Cretaceous Chert-Hosted Microfossils Visualized With Synchrotron Ptychographic X-Ray Computed Tomography (PXCT).

Silicification of microfossils is an important taphonomic process that provides a record of microbial life across a range of environments throughout Earth history. However, questions remain regarding the mechanism(s) by which silica precipitated and preserved delicate organic material and detailed cellular morphologies. Constraining the different mechanisms of silica precipitation and identifying the common factors that allow for microfossil preservation is the key to understanding ancient microbial communities and fossil-preserving mechanisms. Here, we use synchrotron ptychographic X-ray computed tomography (PXCT) as a novel technique to analyze microfossils from the Cretaceous Barra Velha Formation and better characterize their diverse morphologies and preservation styles. Through this technique, we generate 2D and 3D reconstructions that illustrate the microfossils and silica-organic textures at nanometer resolution. At this resolution, we identify previously uncharacterized silica textures and organic-silica relationships that help us relate findings from modern silicifying environments and experimental work to the fossil record. Additionally, we identify primary morphological differences among the microfossils as well as preservational variability that may have been driven by physiological and/or biochemical differences between the different organisms that inhabited the Cretaceous pre-salt basin. These findings help us to better characterize the diversity and complexity of the microbiota in this ancient basin as well as taphonomic processes and biases that may have driven microfossil preservation in this and other silicifying environments throughout Earth history.