Chitinozoan-based paleogeographic patterns in the Lower and Middle Devonian. A review of the similarity coefficient methods

CAMINA, SONIA CLARA; RUBINSTEIN, CLAUDIA VIVIANA; BUTCHER, ANTHONY

Montpellier

Congreso; 56th Annual Meeting AASP-The Palynological Society; 2024

The Palynological Society

The paleogeography during the Devonian included two supercontinents, Gondwana and Euramerica, with smaller continents and terranes separated by oceans. Throughout this period, Gondwana migrated northward to finally collide with Euramerica in the Carboniferous forming Pangea (Torsvik & Cocks, 2017). The proximity of the landmasses, combined with a high sea level, might have created an interconnection between the different basins from the Early to the Middle Devonian. Some authors suggest that during the Early Devonian, Western Gondwana was characterized by a pronounced provincialism across different taxonomic groups (Capel et al., 2023) - acritarchs, however, do not confirm this pattern (Rubinstein et al., 2008). Statistical techniques can be used to quantify the affinities of different assemblages between two geographic areas and analyze the paleogeography. One method commonly used in palynology is the coefficient of similarity, which relies on the presence or absence of taxa.This study aims to review and discuss the different similarity coefficients to identify the most suitable for analyzing the distribution and affinities of chitinozoans. Furthermore, we aim to recognize paleogeographic patterns in the Lower and Middle Devonian from Western Gondwana using chitinozoan assemblages.Statistical analysis was conducted on two chitinozoan assemblages obtained from three boreholes. Two are located in northwestern Argentina, and the third in southern Bolivia. The classical similarity coefficients Jaccard, Dice and Simpson were applied using the statistical program ‘PAST’, the results being compared with the modified Forbes index (Alroy, 2015). The main problem with using similarity coefficients is their high sensitivity to variations in the total number of species within the assemblages being compared. Our results showed that with the same data, the classical indexes suggested either no affinities or great similarity. However, the modified Forbes index (F’) showed more reliable results and therefore was adopted to analyze the paleogeography.The chitinozoan assemblage from the Early Devonian, compared with other chitinozoan assemblages of the same age, have F’ values between 0.1 and 0.3. These low results may indicate the pronounced provincialism recorded during this period. Conversely, the results from the Middle Devonian assemblages indicate a better connection of the basins in general. The F’ between all Western Gondwana basins are between 0.5 and 0.43 and would suggest an interconnection between them. Nonetheless, the F’ between other regions are less than 0.24 which may indicate a provincialism in Western Gondwana for the Middle Devonian. However, the discrepancy between the number of identified taxa and the taxonomic classifications within the different chitinozoan assemblages may have influenced in the low values of the similarity coefficients since the data is not always reliable.As a consequence, the relatively high values of the F’ between the Western Gondwana assemblages probably reflect the interconnection between the basins and the proximity of the landmasses, but the low values, may not be indicating the opposite necessarily. This shows that a taxonomic revision of chitinozoan taxa is necessary to clarify the results and allow more precise paleogeographic interpretations. From these results, the coefficient of similarity F’ appears the most suitable for paleogeographic analysis.