ORGANIC MATRIX IN BRYOZOANS: IMPLICATIONS FOR CALCIFICATION AND FOSSIL DEGRADATION

Studies on organic matrix are essential to an understanding of the formation of calcium carbonate skeletons, as the matrix is both precursor and accompanist to calcification. Despite this, matrix has largely escaped attention, except where its thickness is great enough to stand above etched surfaces. Some workers have distinguished two types of matrix, intercrystalline (where the matrix forms envelopes around individual crystals) and intracrystalline (where tiny ‘shreds' of matrix seem to have been trapped within, and throughout, the crystals). From physical evidence, the intercrystalline matrix seem to be built of the same subunits as the intracrystalline matrix, matted together into lacy or solid sheets. Such details are best seen in polished, etched, and critical-point dried preparations.

Within the mollusks, where most studies of organic matrix have been made, the intercrystalline matrix is abundant and prominent, and the intracrystalline matrix might well be discounted as leftover debris — like sawdust in a lumber mill. It is by turning our attention to bryozoans that we see the dominance, and likely importance, of the intracrystalline matrix.

Examination of Recent specimens of both cyclostomes and cheilostomes has demonstrated the presence of both intracrystalline and intercrystalline matrix, each with essentially the same physical characteristics as in mollusks. The intercrystalline matrix seemed nowhere as well developed or as prominent as in mollusks, but the intracrystalline matrix was widespread. There was a tendency for less density of matrix in zones of spherulitic microstructure, presumably the fastest-growing regions of the skeletons. In general, the distribution of intracrystalline matrix suggested a continuous three-dimensional meshwork, through which crystallization was proceeding, rather than scattered scraps of unwanted organic debris. This could have major implications for understanding the calcification process.

It is also noteworthy that fossil bryozoans, despite their often delicate skeletons, preserve their organic matrix for some time. A random examination of specimens of two cheilostome genera and one cyclostome from the Eocene, as well as one each from the Miocene and Pliocene, found patches of well-preserved matrix in every one.