SHEDDING SYNCHROTRON LIGHT ON THE FOSSIL RECORD OF EARLY PLANT EVOLUTION

The origin and subsequent evolution of land plants transformed our planet utterly. These changes were brought about by key innovations in the land plant lineage, the correct sequence of which is critical to understanding the physiological and developmental evolution that underpin plant evolutionary history. Early land plant fossil remains occur as dispersed spores and compressions of whole plants that inform gross anatomy. The best known record of detailed anatomy is provided by the late Devonian Rhynie Chert, but an older and more diverse record occurs in charcoal lagerstätte from the lower Devonian of Brown Clee Hill, Shropshire. Charcolification produces chemically inert, physically brittle but decay resistant remains preserving cellular level detail. They have typically been studied through destructive dissection, invariably reconstructing gross anatomy from insights afforded by the dissection of disparate specimens. Computed tomography represents a non-invasive non-destructive alternative approach. In particular, Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM) provides a volumetric characterization of fossils to a level of resolution beyond that of fossilization. However, until now, our attempts to characterise remains from Brown Clee Hill failed due to the x-ray absorption contrast between the low attenuating charcoal and the highly attenuating intrinsic pyrite, resulting in ‘bleaching’. Largely through the development of offline data filtering algorithms, we have been able to mitigate these deleterious effects by pyrite crystals and for the first time obtain cellular level tomographic measurements of these fossils. This has allowed us to resolve a comparable level of resolution to SEM techniques of structures including spores within sporangia, connective tissues and vascular canals - all non-invasively - allowing us to dissect these specimens virtually, constraining the sequence of character acquisitions in the assembly of the tracheophyte bodyplan.