Prehistoric ecology and foodwebs - palaeoecology

Once we have determined the diet of all the fossil animals in a fossil locality, it is then possible to fit them into a food web and reassemble the community.

Reconstructing the food web

We have to make sure that all the fossil animals are collected from rocks of the same age and from the same place. A layer of rock running down Britain from Scarborough on the north east coast to Weymouth on the south coast is known as the Oxford Clay. This rock is about 160 million years old and the areas of exposed rock are one of the best sources of fossils in Britain; commonly yielding coiled ammonite shells and bullet-like belemnites.

Primary Producers of the Oxford Clay

Both the fossils and the sediment in these Jurassic aged rocks are typical of a shallow sea, and this is the environmental setting for the ecosystem. All of the energy to fuel the ecosystem comes from the sun, and it is the job of the primary producers to convert this energy into organic matter via photosynthesis. In the Oxford Clay ecosystem, the primary producers were phytoplankton (floating in the water) and benthic microflora (living on the sea bed). This is much the same as shallow marine ecosystems today. These micro-organisms included a variety of biological groups including types of bacteria and algae. They were very abundant, as revealed by their numerous fossil remains in the sediment under the microscope. The benthic microflora must live in shallow water so that the sea bed receives enough sunlight to allow them to photosynthesise.

Top Carnivores of the Oxford Clay

At the other extreme are the top carnivores. In the Oxford Clay ecosystem these are the large marine reptiles such as the ichthyosaurs, plesiosaurs, and marine crocodiles. The other organisms can then be linked into the foodweb, including the invertebrates and fish. When scientists did this, they found that it was actually possible to split the Oxford Clay ecosystem into two separate food webs – the first was in the upper part of the water column where the phytoplankton were the primary producers - The second was in the lower water column, where the benthic microflora on the seabed were the primary producers. A major source of energy in the lower water column was dead organic matter falling from the upper waters.

Simplified food web for the Oxford Clay community.

Sustaining the food web

As well as sunlight, a steady nutrient supply is needed to maintain a complex food web. There are two sources of nutrients. The rivers running off the continents are one source of nutrients. We know these ancient shorelines must have been nearby because land fossils have been found in the marine sediments and there are many fossils typical of shallow water. The second input of nutrients comes from recycling the nutrients contained within the organisms when they die. This ecosystem was a very productive one. This is evident from the huge amount of organic carbon in the sediment, which is a direct result of biological activity.

Next: The Bristol dinosaur

Content last updated: 11/08/2004

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