EXAMINATION OF FOSSILIZED PORE CANALS IN DECAPOD CUTICLE

Fossilization can alter preserved decapod cuticle or obscure its internal structure. However, when differentially mineralized, prepared by etching, and studied in three dimensions, fossil cuticle can provide insight into structures typically observed in two dimensions. The vertical pore canals, which provide a connection between the epidermis and the cuticle, may be particularly well exposed in positive or negative relief in polished and etched preparations of fossil cuticle. In most cases, these canals have the form of twisted ribbons. Examination of models shows a twisted ribbon shape can be composed of a straight central canal surrounded by a double helix. Work of Hegdahl et al. (1977) showed that pore canals observed in cross section consist of a central canal flanked by opposing triangular spaces that are later mineralized; this suggests that the outer parts of the pore canal assume a twisted ribbon shape and yet contain a straight central canal. When observed in thin section, the refractive index of the cuticle surrounding the canal, the central canal, the flanks of the canal, and the embedding medium control the visible components of the structure and allow it to be observed as a straight canal, twisted ribbon, or a double helix. Simple exercises with twisted rubber bands show that a twisted ribbon can quickly transform into a true helix with reduced tension, suggesting that pore canals, reported in the literature as both twisted ribbons and helixes, can likely take on both morphologies and one model explains both. A twisted ribbon could appear as a helix depending on the observational conditions. An understanding of these morphologies is needed in order to understand the true nature of fossil or extant arthropod cuticle. The very process of fossilization adds a layer of complexity in interpretation of the observed structures and their comparison to those of extant material; but, it also provides an added benefit of mineralization during fossilization. An acid etching treatment reveals the exposure of three dimensional views of some of the complex structures within the cuticle. This transformation of pore canal morphology may hold the clue to cuticle changes during the molt cycle.