Chemometric study of the preservation modes of Athabaskia anax (Trilobita, Cambrian Precordillera, Mendoza, Argentina): Implications for taxonomy

Abstract

Selected pygidia of the middle Cambrian corynexochid trilobite Athabaskia anax (Walcott 1916) from the Miaolingian Series of San Isidro (Precordillera, Mendoza, Argentina) are analyzed for the first time using scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM-EDX). Pygidia occur in a single stratigraphic level composed of fine-grained calcareous sandstones and show four types of morphological preservations. The purpose this case study is threefold: (1) to analyze in detail the sample types by means of SEM-EDX in order to explore the chemical changes of the exoskeleton induced by diagenesis; (2) to use principal component analysis (PCA) to focus on groupings as a function of the chemical composition; and (3) to analyze the possible systematic implications of the chemical data. Results show that PCA supports the distinction among the four types of morphological preservations, based mainly on differences in the contents of Ca, Al, and K, as well as Si and Mg. This suggests that diagenesis followed distinct taphonomic pathways in the single stratigraphic level and resulted in contrasting chemical compositions of the four types of morphological preservations. It is interpreted that some morphological characteristics of the pygidia are the result of chemical and structural changes taking place during fossilization and do not represent real morphological differences in the living organisms. Our results dictate caution when using some morphological characteristics of the exoskeletons to erect new taxa, which, in turn, could be used to draw erroneous conclusions on their biostratigraphy, biogeography, and palaeoecology.