Литосфера (Apr 2020)
Composition, structure and model of the formation of conodont lamellar tissue
Abstract
Research subject. This research was focused on the most common mineralized tissue that composes conodont elements. The aim was to investigate the characteristics of the composition and structure of this tissue and to reconstruct its formation process. Materials and methods. The work was based on a collection of well-preserved conodont elements from the Upper Devonian of the East European Platform and the Upper Devonian – Lower Carboniferous of the east of the Pechora Plate. Oriented and polished thin sections made from some of the elements were studied using light and electron microscopy, as well as a microhardness tester. Energy dispersive spectroscopy was used to determine the chemical (elemental) composition of the lamellar tissue. In addition, the carbon isotope ratio was determined for organic matter. Results. The study showed that the lamellar tissue in conodont elements consists of fluorohydroxylapatite crystallites of various morphology, surrounded by organic matter, which makes up 2–3% of the tissue. Variations in the composition of major elements incorporated in fluorohydroxylapatite of the lamellar tissue are insignificant. Organic matter is represented by a collagen-like protein, likely to be of a non-fibrillar type, with a light carbon isotopic composition (–26.2 ‰ PDB). The lamellar tissue has an average microhardness of 2.6 GPa, the variations of which are due to textural and structural features and the distribution of organic matter. In conodont elements, the lamellar tissue is in contact with other types of tissue. Transitions between tissues are relatively sharp at the borders of the lamellae and gradual within the same lamella. Conclusions. A model was developed, according to which the growth cycle of a conodont element covered the sequential formation of two lamellae preceded by the resorption of one external lamella. In the structures formed by the lamellar tissue, both lamellae consisted of this tissue. The lamellar tissue is of interest as a natural model of an organic-mineral composite based on protein and calcium phosphate.
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