Three-Dimensional Morphological (CLSM) and Chemical (Raman) Imagery of Cellularly Mineralized Fossils

Abstract

Of all modes of fossilization, cellular mineralization, whether by the ­non-biologic process of permineralization (“petrifaction”) or by microbially ­mediated mineral precipitation (“authigenic mineralization”), is the most faithful to the preservation of life-like cells and tissues that is known, yielding fossils that are among the biologically and taphonomically most informative available from the geological record. Such preservation spans all forms of life, ranging from vascular plants, such as those permineralized in calcitic coal balls; to organic-walled algae, fungi and ­bacterial prokaryotes, permineralized most commonly in fine-grained quartz; to metazoans that exhibit preserved soft tissues, such as those mineralized in apatite. Though such fossils can be preserved in exquisite cellular detail, two deficiencies have long hampered their study: (1) an inability to document fully their three-dimensional ­morphology at micron-scale spatial resolution; and (2) the lack of a means to analyze in situ and at such resolution the chemistry of the carbonaceous matter (kerogen) that comprises their structurally preserved anatomy. These needs have now been met by two techniques newly introduced to paleobiology, three-dimensional confocal laser scanning microscopy (CLSM) and two- and three-dimensional Raman imagery. We here document the use of these techniques to elucidate the fine-scale structure and kerogenous composition of representative fossils of each of the major biologic groups (animals, plants, fungi, algal protists, and microbes) preserved in phosphorites, cherts, and carbonates, the three principal rock types in which cellular mineralization occurs. The examples presented include an apatite-mineralized ctenophore embryo preserved in a Cambrian phosphorite; quartz-permineralized Eocene fern rhizomes and a fungal-infested Devonian plant axis preserved in carbonaceous cherts; a calcite-permineralized plant stem preserved in a calcareous Carboniferous coal ball; and quartz-permineralized acritarchs (phytoplanktonic algae), cyanobacteria, and especially ancient fossil microbes permineralized in Precambrian cherts. Use of CLSM and Raman imagery can provide new information about the morphology, cellular anatomy, taphonomy, carbonaceous composition and geochemical maturity of organic-walled mineralized fossils, whereas Raman imagery used alone can document the mineralogy of the enclosing matrix and the spatial relations between such fossils and their embedding minerals. Not only can the use of these techniques elucidate the sequence of events and taphonomic processes involved in the cellular mineralization of organic-walled fossils, but the use of Raman to document the geochemical maturity of their kerogenous constituents can provide new evidence of the biases of such preservation over time. Because both techniques are non-intrusive and non-destructive, both can be applied to specimens archived in museum collections. Taken together, the two techniques represent a major advance in the study of ancient fossils.