Wednesday, 23 March 2016

Where are tetrapod fossils preserved?

Last week Dr. Carys Bennett and TW:eed Team colleagues published their first paper from the project in the journal Sedimentology on deposits containing tetrapods from Scotland. We asked the questions: Why do these particular successions preserve such abundant fossil evidence of the early terrestrial ecosystems? And what sedimentary processes were acting to concentrate and preserve tetrapod fossils? The study describes a particular type of sedimentary rock, sandy siltstones, as sites of tetrapod fossil preservation. Sandy siltstones are matrix-supported siltstones with millimetre sized lithic and bioclasts. They host concentrated fossil deposits of tetrapods, fishes, sharks, arthropods, bivalves and plants.
Photographs of sandy siltstones from the main field site and the Norham borehole
Members of the team involved in the paper alongside myself are Sarah Davies, Tim Kearsey, David Millward, Jenny Clack, Tim Smithson and John Marshall. In addition many University of Leicester undergraduate and graduate volunteers have helped examine samples in the lab that contributed to this research, and the following are especially thanked for their efforts: Catherine Caseman, Rachel Curtis, Levi Curry, Rowan DeJardin, Daniel Downs, Hattie Dulson, Deborah Fish, Susan Hammond, Catherine Langford, Graham Liddiard, Jessica Mason, James Mawson, Christopher Stocker, Kirsty Summers and Thomas Worthington. Volunteers helped me examine many hundreds of rock samples and have also worked on the microfossil aspects, with investigations ongoing.
Reconstruction diagram of conditions on the floodplain when sandy siltstones formed

The Sedimentology investigates why these deposits are formed and how tetrapods are preserved. The conditions of their formation can tell us a lot of interesting information about the ancient climate and the environment. Most sandy siltstones were deposited on top of ancient soils or sun-cracked ground surfaces, indicating they were the result of flooding events. The high frequency of these types of rocks throughout the successions we have studied tells us that the floods were regular, perhaps caused by an ancient Monsoonal climate. Tetrapods and arthropods on land were washed away by the floods and transported into temporary pools and lakes on the floodplain which were inhabited by fishes, molluscs and other animals.
This reconstruction diagram by Mark Witton gives a reconstruction of life on the ancient floodplains

This study is the first to recognise these types of sediments as being important sites of vertebrate fossil preservation – importantly for tetrapods, it will give scientists some clues as to where they can look next!

Until next time