UTILIZING THE ORGANIC MATRIX: DISPLAYING 3-D MICROSTRUCTURE, DEFINING 'GOOD PRESERVATION', AND SEEKING ORIGINAL STRUCTURE IN RECRYSTALLIZED FOSSILS

The careful application of mild etching, fixing in alcohol, and careful critical-point drying can expose a 3-D wonderland of organic matrices, which in turn promotes the interpretation of original processes of biomineralization and the later processes of taphonomy. Indeed, the very existence of an organized matrix within shells can be a surprise; although geochemical evidence of proteins and polysaccharides in living and fossil carbonates suggests a matrix was involved in the biomineralization process, its degree of preservation was largely unknown.

With my recent investigations on echinoderms, I can now demonstrate the existence and persistence of organic matrix in recent and fossil representatives of every skeletonized invertebrate phylum. This includes the Porifera, Cnidaria, Bryozoa, Brachiopoda, Mollusca, Annelida, Arthropoda, and Echinodermata, in addition to groups within the Foraminifera and Chordata, and in some cases includes specimens as old as the Lower Cambrian. It is not even essential that the specimens be of exceptional preservation.

The fundamental form of preserved organic matrix is that of nodular strands, with strand thickness around 30 nanometers, bearing nodules at least twice that in diameter. Often the strands will cluster together, or form nets or mesh in a 2-D or 3-D relationship. In many cases, we see thicker accumulations, compressed into sheaths of variable thickness and porosity. Sometimes, as in the nacre, we find uniform sheets of two or three tightly organized layers of heavily nodular networks. Both sheaths and sheets can be seen in fossil material, degrading into networks and strands, and sometimes outlining the original crystals.

Although small segments of matrix are often distributed uniformly throughout a shell, there is commonly more complexity. Echinoderms excepted, the matrix will often form sheets or sheaths that confine growing crystallites and define elaborate microstructures (such as found in the bivalved mollusks). In species that live in stressful environments (variations in temperature, salinity, and other factors), the organic matrix may form concentrations parallel to the growing margin, due to variations in the relative rates of growth of the organic and mineral components, or even due to dissolution of the carbonates during low tides.