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Exceptionally preserved fossils from the Soom Shale of South Africa

September 13, 2002 at 16:00
Dr Sarah Gabbott, Dr of Geology, University of Leicester

Dept. of Geology, University of Leicester
LE1 7RH 
telephone: + 44 (0)116 2523636
fax: + 44 (0)116 252 3819

Session organiser: BA Geology Section 

Key finding of the work
Many people believe that the processes of fossilization take thousands of years. On the contrary, where soft tissues are concerned, fossilization must take place extremely rapidly, within days and weeks after death, or the tissues will decay away. Minerals, such as apatite, pyrite and clay minerals are capable of rapidly replacing soft tissues so that they may survive into the fossil record. A deposit 450 million years old in the Cape Province of South Africa contains beautiful fossils where clay minerals have faithfully replicated soft tissue structures so that we can reveal what life was really like at this time.

What is new and interesting about this research
The Soom Shale is a relatively new deposit which contains fossils that are both beautiful and exceptionally well preserved. In particular the new, large enigmatic fossil 'Sue' is new to science.

Rapid fossilization is essential is soft tissues of organisms are to become part of our fossil record. Without these rare glimpses into the past, that exceptionally preserved biotas provide, we would not have such a complete and wonderful picture of ancient life.

How it is important to a general audience
1. Rare and beautiful fossils 2. Unusual organisms unlike any known today 3. The fact that fossilization of the soft tissues occurs extremely rapidly

The next step in this research
I plan to try to understand why exceptional preservation is confined to certain environments and times in the geological past.

Others working in this specific area
Prof. Richard Aldridge

Details of the presentation

Sex, violence and death in the history of life:
Exceptionally preserved fossils from the Soom Shale of South Africa

The fossil record is invaluable to palaeontologists in the study of the ecology and evolution of ancient life. This record, however, is incredibly biased towards the preservation of the hard and durable parts of organisms such as bones, teeth and shells. Fortunately, for palaeontologists there are rare fossil deposits where the soft parts of organisms such as skin, muscles and organs, become preserved. These exceptionally preserved biotas are of great import to palaeontologists because they provide them with a much more complete picture of life. 

Famous exceptionally preserved biotas include the Chengjiang Formation of China where the organisms which inhabited the sea 520 million years ago are preserved in exquisite detail. The Burgess Shale in Canada is perhaps the world's most famous exceptionally preserved biota. Here organisms of approximately 505 million years ago are also preserved nearly complete. All the fine details such as eyes, appendages (and even the hairs on these appendages), gills, and body fluids are preserved. Furthermore, both of these deposits yield fossils that cannot be easily placed into any of the major animal groups known today. The Burgess Shale, for example, contains an animal named Opabinia which has five eyes, a flexible proboscis with a claw at the end, and a series of overlapping lobes that propelled it through the water. Animals such as these have greatly enriched our understanding of ancient life, but this exceptional preservation does not occur everywhere and is confined to particular areas.

In order to preserve soft tissues the first hurdle to overcome is that of scavengers, which can totally destroy a carcass within days. So that in order for exceptional preservation to occur the carcass must either be quickly covered by sediment, or it must come to rest in an area where scavengers will not go, such as in anoxic or highly saline waters. Even where the carcass is protected from macroscavengers, bacteria will still devour any organic soft tissues very quickly. This process will happen in all marine settings because bacteria are ubiqitous and can live in anoxic, saline or extremely cold environments. So in order to preserve organic tissue it must actually be very rapidly replaced by a mineral so that it will survive bacterial attack and then millenia in the rock until it is discovered. Only a few minerals can do this medusa-like trick of turning soft tissues into stone very rapidly after death. Apatite (the mineral which makes up our bones and teeth), pyrite (fools gold) and common clay minerals are some of the minerals known to faithfully replicate soft tissues.

In my talk, I will introduce these concepts and then move on to talk about a deposit which is 450 million years old from South Africa, the Soom Shale. Here the soft tissues of animals are exquisitely replaced by clay minerals. Eel-like, primitive fish have their muscle fibres preserved in this way, as do sea scorpions which also have their delicate gill structures preserved. In addition, there are strange, enigmatic organisms from this deposit which at present cannot be placed into any known phyla. 

One such animal, affectionately known as Sue (after my mum), has 40 segments containing Y-shaped structures which attach flages of tissue, lateral to these are a series of imbricated lobes. Sue, an animal new to science, is only known because the environment of deposition of the Soom Shale was extremely unusual and was very acidic. Under these conditions clay minerals precipitated and grew onto the decaying tissues of animals and replaced them with a high degree of fidelity.

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Last updated: 12 September 2002 17:00
Created by: Rachel Tunstall

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