1062


Journal of Paleontology


silicic volcanic glass, which is highly active (Hay, 1966; Jones et al., 1969; Sheppard and Gude, 1969). Paleolakes influenced by volcanic activity make available a high abundance of sulfur compounds (Allison et al., 2008), which oxidize into sulfates in aquatic environments. Furthermore, reactive iron minerals are frequently abundant in sedimentary volcanic deposits (Pan et al., 2014). In addition, alkaline media are indicated by both the preservation of phosphatic minerals in ‘conchostracan’ car- apaces (D’Angelo et al., 2013) and cuticle loss in cuticle-free coalified layers of gymnosperm leaves (e.g., Zodrow et al., 2009; D’Angelo et al., 2011). In addition, the presence of Ca and aluminum phosphates


in lake environments is related to pH conditions and phosphate availability. Wavellite can be formed in alkaline to acidic environments with low phosphate concentrations, while mon- tgomeryite prevails with high phosphate availability (Nriagu, 1976). Variscite is unstable relative to the other aluminum hydroxyphosphates and is unlikely to be a major constituent in phosphorite beds. Variscite can be a stable phase in natural environments with significant Ca concentrations and where the values for the activities of participating ions are controlled by other reactions within the system (see Nriagu, 1976). Phosphatic mineralization may occur in alkaline media, even in the case of low P contents (Gierlowski-Kordesch and Kelts, 2000). Other studies have suggested the importance of biological


factors in the preservation of carapaces involving equilibria between the decay and mineralization of carapace remains. Some factors (e.g., microbial activity) produce decomposition and early mineralization in cuticle layers (Allison, 1988; Briggs and Kear, 1994; Briggs, 2003). Algae and bacteria have been observed on the carapace


surfaces of living spinicaudatans. This suggests that algae and bacteria could have played a role in the preservation of fossil carapaces. Stigall et al. (2008) indicated that microbially mediated silica molded onto the inner surfaces of carapace layers and typically preserved a higher fidelity of detail than calcium phosphate layers. These authors proposed that silica precipitation is promoted by the chemical gradient set up by microbial degradation of the spinicaudatan carcass. However, further studies could deepen this idea regarding the role of microbial activity in spinicaudatan carapace preservation. Fromthe Jurassic of Patagonia, spinicaudatanswere recorded


in lacustrine facies (limestones,mudstones, and shales) associated with mud cracks, stromatolites, and organic matter (Cabaleri et al., 2013; Monferran et al., 2016). The Lahuincó creek locality is interpreted as an alkaline-water wetland with pyroclastic material levels represented by tuff and tuffaceous rocks (Cabaleri et al., 2010). The deposits are characterized by clay minerals as well as analcime, kaolinite, smectite, quartz, K-feldspar, plagioclase, and calcite (Cabaleri et al., 2013). The XRD analysis was not useful for the determination of


the phosphate mineral species present in the E. taschi carapace. However, PCA (2) (see supplemental data, Appendix 2), including several mineral samples (standard reference materials) and carapace samples, suggests the likely presence of some phosphate species such as augelite, berlinite, Ca-millisite, crandallite, variscite, and wavellite. In addition, results of PCA (2) indicate that carbonate-fluorapatite and fluorapatite would not be present in the carapace. Furthermore, the PCA model


(supplemental data, Appendix 2) shows that carapace samples are in close affinity to variscite and wavellite species. The latter suggests that relatively low P and high Ca concentrations were available during the mineralization of carapace remains. Overall, our experimental evidence, that is, the information


derived from the XRD analysis, statistical evaluation of EDS data by PCA (2), and sedimentological and ecological inferences, suggests that E. taschi carapace mineralization occurred in alkaline media with low P and high Ca concentra- tions and was influenced by periodic ash fall events.


Taxonomic implications.—The classification of fossil spini- caudatans relies on carapace features, such as morphology, the presence of macroornamentation (nodes and ribs), and the kinds of microornamentation from growth bands (Chen and Shen, 1985). The latter feature represents one of the most important characteristics in the systematic classification of fossil spini- caudatans. In many cases, the preservation does not allow for the identification of the ornamentation, or the original features could be distorted. Frequently, the carapace’s ornamentations are preserved as the reflection (impression-type fossil) of the original morphology. Therefore, the identification of preserved species is important for proper descriptions in fossil spinicau-


datans. Stigall et al. (2008) indicated that silica layers preserved fine details of the carapace microstructure while the recrys- tallized phosphate layers did not. Gallego et al. (2013) published details on general types of carapaces for Martinsestheria codoensis Cardoso, 1962, showing lamellae of different thick- nesses that make up the carapaces, each with a different kind or presentation of ornamentation or morphological structure (e.g., Gallego et al., 2013, p. 53, fig. 4). In our study, the carapace exhibits parts with reticular


ornamentation in positive relief that present high phosphorus and calcium concentrations (highlighted picture, Fig. 4.2, 4.3). The same carapace also presents reticular ornamentation in negative relief, corresponding to the internal layer of the carapace, related to the mixture of silicate minerals with phosphorus or calcium (white arrows, Fig. 4.2, 4.3). Therefore, the same fossil carapace could exhibit two modes of ornamenta- tion according to the preservation of different layers, and the carapace exhibits different planar views of the ornamentation. These observations also support the idea that the complete carapace was present during the burial and that some layers of the carapace were exfoliated during later stages of diagenesis or postdiagenesis.


Types of spinicaudatans preserved at the Cañadón Asfalto Formation.—Spinicaudatans are abundant in several localities in the Cañadón Asfalto Formation, and their preservation is specific for each locality (Fig. 5). For example, the carapaces are preserved as black remains at Las Chacritas Creek (Fig. 5.1), brown remains at Carrizal Creek (Fig. 5.2), and white remains at Miyanao Creek (Fig. 5.3). However, sometimes only silicified spinicaudatan molds are preserved. This is the case for spini- caudatan carapaces found at Caracoles Creek (Fig. 5.4). There- fore, differences in the carapace color are mainly due to different preservation types. Tasch (1982) related the preservation color of carapaces to the temperature during fossilization. According to this experimental study, a transparent color is obtained


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