ORGANIC MATRIX PRESERVATION IN LOWER PALEOZOIC BRACHIOPODS IS MORE COMMON THAN WE THINK
The fundamental unit of organic matrix in all these organisms is remarkably similar among phyla, consisting of single or anastomosing strands with diameters of 30-40 nanometers, linking closely spaced nodules of perhaps twice that diameter. This type of matrix is found, with various degrees of organization, distributed throughout the mineralized skeletons. In those phyla with more organized skeletal microstructures, some of these fundamental units can be found matted together as sheaths surrounding the crystal elements of the skeletons, forming a composite material with both elastic and compressive strength. But even in those groups, lacy strands of matrix persist within the crystals.
The organic matrix can respond in different ways to taphonomic influences, in some conditions protecting the original microstructures, and in other cases degrading and providing pathways to more rapid destruction of a shell. Partially degraded sheaths separate into meshworks and strands, confirming that only one fundamental matrix type is present.
Living representatives of terebratulid brachiopods show these features well, with thick organic sheaths enclosing crystals, which in turn enclose lacy strands of matrix. Fossil brachiopods are much the same, and exceptional preservation is not essential to see this. A random sampling of ten lower to middle Paleozoic brachiopods found that all ten had recognizable strands of matrix preserved within the crystal structure, and that four specimens (one Rhynchotrema, two Dinorthis, and one Dalmanella, all Ordovician) had some patches of sheaths preserved as well.
Chemical and biochemical taphonomic studies on Paleozoic fossils should consider the strong possibility that organic components are still present, and possibly in significant amounts. A physical search, using the above techniques, can be useful in interpreting chemical results.