Thomas Clements, PhD student

Decay, preservation and environmental controls on non-biomineralised fossil anatomy: the taphonomy of the Carboniferous Mazon Creek, USA

Supervisors: Professor Sarah Gabbott and Professor Mark Purnell.

Project overview:

Thomas ClementsWhilst discovering and describing fossils is still the major core of palaeontology, my interest lies in the study of the processes that control fossilisation, known as ‘taphonomy’. Turning organic material into stone over millions of years is only part of the story and there are three main stages where important information that a fossil contains may be altered or lost. The first follows the death of the organism; post mortem events, such as being transported by water currents, predated or scavenged upon, may remove or damage the remains of an organism or destroy it all together. Subsequently, in order to become a fossil the remains must be buried. Factors such as the surrounding environmental conditions, type of sediment and the speed of burial may for example affect the amount of tissue decay. Lastly, after burial, geological processes including heating, pressure or chemical changes (known as diagenesis) may alter, remove or ultimately destroy the remains.

As you can see, the odds of an organism becoming a fossil are heavily stacked against it – and these are but a handful of examples of what might prevent fossilisation. In the event that a fossil is formed, then any one of these factors can dramatically reduce our ability to correctly interpret a fossil and therefore may bias our knowledge of ancient life. Understanding and constraining these factors and how they influences fossilisation are the main aims of my research.

Implementation: The Mazon Creek

Around the world there are very rare and special areas where fossils demonstrating exceptional preservation of soft tissues are found. These sites are known as Lagerstätte from the German words: Lager 'storage' and Stätte 'place'; plural Lagerstätten. My research focuses on an exciting and unique 300 million year old fossil Lagerstätte in Illinois, USA, called the Mazon Creek. Although fossils have been found since the 1800’s, much of the fossil material available today was discovered during a boom in coal mining between the 1900’s to the 1970’s. The fossils are found within nodules of a mineral called siderite, which was considered waste material by the coal miners. Collectors would often take the nodules from spoil heaps and over time large museum collections have been accumulated in many countries worldwide.

The Mazon Creek nodules exceptionally preserve a variety of soft tissues such as animal guts, worm integument (skin) and even fish eyes and gills. Animals like worms are very rarely preserved in the fossil record because they don’t have any hard parts. In the Mazon Creek however, they are so numerous that you can buy them on eBay! A researcher called Ida Thompson (1979) said of the polychaete worms found in the Mazon Creek Lagerstätte: “The impression one has is that these animals, after being dead for 300 million years, could, if given the right motivation, lift themselves up and crawl off the concretions”. This gives you an idea of how well preserved these animals are.

Uniquely, the Mazon Creek fossils also represent multiple habitats. The depositional environment of the Mazon Creek was thought to be swampy river delta which terminated in an estuarine (salt water) bay – not dissimilar to the Mississippi river today. These habitats were rich in trees, insects, fish, crustaceans, worms, amphibians and all sorts of beasties which are very different to any animal groups we see today. During some form of natural disaster many of these animals were killed and turned into fossils within nodules.

My research will focus on what these fossils are made of now compared to when the animal was alive, how they preserve soft tissues and what the mode of preservation is. Furthermore, by comparing the fossils of organisms that are found in both fresh and salt water within the Mazon Creek , it will allow us to test for the first time if environmental differences within one Lagerstätte effect the decay of carcasses and therefore the information preserved as a fossil.

 

References:

Baird, G. C., Sroka, S. D., Shabica, C. W., & Kuecher, G. J. (1986). Taphonomy of Middle Pennsylvanian Mazon Creek area fossil localities, northeast Illinois: Significance of exceptional fossil preservation in syngenetic concretions. Palaios, 271-285.

Baird, G. C., Sroka, S. D., Shabica, C. W., Beard, T. L., Scott, A. C., & Broadhurst, F. M. (1985). Mazon Creek-Type Fossil Assemblages in the US Midcontinent Pennsylvanian: Their Recurrent Character and Palaeoenvironmental Significance [and Discussion]. Philosophical Transactions of the Royal Society B: Biological Sciences, 311(1148), 87-99.

Johnson, R. G., & Richardson Jr, E. S. (1966). A remarkable Pennsylvanian fauna from the Mazon Creek area, Illinois. The Journal of Geology, 626-631.

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