Transparency of calcite. The nature of the Herefordshire material necessitates guarding against two potential pitfalls. The first concerns the assumption that each image represents the intersection of the fossil with a single plane. The calcite that comprises the fossils is normally dark and opaque, but translucent regions can reveal features beneath the surface plane or be relatively light in shade when matrix lies only a short distance below the plane. Such occurrences, which have the potential to invalidate reconstructions partially, are detected by inspection of the slice images and corrected by manual retouching (see Appendix 1, Section 3).
‘Halo’
effects . Light-coloured regions, presumed to be
reduction haloes, often occur around specimens of certain taxa (Figure
12). Fossil material within these haloes is often no darker than the matrix
outside them, making the selection of a single isosurface threshold level for
the whole fossil impossible. Reconstructions may therefore fail to include
extremities within these regions that are nonetheless clearly visible on the
slice images. While this problem can be overcome by careful manual darkening of
these regions, a simpler alternative is often to prepare a dissection