Palaeonvironmental and palaeoecological significance of flamingo-like footprints in shallow-lacustrine rocks: An example from the Oligocene-Miocene Vinchina Formation, Argentina

Abstract

This article describes the lacustrine trace-fossil assemblage of the Late Oligocene-Early Miocene Vinchina Formation of northwest Argentina. The thick (> 5000. m) continental succession of the formation is essentially fluvial with thin aeolian and lacustrine intervals. The trace-fossil assemblage from the lacustrine facies is dominated by bird footprints, notably flamingo-like palmate bird footprints, and also contains invertebrate traces. The assemblage is of contrasting composition with those of the ubiquitous fluvial facies of the Vinchina Formation. We identify the possible trace maker of the web-footed bird footprints and infer the palaeoenvironmental and palaeoecological meaning of the lacustrine trace-fossil assemblage of the Vinchina Formation. The lacustrine succession is mainly composed of wave-rippled fine-grained sandstones and siltstones and wave-rippled heterolithic facies, with subordinate laminated mudstone and medium-grained sandstone with parallel lamination or cross-bedding. Metre-scale shallowing-upward cycles are typical and desiccation cracks were observed in a single bedding plane. The analysed interval was deposited in a wave-dominated littoral zone of a shallow permanent lake basin. Ichnofossils were found mostly in three trampled horizons of the upper part of the analysed succession. The trace-fossil assemblage is dominated by Phoenicopterichnum isp. (flamingo-like footprints), slender bird tracks without web traces, and triangular and fusiform hypichnial ridges (interpreted as bird undertracks). The remaining trace fossils are Lockeia siliquaria (bivalve resting trace), Rusophycus isp. (shallow arthropod burrow), Taenidium barretti (endostratal meniscate burrow) and small hypichnial double knobs (possible beak probing marks). Neoichnological observations on modern Chilean flamingo footprints (Phoenicopterus chilensis) suggest that Phoenicopterichnum isp. is morphologically akin to modern flamingo footprints, although its average size is slightly smaller. The likely producers of the fossil footprints are members of the Palaelodidae and Phoenicopteridae, which are recorded in South America since the Late Oligocene. Phoenicopterichnum isp. appears in cuasi-parallel trackways with short pace length and forming a low angle with ripple crests. By comparison with observations of modern flamingo behaviour during this study and of shorebird studies from the literature, these tracks and trackways likely reflect flock-feeding behaviour (probably filter feeding) in a subaqueous belt parallel to the waterline. Invertebrate ichnofossils that appear on the same slabs from trampled horizons are mainly triangular and fusiform hypichnial ridges, while Phoenicopterichnum isp. and sparse small hypichnial double knobs also occur. In consequence, it is possible that these birds were foraging on small invertebrates (represented by Lockeia siliquaria, Rusophycus isp. and probably Taenidium barretti) in the coast of the Vinchina lake. The only fossil evidence for these invertebrates is the impression of articulated valves probably belonging to a small mollusc (potential producer of Lockeia). Trace-fossil assemblages dominated by flamingo-like footprints (including ichnogenera Phoenicopterichnum, Culcitapeda and similar unnamed tracks) are indicative of alkaline and/or saline waters, as suggested by the specialised filter-feeding mechanism of Late Oligocene to modern Phoenicopteriformes. These particular assemblages are distinguished as a subset within the "shorebird ichnofacies" with a definite palaeoenvironmental and palaeoecological meaning: the Phoenicopterichnum ichnosubfacies.