1060
Table 2. Results of XRD analysis. Material
Rock Matrix * * * *
Journal of Paleontology
Visible Ref. code Score Compound name 01-078-2315 59 Quartz
Carapace * * * * * * *
01-076-0824 46 Potassium-feldspar P2A 01-071-0785 30 Cristobalite low 00-013-0135 24 Montmorillonite-15A 01-078-2315 67 Quartz 01-074-1904 24 Gypsum 01-071-1776 33 Alunite
00-013-0135 32 Montmorillonite-15A 01-075-0923 12 Cristobalite low 00-019-1175 24 Dawsonite
01-076-0828 43 Potassium-feldspar CA1B
Displacement (°2theta) Scale factor Chemical formula 0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.127 K.931 Na0.055 Ca.009 Ba.005 Al0.97 Si3.03 O8 0.032 Si O2
0.773 Si O2
0.047 Ca0.2 (Al, Mg )2 Si4 O10 (OH)2 !4 H2 O 0.961 Si O2
0.128 Ca (SO4) (H2O)2 0.073 K (Al3 (SO4)2 (OH)6)
0.033 Ca0.2 (Al, Mg)2 Si4 O10 (OH)2 !4 H2 O 0.022 Si O2
0.200 K0.94 Na0.06 Al0.99 Si3.01 O8 0.040 Na Al C O3 (OH)2
Table 3. Mean values of elemental composition (w/w %) of six individual spinicaudatan carapaces; 1–5 are according to Stigall et al. (2008), and the species E. taschi was analyzed in this work. n.d.=not detected.
Species
5. Carapacestheria disgregaris † 6. Eustheria taschi †
1. Leptestheria compleximanus Packard, 1877 46.64 2. Leptestheria compleximanus 3. Eulimnadia sp. 4. Eulimnadia sp.
43.28 43.73 67.34 45.47 59.22
O Na n.d.
P
n.d. n.d. n.d. n.d. 1.3
11.26 16.18 19.2 3.5
18.39 28.5 18.2
12.42 15 5.71
5.88 1
Ca Mg Al 3.79
n.d. n.d. n.d. n.d. 1.33
7.15 3.31 1.75 0.57
10.5
1.06 2.04 2.73 0.45
1.14 1 0.81
S K Fe 1.23
n.d. n.d. n.d.
n.d. 1 0.74
Si
0.49 0.35 0.06 0.49
n.d.
6.93 3.87 0.54 n.d.
15 2.99 F
n.d. n.d. n.d. n.d. n.d. 17
Figure 3. Principal component analysis (PCA) of energy-dispersive X-ray spectrometry (EDS) data obtained from spinicaudatan carapace remains and rock matrix. (1) Plot of PC1 vs. PC2 component loadings; (2) plot of PC1 vs. PC2 component scores; (3) scree plot.
not found or are at low concentrations (e.g., Al, K, F, and Mg; Table 3). These chemical differences are attributed to the variation of the mineralization process for each carapace fossil. The secondary phosphatic mineralization in C. disgregaris carapace remains would have been higher. The SEM-EDS spectra and PCA analysis of the E. taschi
carapace revealed variable chemical compositions according to the measured points (Fig. 3). In specific zones, the carapace remains showed high contents of Ca and P. However, in some other parts of the carapace, the concentrations of Ca and P
decreased, while Si contents increased. This finding suggests a different diagenetic pattern for external and internal carapace layers, as in the patterns obtained by Stigall et al. (2008) in C. disgregaris, where a single valve showed different layers or regions preserved as either calcium phosphate or silica. These authors explained that the silica layerswere present where calcium phosphate layers had been
exfoliated.Therefore, considering these data, the continuum of variation in the chemical compositions in the carapace remains of E. taschi indicated that different modes of the layers had been preserved, with internal layers preserved as
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