NOVEL INSIGHTS ON CONODONT CHEMISTRY

Conodont elements are the preserved mineralized tissue of an extinct group of marine organisms of Late Cambrian to Late Triassic age. Despite over one hundred and fifty years of study, the phylogenetic position of conodonts has been controversial. However recently, conodonts have been considered to belong to an extinct group of primitive chordates or possibly vertebrates such as eel-like fishes. The stimulus of early research in elucidating the chemical composition of conodonts was to provide a better knowledge of conodont biological relationships to other organisms. Currently, our knowledge of conodont chemistry is that these microfossils are almost entirely inorganic carbonated hydroxylapatite (CO₃OHAp) with trace quantities of organic matter disseminated throughout the CO₃OHAp crystallites. Different organisms synthesize apatites of various mineral crystallinity, orientation of crystallites, and carbonate to phosphate ratios, thus allowing a determination of biological affinity. Here I show a novel application of polarized Raman spectroscopy to provide a better understanding of apatite chemistry to establish biological source. The Raman intensity of Raman-scattered radiation from anisotropic minerals strongly depends on the orientation of the crystal and the polarization of incident and scattered light. Raman scattering of plane polarized light can result in scattered radiation that is either parallel or perpendicular to the c-axis of anisotropic crystalline solids, which can result in an enhancement of Raman bands depending on c-axis parallel or c-axis perpendicular analysis. Polarized Raman spectroscopy can give an insight into crystal orientation by investigation of the depolarization ratio of polarized Raman spectra. This study elucidates the orientation of apatite crystallites by examining the depolarization ratio of the phosphate (ν₁P-O symmetric stretch) mode to spectroscopically determine the phylogenetic relationship of conodonts.