The ichnotaphonomic window in lacustrine subaqueous littoral deposits: an example of bird and mammal footprints from the Miocene of northwest Argentina

This study includes the ichnotaxonomic and taphonomic analysis of bird and mammal footprints from an Upper Miocene lacustrine sequence of the San Juan Province, northwestern Argentina. The interval hosting the trace fossils represents the interaction between unconfined fluvial flows and lacustrine deposits. Recorded trace fossils include a diverse assemblage of bird footprints (12 ichnotaxa), mammal footprints (2 ichnotaxa, including a new one), swimming traces (1 ichnotaxa) and invertebrate trace fossils (7 ichnotaxa). This case study is peculiar because of the dominance of bird footprints, of moderate to large size, preserved in full relief (77% of specimens) that are interpreted as shallow to deep depressions left by foraging birds in the subaqueous littoral zone of the lake. This type of preservation of bird footprints is indicative of a subaqueous littoral ichnotaphonomic window in lakes distinct from the “shorebird ichnofacies” and related to the partitioning of shallow-water habitats by waterbirds. Two littoral depth zones are recognized: 1) the shallow littoral (∼5–25 cm) represented by cf. Ornithotarnocia isp., Chraradriipeda isp., Gruipeda isp., Uhangrichnus isp., indeterminate tridactyl footprints, and tridactyl footprints with elongate posterior traces type 1 and 2; and 2) the wading-depth littoral zone (∼25–40/50 cm) that includes cf. Gyeongsangornipes isp. and Phoenicopterichnum isp. The bird assemblage also has footprints preserved in positive hyporelief and negative epirelief (23% of specimens) from shore margin zone (∼0–5 cm) containing the smaller footprints ( Avipeda phoenix , Gruipeda dominguensis and cf. Avipeda circumontis ). The latter type of preservation is commonly described in the literature and belongs to the “shorebird ichnofacies”. • Diverse bird and mammal footprint assemblage from Upper Miocene lacustrine deposits. • Preservation of most fossil bird footprints in full relief corresponds to the subaqueous littoral zone. • Ichnotaphonomic window related to partitioning of shallow-water habitats by waterbirds. • Different from the “shorebird ichnofacies” that belongs to the shore margin zone. • A new mammal (Caviidae?) ichnotaxon: Calingastapodus littoralis nov. igen. et isp.