees) or the mudflats near the Severn river or the Gower pennisula.
Severn Mudflats
Mud flats are not really flat, but tend to have little pockets where biological detritus collects.
a horseshoe crab These pockets become hotspots for fossil collection when covered with silt by tidal floods. There are occasional quasi-stable areas and these can also support a colony of animals that is subject to being covered in a local flood event and we have found a few dense patches of Crinoid stems – including one with a trilobite in it that I don’t have a picture of (yet). Shallow water above mud flats is a great location for small crustaceans and various small mollusc’s which is just what we find in the fossil evidence. Some of them were just unlucky (or lucky – it depends on your viewpoint) to get caught in a small pool at low tide and have their remains preserved for us.

One of the stronger pieces of evidence that this was an estuarine environment comes from about 100-1000 layers above the fossil rich layer(if each major layer in the mudstone reflects a season – then maybe only about 1000 years later) the river shifted and deposited a layer of gravel on top of the mudstone. There is a continuous shift in the color of the sediment from a dark gray – where the fossils are found to a lighter gray or tan and we don’t see visual evidence of a discontinuity event. It looks just like a gravel bank deposited on the side of a river. In some ways it is a pity that it is so old -because it would be a great place to look for land animal fossils – just that there aren’t any from the Cambrian.

Written by Rob in: outdoors,science,Uncategorized,Wildlife |

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