Albid tissue of the conodont elements: composition and forming model

Conodont elements are composed of the following hard tissues: lamellar, paralamellar, interlamellar, and albid. The albid tissue is considered as specific for conodonts and seems to be the most evolutionary advanced hard tissue in this extinct group of animals. Study of the albid tissue may be a key to deciphering conodont affinities. Collection of hardly altered conodont elements (CAI=1) was studied with optical microscopy, SEM, TEM, micro-CT, XRD, and biochemical methods. The methods used allow to describe the tissue structure and texture. The albid tissue forms only cores of denticles of the carina of the primary processes of conodont elements. The tissue is composed of well-ordered crystallites of apatite-(CaF) (fuorapatite), which form the porous mesocrystal. The size of the pores varies from 0.6 mkm to 4 mkm. The albid tissue has gradual boundaries with lamellar and paralamellar tissues, and demonstrates weak lamellar structure. Cores of albid tissue are covered with a single lamella of the lamellar tissue. Demineralization of the conodont elements shows that the organic matrix has the same structure and composition in different tissue types. According to the results of the gel electrophoresis (SDS PAGE) and Van Gieson trichrome staining, the organic matrix is composed of collagen. Elaborated model of the albid tissue formation comprises several stages: resorption of the outer cover of the lamellar tissue and part of the albid tissue lamellae; forming of the organic matrix of the new albid tissue lamellae; initial crystallization of the disordered crystallites of the lamellae; maturation and forming the mesocrystal; sorption of the covering lamella of lamellar tissue. The model of the albid tissue generation demonstrates similarity with bone growth of the recent Vertebrata. This similarity may be caused by similar process of biomineralization, not by synplesiomorphy.