On the external indicating status of zonal biochronological scales

Research subject . The possibility of creating an external biochronological scale in stratigraphy is discussed. Materials and methods . Factors that ensure the independence of the events selected for constructing a biochronological scale from the geological setting of their occurrence are analyzed. Results . A category of biochronological scales, which are constructed based on studying evolutionary changes in a certain part of the skeleton, homologous to members of a long-existing group of organisms, was distinguished. Such scales are proposed to be referred to as meronomical. The specifics of constructing meronomical scales based on the study of conodonts is discussed. The evolutionary sequence of fossil species ensures the continuity of the scale and its external indicating status, which implies the independence of the scale from the facial features of the sections compared with its help. The main features of divergence arising in the process of evolution in the competitive relationships of related species are discussed. The correspondence of the main features arising in the development of skeletal elements during the evolution of conodonts with the prediction based on the accepted model of an ensemble development of related species is shown. In particular, such features include an increase in the variability of platform elements and stabilization of the morphology of ramiform elements, as well as a directed change in Pa elements, which serves as the basis for constructing conodont meronomical scales. The evolutionary morphological trend of the Pa element, which arises in the process of competitive relations between related species, does not depend on the particular external abiotic conditions in which these relationships are carried out. Accordingly, the biochronological scale built on the basis of the evolutionary trend of the Pa element states can also be considered as external in relation to the measured geological events captured in specific sections. The facts of the brief existence and wide distribution of periodically occurring Pa element morphotypes are explained by the phenomenon of evolutionary oscillations. This phenomenon implies that, at certain time periods in spatially separated populations of related species, their gene pools change simultaneously and in a similar way. The process of a synchronous increase in the frequency of a certain trait covers isolated and semi-isolated populations of related species in an area stretching hundreds and thousands of kilometers and almost simultaneously. A similar picture can be observed in the morphological transformations of conodonts at certain stratigraphic boundaries. Although the reasons for such evolutionary oscillations are still unclear, the established fact of their synchronous manifestation in large areas excludes the dependence of related populations on the abiotic conditions of specific locations. This circumstance represents another condition that makes it possible to consider the historical morphogenesis of Pa elements observed in the process of the described ensemble evolution of related conodont species as independent of abiotic causes and determined only by internal factors. Conclusion . Biochronological scales, which are based on the stages of evolutionary changes in a certain part of the skeleton, homologous to members of a long-existing group of organisms, are largely protected from the influence of abiotic conditions and can be considered as external scales.