Fish taphonomy as a tool for interpreting water temperature in the Agua de la Zorra Formation

Abstract

The Agua de la Zorra Formation (Middle–Upper Triassic) consists of a lacustrine-deltaic environment with episodic incursions of the lava flow into the aquatic environment. Moreover, based on the exclusive dominance of progradational stacking of siliciclastic lithologies, organic geochemistry data, and low primary productivity Agua de la Zorra was interpreted as an overfilled lake-basin palaeolake (Harrington, 1971; Brea et al., 2009; Ottone et al., 2011; Pedernera et al., 2019). The fossil record includes plants (macro- and microflora), invertebrates (branchiopods, spinicaudatans, and insects), trace fossils, and vertebrates (actinopterygian fish and archosaurs). Taphonomy is crucial for understanding the processes affecting dead organisms, the ecology of organisms before their death, and reconstructing past depositional environments (e.g., Behrensmeyer and Kidwell, 1985; Brett and Baird, 1986; Elder and Smith, 1988; Mancuso, 2012). In this sense, fish taphonomy and its preservational modes can be sensitive environmental indicators of conditions that characterize sub-environments and depositional processes in lacustrine systems (e.g., water temperature). Our research presents a case of fish taphonomic study from the Agua de la Zorra Formation to characterize the environmental conditions of the overfilled lacustrine-deltaic system. Fish remains are housed in the Palaeovertebrate Collection of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Mendoza city, Argentina. Following the currently accepted methodologies, we defined and analyzed taphonomic attributes, focusing on the degree of articulation, which is useful as an indicator of conditions such as water temperature. Consequently, we established four taphonomic modes, Mode A is characterized by isolated and dispersed fish remains, dominated by scales. Mode B contains associated but dispersed fish remains including mostly scales. Mode C includes disarticulated but associated fish remains with scales and scarce bone remains. Mode D is dominated by articulated specimens, such as complete or almost complete individuals. Therefore, the studied fish assemblages document different taphonomic histories, Mode A and B, suggesting that specimens came under flotation decay, indicating water temperatures warm enough to accelerate decay and gas concentration (full flotation). On the other side, Mode C indicates that specimens were under intermediate water temperatures (partial flotation). Finally, Mode D contains remains of articulated fish, suggesting that the specimens did not suffer flotation decay, which probably was inhibited by low temperatures of the water or low oxygen concentration. Until now, based on a fish taphonomy study, we can propose that the Agua de la Zorra Formation represents a deltaic lacustrine environment that experiences fluctuating warmer to cooler lake water column conditions.