Monferran et al.—Chemical taphonomy of Jurassic spinicaudatans from Patagonia, Argentina 92(6):1054–1065 1063


different carapace layers (with different chemical compositions) were preserved. This preservation is the result of diagenetic modifications of the carapace, while minimal (or no) bios- tratinomic factors may have been involved. The ornamentation modes of carapace growth bands are


correlatedwith the preservationmodes of their layers. In this case, the original mode is the reticular ornamentation, which is visible in positive relief when the complete carapace morphology (i.e., external layers) is preserved. On the other hand, when incomplete carapace morphological characteristics (i.e., inner layers) are pre- served, the reticular ornamentation is seen in negative relief. Our paleoenvironmental evidence (i.e., sedimentological,


Figure 5. Remains of spinicaudatans carapace showing characteristic colors of different preservation modes; (1) black (Las Chacritas Creek); (2) brown (Karrizal Creek); (3) white (Miyanao Creek); (4) silicified carapace molds (Cañadón Caracoles locality). Scale bars=1mm.


between 300°C and 400°C, a light-brown crust appears in the carapace at 450°C–600°C, a superficial white color forms from 600°C to 1,100°C, and carapaces acquire a transparent glassy characteristic at 1,200°C. The fossil conchostracans analyzed by Tasch (1982) are from Blizzard Heights and Mauger Nunatak of Antarctica, where the overlying Kirkpartrick basalt flow thermally affected the sedimentary sequences of fossil con-


chostracans. The Cañadón Asfalto Formation presents a basalt flow between sedimentary sequences, where the temperature from magmatic activity could have affected the preservation (including the colors) of spinicaudatan carapaces. Nevertheless, several complementary studies including X-ray diffraction analysis, Fourier transform infrared analysis, the determination of rock mineral composition, and data on paleoenvironmental inferences are needed to better understand the relationships between carapace color and physical geochemical preservation conditions.


Conclusion


In this contribution, we present the first chemometric approach to the study of the chemical preservation modes of spinicauda- tan carapaces from the Jurassic of Argentina. Carapaces of Euestheria taschi from the Lahuincó creek locality were ana- lyzed using XRD and EDS. Semiquantitative elemental com- position data obtained by EDS analysis were subsequently evaluated using a principal component analysis. The EDS results clearly indicate that spinicaudatan car-


apaces were not uniformly preserved. These results show com- plex distribution patterns of elemental compositions in which variable amounts of major constituents (i.e., Ca, P, Al, and F) are detected in different zones of the carapaces. Although the preservation is influenced by diagenetic recrystallization, the recorded values of the elemental compositions are considered part of the original chemical composition of the carapace materials. On the other hand, some other zones of the carapace show that Ca and P contents decrease, while SiO2 (from the underlying rock matrix) contents increase. Therefore, the che- mical composition variability found in E. taschi indicates that


ecological, and chemical data) suggest that mineralization of E. taschi carapaces likely occurred in alkaline media with low P and high Ca concentrations and was influenced by periodic ash fall events. The sediments at Lahuincó creek locality are inter- preted as wetland paleoenvironments consisting of alkaline brackish water with pyroclastic material levels represented by tuff and tuffaceous rocks. Therefore, ash fall deposits may have played an important role in the fossil preservation at Cañadón Asfalto Formation, while the alkaline media may have favored the preservation of spinicaudatans. Overall, our results clearly indicate that diagenetic factors


played an important role in the preservation modes of the spi- nicaudatans carapaces. As we show in this contribution, tracking carapace chemi-


cal signatures using chemometric tools opens new possibilities to achieve a better understanding of preservation mechanisms. Future contributions will include studies on the preservation modes of spinicaudatan remains from other Jurassic localities of Argentina.


Acknowledgments


This research was supported by grants PIP 5760 and PIP-112/ 201001/00034 to NGC from Argentina’s Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Thanks to Dr. F. Cravero (Instituto Centro de Tecnología de Recursos Minerales y Cerámica CETMIC-CONICET) for RX diffraction analysis. The Comisión Nacional de Energía Atómica (CNEA) provided logistical support at the Campamento Los Adobes


during fieldwork, and G. Giordanengo produced the digital figures.


Accessibility of supplemental data


Data available from the Dryad Digital Repository: https://doi. org/10.5061/dryad.67sf15d.


References Allison, P.A., 1988, Phosphatized soft-bodied squids from the Jurassic Oxford Clay: Lethaia, v. 21, p. 403–410.


Allison, P.A., Maeda, H., Tuzino, T., and Maeda, Y., 2008, Exceptional pre- servation within Pleistocene lacustrine sediments of Shiobara, Japan: PALAIOS, v. 23, p. 260–266.


Anderson, T.W., 2003, An Introduction to Multivariate Statistical Analysis (third edition): Hoboken, Wiley, 752 p.


Astrop, T.I., and Hegna, T.A., 2015, Phylogenetic relationships between living and fossil spinicaudatan taxa (Branchiopoda Spinicaudata): Reconsidering the evidence: Journal of Crustacean Biology, v. 35, p. 339–354.


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