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Fossil bonanzas

Updated Monday, 15th November 2004

Taphonomy - we explain fossil bonanza, and why fossils are plentiful in certain rocks

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Types of fossil bonanza

 

There are particular rocks throughout the world known as ‘fossil bonanzas’. The scientific word for these rocks, lagerstatten, comes from the German words for ‘deposit’ and ‘place’, referring to the places where fossil deposits are found. These rocks are important because they formed under exceptional circumstances. There are two types of lagerstatten, those containing a huge abundance of fossils, and those containing fossils with exceptionally preservation, often including soft parts. Both are incredibly useful scientifically.

British fossil bonanza

One example of a British fossil bonanza is a band of blue coloured limestone running from the north east coast down to the south coast. This rock, known as the Lias, contains many fossils. The first fossil reptiles were discovered here, before anyone had even contemplated the existence of dinosaurs. One of the best ways to find fossils is to look for hard lumps of rock called nodules, as these often contain beautiful and uncrushed fossils. Part of the reason for the good preservation is the formation of minerals in the rock, very early in its history.

What is a nodule?

One of the best ways to find a fossil is to look for nodules. The formation of nodules within sediments is often vital for good fossil preservation. Nodules are rounded lumps of hard stone found within mudstones and limestones, which are often small but may reach metres in size. They are clearly distinct from the surrounding rock known as matrix. Nodules form due to chemical reactions in the sediment often very soon after burial, whilst the rest of the sediment is still soft. Localised chemical conditions cause the rock to harden (cementation). Cementation initially occurs around a centre point, known as the nucleus, upon which mineral particles may be precipitated to form the nodule.

Nodules protect the fossils

The nodule gradually grows outwards from the nucleus, which may be a small grain of rock but is very often the remains of a dead organism. In such cases, the organism is quickly encased in a protective rock ‘shell’. When the rock is later buried at depth and subjected to crushing pressure and destructive temperature, the fossil within the nodule is protected and survives. Fossils in nodules are so well protected, that they usually even retain their three-dimensional structure.

 

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Next: Fossil preservation

 

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