X-RAY DIFFRACTION AND NANOINDENTATION ON CONODONT MICROFOSSILS REVEALED VARIATION OF MECHANO-STRUCTURAL PROPERTIES ALONG THE LENGTH OF CONICAL ELEMENTS

Conodonts are enigmatic early vertebrate whose teeth-like elements have been extensively used to probe paleoenvironmental conditions of sedimentary basins and oceans. One of the tools used for petroleum exploration is a semiquantitative technique based on visible color and known as Color Alteration Index (CAI). Personal assessments and lithological difference can induce error on CAI estimation which suggests a complete chemical picture for changes in CAI values is needed to determine the level of consistency in natural samples. Although a handful of chemical characterization studies have been carried out on conodonts in the past, a systematic structural investigation is still lacking. Mechanical properties at different regions of conodont elements have also been calculated using computational model to imply their function. But an experimental analysis of mechanical properties is yet to be done. A systematic investigation of structural and mechanical properties of conodont element is necessary not only for improving geo-environmental analysis but also evaluating their biological function.

In the present study, we analyzed 20 coniform Dapsilodus obliquicostatus conodont elements (S_a, S_b-c and M elements) that all displayed lower CAI values (< 2). Crystalline structure and mechanical properties at different sections along the length of each element were analyzed using a microfocused X-ray diffractometer and Atomic Force Microscopy (AFM). The X-ray diffraction pattern revealed variability of crystalline structure ranging from single crystalline to polycrystalline in different sections of each specimens. Albid of juvenile S_a and S_b-c elements were found to have more ordered single crystalline structure than ontogenetically older elements. The basal body of all elements displayed characteristic diffraction patterns of polycrystalline material. Variation of mechanical properties were also observed along the length of each element through measuring Young’s modulus by AFM nanoindentaion experiments. The tip of albid and basal body have lower Young’s modulus compared to middle sections of elements suggesting materials at the middle sections are more stiff and resistance to deformation in response to an applied force. Moreover, the Young’s modulus for older elements were found to be higher than the same element type in younger specimens.