Continental fish taphonomy: a case study in the Triassic of Argentina

The Triassic Los Rastros Formation, Ischigualasto-Villa Uniَn Basin, consists of alternating cycles of lacustrine black shales and deltaic sandstones. In each cycle, two facies assemblages have been defined: the lake (Ffl and SFm) and the deltaic (Fm, Sr, Sh, Sp/St, and FSC). The fossil content includes plants (macroflora and microflora), invertebrates (conchostracans, insects, and bivalves), and vertebrates (actinopterigian fish, temnospondyl amphibians, and archosaur footprints). Fossil fish occur in three facies (Ffl, SFm, Sh), and their remains have different taphonomic features in each. Three taphofacies were recognized, namely, A, B, and C. Taphofacies A is characterized by disarticulated fish material that tends to be dispersed or in loosely packed clumps; the remains have a low density of accumulation and a high degree of sorting. This taphofacies is related to distal lake deposits. Taphofacies B is characterized by densely packed, partially articulated remains, has a moderate density of accumulation and poor sorting, and is related to distal turbidity current deposits. Taphofacies C is characterized by partially to totally articulated remains that are densely to loosely packed with a moderate density of accumulation and poor sorting; this taphofacies is related to the mouth-bar lateral margin deposit. The fish assemblages studied document different taphonomic histories, and it is proposed that they were allochthonous to the lake, probably inhabiting instead the affluent fluvial system.