PITFALLS FOR THREE-DIMENSIONAL RECONSTRUCTIONS

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 (see Dissection of models) of the inside of a light region, and to reconstruct this section separately.

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