ORGANIC MATRIX PRESERVATION IN TRILOBITE CUTICLES FROM THE LOWER AND MIDDLE PALEOZOIC

Organic matrices are found in the calcium carbonate shells and skeletons of a wide range of major invertebrate groups, and in their simplest form, consist of nodular strands with a diameter of 30 to 40 nanometers. These strands are usually matted into a lace-like meshwork, and in the more advanced taxa are matted further to form sheaths that enclose and bind individual crystallites. They have been reported in Recent specimens from the Porifera, Cnidaria, Bryozoa, Brachiopoda, Mollusca, and cirriped Arthropoda.

This organic matrix is surprisingly resistant to taphonomic degradation, and although the thicker sheaths seem to readily degrade into lacy meshworks, these, their fragments, and single strands can persist for very long periods of time. One factor that probably contributes to this is the frequent preservation of carbonate hard parts by recrystallization, which can convert small aragonite or calcite crystals into blocky calcite, while leaving much of the organic matrix in place. Once sealed in stable calcite, the matrix is chemically isolated. Other factors, such as preservation in tightly sealed rocks, are often involved.

Earlier reports have shown recognizable remnants of matrix preserved in fossils representing nearly every taxonomic group mentioned above, and in some, notably the rugose corals, calcisponges, and archaeocyathids, matrix has been found as old as the Early Paleozoic.

I now report recognizable organic matrix from trilobites from four localities and ages. These include Phacops rana from a Middle Devonian shale in Geneseo, New York; Isotelus sp. from an Ordovician shale in Oxford, Ohio; Peronopsis sp. from the Middle Cambrian of Utah; and a fragment of an unidentified trilobite from the Lower Cambrian of Nevada.

In each case the matrix was exposed by etching, and careful examination made with a scanning electron microscope to ensure that the strands continued into the skeletal material, as shown by partial exposures of matrix at the base of the visible strands.

I have done no work on the chemical nature of these fossil matrices, although molluscan organic matrix seems to involve both proteins and polysaccharides. It would be interesting to see if variations in the chemistry of fossil matrices could be used to cast more light on the taphonomic processes involved in their preservation.