Fossil hunting? Clay holds the key
Researchers at the Universities of Oxford, Yale, Harvard, and Massachusetts Institute of Technology, and at the Diamond Light Source in the UK, have discovered that kaolinite, a mineral found in certain fine-grained rocks around the world, is a key ingredient for preserving some of the earliest forms of complex life.
The study, published today in Interface Focus, a Royal Society journal, reveals how the importance of kaolinite in the fossilisation process has led to a bias in the early fossil record toward organisms that lived in places where kaolinite forms.
The study, led by Oxford Earth Sciences post-doctoral researcher Dr Ross Anderson, wanted to understand the fossilisation process for complex life forms that lived before the evolution of diverse skeletons during the so-called Cambrian era, which began about 541 million years ago. The researchers investigated microscopic, soft-bodied pre-Cambrian organisms as old as around 800 million years from sites in Russia, the Canadian Arctic, and the Norwegian archipelago Svalbard.
First author Ross Anderson in Svalbard. Photo credit: Alexie Millikin/Yale University.
The researchers discovered that such traces of early life were encased in halos of the clay known as kaolinite. The presence of kaolinite suppressed the decay of the cells of these organisms — which include bacteria, fungi, and algae — enabling their fossilisation. A similar association promoted the preservation of soft-bodied Cambrian animals such as those in the Burgess Shale of British Columbia.
Dr Anderson notes: ‘It would suggest that the early fossil record is biased to environments where kaolinite is likely to be common – typically tropical environments. Interestingly, all the fossils studied come from rocks that were formed in the tropics.
‘Our discovery that animals are absent in 800-million-year-old rocks where conditions are ideal for soft-tissue preservation suggests they had yet to evolve and constrains the timing of animal origins.’
Another important implication of the protective kaolinite haloes, is in the discovery of fossils on other planets. Having a rich understanding of how microscopic life can be fossilised is key to finding proof of life on other planets. In fact, NASA plan to send a rover to Mars in July 2020 to collect rock and soil samples to uncover signs of possible life. It is believed its Martian landing site, the Jezero Crater, could have kaolinite, which would hopefully improve the chances of finding intact microfossils.
Fossilized proterocladus — an ancient, seaweed-like organism — is visible in this clay sample from Svalbard, Norway.
In the future, Dr Anderson and the rest of the team intend to study microfossils from other deposits of different time periods. He concludes: ‘The method we have developed really enhances the tools with which palaeontologists can characterise the composition of exceptionally preserved fossils and their surrounding rock. It will be applicable to a wide range of questions across the geological record.’
The paper, ‘Aluminosilicate haloes preserve complex life approximately 800 million years ago‘ is available to download here: https://doi.org/10.1098/rsfs.2020.0011
Featured image: Ostiana from the Wynniatt Formation, Canada; either a colony of a cyanobacterium or another green alga (an individual cell is ~12 µm). Picture Credit: Dr Ross Anderson. Text adapted from Yale news article by Jim Shelton.