Chemical taphonomy and preservation modes of Jurassic spinicaudatans from Patagonia: a chemometric approach

Abstract

Spinicaudatans (‘clam shrimps’) are small branchiopod crustaceans enclosed in a chitinous bivalved carapace that is often the only preserved element in the fossil record. However, few studies have analyzed the preservation of these carapaces, which have been found in continental facies from the Devonian to the present. The aim of this study was to contribute to a better understanding of the chemical preservation of fossil spinicaudatan carapaces, and it focused on spinicaudatan carapaces of the Cañadón Asfalto Formation from the Jurassic of Argentina. Semiquantitative energy-dispersive X-ray spectrometry (EDS) analysis provided elemental composition data that were interpreted using principal component analysis (PCA). The results showed a complex chemical mode of preservation for spinicaudatan carapaces. In some parts, EDS spectra of the specimens exhibit peaks of calcium, phosphorous, aluminum, and fluorine, representing the retention of original carapace material with some diagenetic recrystallization. Certain zones of the carapace show low-intensity peaks of the elements mentioned, while silicon and oxygen peaks (from the rock matrix) become the dominant spectral signals. These modes of preservation modify the interpretations and observations of the ornamentation of the carapace, which are used as taxonomic features. Our results suggest that specific diagenetic processes play a fundamental role in the preservation of spinicaudatans.

Taphonomic analysis of Spinicaudata from two Triassic lacustrine systems affected by different volcanic processes

“…Taphonomic studies on spinicaudata are mainly based on mechanical features (Krishnan, 1958;Rieder et al, 1984;Zhang et al, 1990;Orr et al, 2002Orr et al, , 2008Olempska, 2004;Mancuso and Marsicano, 2008;Astrop and Hegna, 2015;Bustos Escalona et al, 2017;Jenisch et al, 2017), or chemical composition of post-buried shells (Briggs, 2003;Astrop and Hegna, 2015;Monferran et al, 2018;Hu et al, 2020). The spinicaudata shell is mainly composed of a calcium-phosphate complex (e.g., , and the multi-laminar structure of shells makes it resistant to taphonomic processes such as decay, desiccation, and compaction (Krishnan, 1958).…”

“…The resistance of the elements to dissolution is related to the stability of their mineral components, effective surface area, and other factors such as their microstructure and permeability (e.g., Plotnick, 1986;Wilson, 1988;Behrensmeyer, 1991;Behrensmeyer and Hook, 1992;Behrensmeyer et al, 2000;Karr and Clapham, 2015). Most of the shell spinicaudatan fossils, which were chemically analyzed, have been preserved as recrystallized calcium phosphate or silica replaced molds (Stigall and Hartman, 2008;Stigall et al, , 2014Hethke et al, 2013;Monferran et al, 2018;Hu et al, 2020).…”

Plant Taphonomy in a Lake Affected by Volcanism (Agua De La Zorra Formation, Upper Triassic) Mendoza, Argentina

“…The chemometric approach conducted using a Scanning Electron Microscope (SEM) in combination with energy-dispersive X-ray spectrometry (EDX) has proved to be useful in providing diagenetic information related to different modes of preservation of fossil remains (e.g., Previtera et al 2013;Benavente et al 2014;Monferran et al 2018;Previtera 2019). Although the results provided by the standard procedure are qualitative and semiquantitative, SEM-EDX allows the determination of major and minor components of different materials.…”