1058
Journal of Paleontology
Paleontological Collections, Paleozoology section CTES-PZ (Centro de Ecología Aplicada del Litoral, Centro Científico Tecnológico Nordeste, Consejo Nacional de Investigaciones Científicas y Técnicas, Corrientes, Argentina).
Results
The EDS analysis allowed for the discernment of differences in elemental chemical compositions between carapaces of Eues- theria taschi Vallati, 1986 (CTES-PZ –7.678) and the sur- rounding rock matrix. The carapace EDS spectra showed high- intensity peaks of P and Ca, indicating a predominantly calcium phosphate composition (Fig. 2.1, 2.2). By contrast, the rock- matrix-EDS spectra exhibited O and Si peaks, indicating the presence of the major components of silicate compounds. However, minor elements (i.e., Mg, K, and Al ) were also recorded in the carapace samples. The point analyses A1–A4 were taken on an anteroposterior carapace zone, and the points A5 and A6 were assigned to the rock matrix (Fig. 2.2; Table 1). The points B1–B8 correspond to the ventral carapace zone (Fig. 2.3). In all cases, the chemical compositions exhibited differences and similarities among analyzed points. Differences in mineral phases were confirmed by XRD
analysis, showing that rock matrix surrounding the carapaces predominantly had quartz-cristobalite, K-feldspar, and mon- tmorillonite (Table 2). Furthermore, the diffractogram of car- apace samples showed the presence of minerals that were associated directly with the surrounding matrix material (quartz- cristobalite and montmorillonite), the presence of mineral pha- ses such as gypsum and alunite, and other unidentified peaks that can be attributed to phosphate minerals. Unfortunately, the complete elucidation of the diffraction pattern associated with the carapaces was not possible because of: (1) the small amount of material obtained from the carapaces, and (2) the impossi- bility of obtaining this material without contamination from the surrounding rock matrix. Such limitations were partly overcome by a multivariate evaluation PCA [2], see supplemental data, Appendix 2) of the elemental composition data obtained from the carapace, rock matrix, and mineral specimens (see the fol- lowing section). These results were compared to the XRD data.
PCA of fossil samples.—The EDS technique provided 14 data points (Table 1), which were evaluated using PCA (Tables 1–3; supplemental data, Appendix 1). Cumulatively, two compo- nents account for 79.73% of the variance (Fig. 3). The first PC (63.14% of variance) exhibits predominantly positive loadings on Al, F, and P, with the exception of Si and K, which empha- sizes the relative abundance of silicate minerals (Fig. 3.1). The second PC (16.59% of variance) shows positive values for O and negative values for Ca and Mg. The points measured on rock matrix (A5–A6) exhibited the most negative scores in PC1 (Fig. 3.2), indicating a low content of P, Ca, and F and the importance of compounds containing Si and K, as shown in Table 1. The plot of scores (Fig. 3.2) shows the grouping of data as a
function of the chemical compositions from the points analyzed on the spinicaudatan carapace remains. The rock matrix analyzed (A5–A6) exhibited the most negative scores in PC1, indicating the low content of P, Ca, and F and the importance of
compounds containing Si and K, as shown in Table 1. The points measured on the carapace remains (A1–A4, B1, B3, B5, B6, and B8) exhibit the most positive scores (PC1), reflecting their high contents of P, F (except B8), Al, and Mg (except for B1 and B6). However, the other points measured (B2, B4, and B7) show a composition mixture between carapace and rock matrix, and some of these points (B2, B4, and B7) have a composition more similar to the rock matrix than to the carapace. These different chemical compositions of the carapace remains could represent the thickness of layers preserved or different conditions of layer preservation. A second principal component analysis (PCA (2), see
supplemental data, Appendix 2) was performed using data points measured on both the carapace remains and selected mineral specimens (high purity, 99.5%, standard reference materials), which were included for multivariate comparison of elemental compositions: albite, augelite, berlinite, biotite, calcite, ca- millisite, carbonate-fluorapatite, crandallite, fluorapatite, goethite, gypsum, hematite, limonite, margarite, montmorillonite, musco- vite, quartz, variscite, and wavellite (Table 1). Results showed that two principal components accounted
for 49% of the cumulative variance (see Tables 1–3; supple- mental data, Appendix 2). The plots of the component loadings and component scores are shown in supplemental data (Appendix 2). Considering the first three principal components, ~40% of the variance remained unexplained. Therefore, it is important to emphasize that PCA (2) of fossil and mineral samples was intended only as a preliminary analysis, which will be fully developed in future studies. PCA (2) model is a preliminary analysis that indicated a considerable correlation between EDS-derived data measured from carapace and those corresponding to phosphate species (margarite, variscite, ca- millisite, wavellite, augelite, and berlinite). The latter were in agreement with the results obtained by XRD analysis, which unequivocally indicated the presence of phosphate species.
Discussion
Data interpretation.—The EDS analysis is a tool for the study of the modes of chemical preservation in spinicaudatan car- apaces. Stigall et al. (2008) analyzed modern spinicaudatan carapaces and revealed a calcium-phosphate composition with minor amounts of Al, Si, S, and Fe (Table 3). Astrop et al. (2015) analyzed the elemental composition of five living spini- caudatan specimens and showed the presence of C, O, P, and Ca (Na was recorded in only one specimen). These results indicated that the spinicaudatan carapaces are composed of the chitinous component (C, O) of the cuticle and the mineralization of a calcium-phosphate complex (Ca, P). Trace amounts of Zn, S, and other elements also occurred in very low quantities (Astrop et al., 2015). Carapacestheria disgregaris Tasch, 1987 from the Lower Jurassic of Antarctica was also analyzed by EDS, showing the original skeletal mineralogy (Stigall et al., 2008). In our study, the elemental composition (major components) of E. taschi carapace exhibits chemical elements such as F, P, Ca, Al, and Mg. Table 3 shows the similarities and differences in che- mical compositions between living and fossil spinicaudatan carapaces. Therefore, our results confirm that the carapace of this species preserved part of the original chemical composition
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