Clay-organic Bonds Exposed as Key to Fossilisation in the Iconic Burgess Shale

Oxford Earth scientists have uncovered how chemical bonds between clays and decaying animal soft tissues played a key role in rare soft-tissue fossilisation. Such fossils are crucial to our understanding of ancient life on Earth.

The Cambrian Explosion of animal life over 500 million years ago was one of the most significant events in the evolutionary history of life on our planet. Understanding this major turning point for life relies on rare fossils which preserve soft tissues. Unfortunately, most early animals lacked hard shells and skeletons which make up the majority of fossilised remains. Instead many of these animals had entirely soft bodies like worms today.

Soft tissues normally decay too quickly to be fossilised. However, in the Burgess Shale, an iconic fossil deposit in British Columbia, Canada, a bounty of these soft tissue fossils is known. These have provided a wealth of information on early animal ecosystems.

Now, in a study published in Geology, researchers have uncovered the key to this exceptional fossilisation – clay. The team analysed a variety of fossils from the Burgess Shale with a new non-destructive X-ray diffraction technique in Oxford and with X-ray spectroscopy in Oxford and Bristol. They revealed that fossils are distinguished from the rock which surrounds them by the presence of the clay mineral kaolinite.

Chemical map of the most common fossil in the Cambrian Burgess Shale (British Columbia, 508 million years old), Marrella, a small arthropod (i.e., relative of shrimps) less than 2cm long. Red indicates regions rich in Aluminium, a constituent of the clay mineral kaolinite.

“Kaolinite is known to have antibacterial properties which likely slowed the decay of these early animals once they died” said lead author Ross Anderson, a Fellow of All Souls College and Post-Doctoral Researcher in the Department of Earth Sciences.

The research suggests that kaolinite bonded to the soft-tissues shortly after death. The mineral’s chemical structure made it uniquely suited to these bonds.

“The study has wide implications for our reading of early animal evolution. It suggests evidence provided by the fossil record is biased to regions where clay production was high, namely the tropics” said Anderson.

The study is co-authored by Nicholas Tosca, formally at Oxford but now at the University of Cambridge, Erin Saupe and Jon Wade both at Oxford, and Derek Briggs of Yale University.

The Leverhulme Trust, a Mid-America Paleontological Society Student Award, the NASA Astrobiology Institute, a NASA Earth and Space Science Fellowship, the Yale Institute for Biospheric Studies, and the Yale Peabody Museum of Natural History provided support for the study. All fossils were loaned by the National Museum of Natural History, Smithsonian Institution, USA.

The Geology publication can be accessed here:

Anderson, R.P., Tosca, N.J., Saupe, E.E., Wade, J. & Briggs, D.E.G. Early formation and taphonomic significance of kaolinite associated with Burgess Shale fossils. Geology. doi: https://doi.org/10.1130/G48067.1

Featured image: Waptia, a shrimp-like fossil from the Cambrian Burgess Shale (British Columbia, 508 million years old) is about 5.5 cm long. (Photo credit: Susan Butts, Yale University)