Two commonly used medical scanning technologies, Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), use non-invasive techniques to produce ‘slice-based’ datasets representing respectively, the X-ray absorption, and degree of magnetically induced nuclear resonance of materials in the sample. The latter technique is not well suited to solid geological materials, but CT has been used successfully to provide three-dimensional data on fossils (Hamada et al. 1991; Torres 1999). Although our preliminary experiments have shown that the Herefordshire fossils within nodules can be detected by CT scanners, generally available equipment is capable at best of slice spacings and pixel sizes of around 0.3 mm to 0.5 mm. Specialised CT equipment (e.g., the University of Texas High Resolution X-ray Computed Tomography Facility) can, under ideal conditions, achieve resolutions over an order of magnitude finer than this, and might thus in principle be a viable tool for studying the fauna. However, time on such equipment is expensive and its ability to consistently distinguish fossil from matrix in the Herefordshire material at high resolutions is as yet unproven. While CT may provide a valuable tool for investigating large or extremely rare taxa, we do not consider it to be a viable approach to the routine study of this fauna.
Serial sectioning of fossils is the normal approach to the collection of ‘slice-based’ datasets. However, the term ‘serial sectioning’ is commonly used to encompass both serial slicing by saw or blade and serial grinding (Ager 1965, p. 213). These are substantially different techniques, and we prefer to use the separate terms to distinguish between them.
Serial slicing is less commonly used on fossils than serial grinding, although it does allow much of the original material to be retained whilst providing the required multiplanar views. The method is best suited to larger specimens than the Herefordshire fossils because intact slices substantially thinner than 1 mm are extremely difficult to produce by sawing. In contrast, biological microtome equipment can generate exceptionally fine slices (as small as 1 µm). However, microtomy requires a non-brittle sample capable of forming a thin cohesive slice—properties not associated with crystalline materials. Almost certainly for these reasons, experiments on the use of microtomy on Herefordshire fossils were unsuccessful, despite their vacuum impregnation with resin.
Serial grinding involves the sequential removal by abrasion of small thicknesses of material from a single planar surface which is photographed at each stage. Although this approach is destructive, it suffers from none of the problems of a lack of cohesiveness associated with slicing, and inherently produces very flat polished surfaces which are ideal for photography. For these reasons we consider it the only viable approach to obtaining high-quality serial images from the Herefordshire fossils. Serial grinding is not a new technique and has been used extensively on fossils of widely differing sizes (e.g., Sollas and Sollas 1913; Stensiø 1927; Ager 1956-67; Baker 1978; Hammer 1999). Grinding has been carried out by other researchers with a variety of pieces of dedicated equipment, the most popular of which have been variants of the ‘Croft Grinder’ (Croft 1950; see also Ager 1965; Sandy 1989). The principle differences between our data acquisition technique and those used by most previous authors are the relative simplicity of the grinding device, and the method of data capture—photography at regular intervals rather than manually prepared drawings often at irregular intervals.
Photography. The destructive nature of serial grinding places special demands on the photography of each freshly exposed surface. The photographic technique used must not only capture as much information as possible, but it must have a very low failure rate, because images cannot be re-taken after grinding has progressed. Digital rather than conventional photography is therefore a key element of our method since digital images can be downloaded instantly to a computer where they can be checked for quality before grinding continues. The use of a digital camera also offers significant economic savings as well as removing the need for images to be scanned into a computer for reconstruction.
Registration. Although the processing of images from serial grinding is discussed below, one aspect must be considered here as it impacts on the early stages of the technique. Before any reconstruction can be attempted images must be ‘registered’ (or aligned) so that the arrays of pixels line up correctly. This can be accomplished in one of two ways: 1) the sample can be placed in exactly the same position relative to the field of view of the camera for each image capture; or 2) the sample can be processed so that it contains physical ‘fiduciary’ (or alignment) markings of some sort, that can be used subsequently to align the images. The second approach is simpler in practice and is preferred in our work. Fiduciary markings used in similar work by previous authors are, typically, straight holes drilled perpendicular to the grinding surface so that they appear in the corners of the image (e.g., Herbert 1999, figure 3.2; Baker 1978). However, holes less than 0.5 mm in diameter cannot be produced reliably with current drilling technology and larger holes would be unacceptable in the context of this study. Consequently, we use two perpendicular edges (see below) as fiduciary structures, which are roughly aligned in the field of view when images are first captured. These edges are used to guide precise digital alignment during the registration phase of post-processing (see Appendix 1, section 1).