Using X-ray images to analyse graptolite distribution and alignment in Welsh mudrocks

X-ray imaging of graptolites is here shown to enhance greatly the amount and type of fossil evidence available, both for biostratigraphical study and for palaeoceanographical and palaeoecological analysis. The technique requires there to be sufficient density contrast between the fossils and the enclosing rock, as with the pyritized graptolites within the Silurian (Llandovery, Aeronian) mudrocks of central Wales investigated herein. Graptolite species previously not encountered using traditional collecting techniques were recorded in significant numbers using X-ray imaging and include two species recorded for the first time in Britain. Stratigraphic variation has been found in the numbers of pyritized graptolites, and this is inversely related to the amount of free aggregated pyrite within the rock. Biserial graptolite specimens show systematic patterns of alignment by currents, and construction of stereo and anaglyph images enables 3-dimensional visualisation of spatial information, allowing the geometry and microstratigraphic relationship of one graptolite to another to be discerned. CT-scan techniques show better-resolved 3D morphology, but digital X-ray techniques, being more rapid, may remain optimal for many routine uses in palaeontology.