Abstract A wide spectrum of tomographic techniques now exists for studying palaeontological specimens, but the suitability of these methods for assessing Earth’s oldest prokaryotic life has not been comprehensively investigated. We evaluated the ability of X-ray computed tomography – specifically X-ray microtomography – to reveal the morphology and petrological context of Precambrian microfossils, pseudofossils and biosedimentary structures, all of which are important in the origin and early evolution of life of Earth. The materials tested came from the Pilbara Craton of Western Australia (the 3.49 Ga Dresser Formation, the 3.46 Ga Apex chert and the 3.43 Ga Strelley Pool Formation) and the 1.88 Ga Gunflint Formation of Ontario, Canada. These units chart key developments in palaeobiology. The oldest formations contain profoundly controversial microfossil-like objects and microbially-induced sedimentary structures, whereas definitive prokaryotes are found in the youngest formations. We demonstrate that the imaging of individual microfossils and pseudofossils currently lies at the limits of the capabilities of laboratory-based X-ray microtomography and requires beneficial taphonomy. However, microtomography does provide a good overview of their petrological context at flexible spatial scales, although the quality of the data obtained from mesoscopic microbially-induced sedimentary structures and stromatolites depends largely on their style of preservation. Supplementary material: A zipped Drishti volume for all CT scans, a 0.7z split zip file of one Drishti volume and a HDMI supplementary movie showing digital visualizations for all of the scans are available at https://doi.org/10.5281/zenodo.58161

Abstract The monotypic scombrid fish Micrornatus is represented by a single skull from the early Eocene (Ypresian) London Clay Formation of southeastern England. Although Arthur Smith Woodward substantially increased the diversity of scombrids and scombrid-like taxa known from this deposit, he seems not to have studied the fossil that would later be recognized as Micrornatus . Here we re-examine this specimen using computed microtomography, with two principal aims: first, a revised anatomical account with an emphasis on concealed features; and, second, ‘proof-of-concept’ for the tomographic study of fish crania from the London Clay. Scans reveal new details of the braincase, suspensorium and ventral hyoid arch. We compare the cranial anatomy of Micrornatus with other members of Eocoelopomini, a group also containing the genera Eocoelopoma , Palaeothunnus and possibly Landanichthys . Clarification of the taxonomy and phylogeny of early fossil scombrids is needed, and we suggest that computed tomography will be a useful tool for revealing the anatomical evidence needed to accomplish this goal. Supplementary material: Micrornatus hopwoodi NHMUK PV OR 36136 PLY files are available at: http://doi.org/10.6084/m9.figshare.1561381

Abstract Computed X-ray tomography is a technique that produces cross sections of an object from a series of projections at different angles. The technique has found widespread use in medical CAT scanners, which typically have resolutions of ~1 mm. Microtomography is the extension of this technique to smaller spatial resolution down to <1 μm. In the last 15 years the development of high-brightness synchrotron X-ray sources, high resolution CCD detectors, and high-performance computing have allowed the field of microtomography to progress rapidly. It is now being applied widely in Earth and soil science, where it is used to image the 3-D distribution of minerals, fluids, and pores. By exploiting X-ray absorption edges, 3-D images of the distribution of specific chemical elements can be produced. This is used to image the distribution of aqueous and organic fluids that have been doped with contrast agents such as iodine and cesium. The method is also being used to locate trace-mineral phases containing high-atomic-number elements such as zirconium and cerium. With fluorescence tomography 3-D images of trace element abundances and even oxidation states can be produced. This is being applied to understand the chemical contamination and remediation by plants in the environment. Diffraction tomography images the 3-D distribution of crystalline phases based on their powder diffraction peaks, and is very useful for imaging materials with similar X-ray absorption and composition but different crystalline structures.

We present a new technique for X-ray microtomography under high pressure. By modifying an opposed-anvil high-pressure cell known as the Drickamer cell, monochromatic X-ray radiographs can be collected through the entire cell assembly and a thin-walled containment ring. We designed a rotation mechanism to rotate the Drickamer cell from 0 to 180° under hydraulic load, and examined pressure-generation efficiencies of the Drickamer cell up to 8 GPa at room temperature using the energy-dispersive technique through the containment ring, which allowed us to conduct a detailed evaluation of effects of geometric factors of the Drickamer anvils for tomography application. The maximum attainable load supported by the containment ring is proportional to the anvil diameter. Cells with larger anvil diameters are less pressure efficient, although they can reach higher pressures with much higher loads. Pressure efficiency generally increases with the tapering angle and decreases with tip diameter of the anvils. However, cells with larger tapering angles are more unstable, causing blowouts beyond a certain pressure. We evaluated the quality of X-ray images using the optical setup for conventional tomography at the GSECARS (GeoSoilEnviroCARS [Consortium for Advanced Radiation Sources]) beam line, the Advanced Photon Source. Noise level in the images depends on the material used for the containment ring. Containment rings made of either cubic boron nitride or silicon carbide allow us to better observe the images, but these materials are brittle and prone to mode-1 failure and are not suitable for high-pressure generation. The noise level of aluminum-alloy rings is somewhat higher, but the material is much more ductile, and hence it is capable of supporting higher loads. Using the aluminum-alloy containment ring, we conducted a commissioning run of tomography up to 3 GPa. We demonstrate that the high-pressure tomography setup is useful for studying internal structure of objects and density of melt and fluid under pressure.