X-ray nanotomography and electron backscatter diffraction demonstrate the crystalline, heterogeneous and impermeable nature of conodont white matter

Abstract

Conodont elements, microfossil remains of extinct primitive vertebrates, are commonly exploited as mineral archives of ocean chemistry, yielding fundamental insights into the palaeotemperature and chemical composition of past oceans. Geochemical assays have been traditionally focused on the so-called lamellar and white matter crown tissues; however, the porosity and crystallographic nature of the white matter and its inferred permeability are disputed, raising concerns over its suitability as a geochemical archive. Here, we constrain the characteristics of this tissue and address conflicting interpretations using ptychographic X-ray-computed tomography (PXCT), pore network analysis, synchrotron radiation X-ray tomographic microscopy (srXTM) and electron back-scatter diffraction (EBSD). PXCT and pore network analyses based on these data reveal that while white matter is extremely porous, the pores are unconnected, rendering this tissue closed to postmortem fluid percolation. EBSD analyses demonstrate that white matter is crystalline and comprised of a single crystal typically tens of micrometres in dimensions. Combined with evidence that conodont elements grow episodically, these data suggest that white matter, which comprises the denticles of conodont elements, grows syntactically, indicating that individual crystals are time heterogeneous. Together these data provide support for the interpretation of conodont white matter as a closed geochemical system and, therefore, its utility of the conodont fossil record as a historical archive of Palaeozoic and Early Mesozoic ocean chemistry.

Sr/Ca and Ba/Ca ratios support trophic partitioning within a Silurian conodont community from Gotland, Sweden

“…White matter is often found in the denticles of conodont elements, but the distribution and volume vary greatly between taxa. It has different crystallographic properties (Trotter et al 2007; Atakul-Özdemir et al 2021) and elemental composition than the lamellar tissue (Trotter and Eggins 2006). The compositional variation is therefore very important to determine; if white matter contains varying amounts of Sr, Ba, or Ca in comparison to the lamellar tissue, it could have a significant impact on the measured Sr/Ca and Ba/Ca ratios measured in taxa with systematically different proportions of these tissues.…”

Increasing control over biomineralization in conodont evolution

“…Yet highly porous bioapatite tissues, such as bone and dentine, are still known to be susceptible to substantial diagenetic changes 45 , 46 . Conodont crown and its analog tissue, enamel, are the least susceptible tissues to diagenesis 9 owing to their low porosity 47 and low organic contents, but even in these tissues diagenetic exchange of ions and isotopes 9 , 40 and changes in crystallinity 42 can occur. As there are no other studies yet that would assess the crystallographic texture of fossil bioapatite quantitatively, no insights are available yet as to how diagenesis might affect our conclusions.…”

“…Testing of the hypothesis on functional adaptation of conodont ultrastructure has not been possible until recent improvement in in situ quantification of crystallography of bioapatite, which is sensitive to the electron beam. Until now, the only successful EBSD study on conodonts 47 obtained EBSD diffraction signal from the white matter in two taxa, focusing on the crystallinity of this tissue without analyzing the implications for material properties of the elements. Thanks to the quantitative orientation data obtained from the hyaline tissue of conodont dental elements, we identify two quantifiable criteria for testing this hypothesis: (1) parallel orientation of c -axes to the occlusal surface, and (2) the degree of crystallographic order expressed using the Texture Index (TI).…”

New Insights on Micro‐Scale Variations of Geochemical and Oxygen Isotope Compositions in Conodont and Shark Tooth Bioapatite

“…Detailed microstructural investigations of the albid crown have characterised these tissues as relatively porous, with pore spaces of hundreds of nanometres (about 160 nm) being dominant (Atakul‐Özdemir et al . 2021). In the specimens imaged here, estimates of the pore sizes of albid tissue are about 1 μm in size (Figures 1, 3a).…”