ORGANIC MATRIX IN THE ECHINODERM SKELETON: PHYSICAL NATURE

The microstructure of echinoderm skeletons is unique among invertebrates, as every individual skeletal component is essentially a single crystal of calcite, growing incrementally with little variation in crystalline orientation. Moreover, the skeleton is extremely porous, consisting of a labyrinthine structure, the stereom, with living tissue filling the openings. Depending on the strength required at different points, the ratio of organic to mineralized tissue can vary considerably. Interestingly, the most common taphonomy expressed by echinoderm fossils is the infilling of the porous skeleton with calcite in crystallographic continuity with the original biomineral.

Numerous studies on the chemistry of invertebrate skeletons, especially on mollusks, have shown that there is a significant organic component in calcium carbonate shells, consisting primarily of proteins and polysaccharides. A few studies, some from the earliest period of use of electron microscopes, have also established that the organic component can have its own three-dimensional structure, the organic matrix.

I have managed to extend these studies to the point of showing that an organic matrix is present in all invertebrate phyla with extensive skeletons, including the Porifera, Cnidaria, Bryozoa, Brachiopoda, Mollusca, Arthropoda, Annelida, and now the Echinodermata, as well as the Foraminifera and Vertebrata.

Recognition of an organized matrix within the echinoderm skeleton proved to be very difficult. The single-crystal nature of entire skeletal segments necessarily precludes the presence of the sheets and sheaths bounding the crystallites of other phyla, and it appears that only nodular strands and loosely connected nets or mesh are present here. Moreover, these are indifferently exposed by etching, as the etching agents themselves seem to weaken or break up the strands. Nonetheless, in many cases such nets seem to mark intervals of growth, if only by closely spaced lines of dots in the sectioned stereom.

The matrix found in fossil echinoderms seemed, surprisingly, to be better expressed than in the recent specimens. This might suggest that the stability of echinoderm organic matrix increases with time, or that these ancestral forms built more stable matrix than their modern descendants.