Taphonomic analysis of fish in rift lacustrine systems: Environmental indicators and implications for fish speciation

Abstract

Two rift-basin lacustrine successions (Cacheuta and Los Rastros Formations) in the Middle Triassic of Argentina were studied to test two different hypotheses: 1) fish taphonomy is useful as an indicator of environmental conditions for these two paleolakes; and 2) fish taphonomy is useful for understanding environmental effects on speciation. The fish fossils are found, in two lacustrine deposits, as isolated and disarticulated scales and cranial bones, clumps and stringers of scales, articulated body segments, and complete specimens. Four taphofacies are defined for Cacheuta specimens (Ca-A to Ca-D), and three taphofacies are defined for Los Rastros fish assemblages (LR-A to LR-C). A Principal Components Analysis was performed, with the taphonomic features supporting the seven taphofacies defined. A taphonomic model is proposed that characterizes a lacustrine-deltaic environment developed in the rift basin and identifies three sub-environments, each one with its particular fossil assemblages. The fish assemblages in the Los Rastros Formation suggest that the fishes preserved in delta-front and turbidity-current deposits (LR-B and LR-C taphofacies) were allochthonous to the lake and probably inhabited the delta plains and affluent fluvial system. In contrast, the isolated fish remains, preserved in open-lake deposits (LR-A taphofacies), likely represent both autochthonous and/or allochthonous specimens. In the Cacheuta Formation, articulated and isolated fish remains preserved in open lake deposits (Ca-A and Ca-D tafofacies), might represent autochthonous lacustrine specimens, based on the good preservation of the material. The fish remains preserved in underflow and turbidity current deposits (Ca-B and Ca-C taphofacies) were allochthonous to the lake and probably inhabited the delta plain and affluent fluvial system. The fish taphonomic study of two different rift-basin lacustrine successions allows test of the two proposed hypotheses. Fish taphonomy was useful as an indicator of environmental conditions beacuse the differences in fish preservation allowed differentiation of the sub-environments and reconstruction of the environmental conditions and dynamics of the rift lake basins. Also, fish taphonomy was useful for understanding environmental effects on speciation by characterizing the environments and dynamics that promote the development of newfish species, recognizing fish-inhabited environments, and identifying the generalist and specialist species.