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Paleoenvironmental and paleoecological aspects of the Río Loro Formation (Paleogene, northwestern Argentina) were analyzed. In the type locality, four main sedimentary facies were recognized. Facies 1 (coarse-grained to pebbly amalgamated sandstones) record sedimentation in braided channels. Facies 2 (structureless to faintly laminated mudstones) and Facies 3 (interbedded mudstones with sandstones) are ascribed to deposition in distal to proximal floodplain settings, respectively. Facies 4 (coarse to medium-grained sandstone) is interpreted as deposited in meandering channels. Insect trace fossils, ascribed to nesting structures, were recognised particularly in facies 1 and 4. The recognized facies were grouped in a main facies association which allows interpretating a deposition in a complex multi-channel fluvial system with relatively stable muddy floodplain areas. Within this system, seasonal humid to dryer conditions favored biota substrate colonization and soil-forming processes. Well-preserved mammalian and reptilian remains were recognized in sandy mudstone deposits of Facies 3, showing low weathering stages (mainly stages 1 and 2) and the absence of signs of abrasion. These aspects, along with the presence of some articulated remains and their integration with the paleoenvironmental framework, suggest that skeletal pieces suffered an early burial after partial disarticulation, possibly by splays during major floods. The inferred habits of some of the taxa recorded are consistent with proximity to freshwater sources, vegetated areas, and seasonality. This multidisciplinary approach contributes to the understanding of paleoenvironmental and paleontological attributes of the Río Loro Formation and represents a basis for future surveys on this unit and its remarkable fossil record.

The Río Loro Formation (early Paleogene, Tucumán Province, northwestern Argentina) has yielded several endemic vertebrates, including turtles, crocodiles, and mammals. The age of this unit is currently under discussion given the lack of absolute chronological data, although recent works favored a middle Paleocene–early Eocene age. In this contribution, we present a left maxillary fragment (PVL 5901), preserving part of the M1 and the M2. A first analysis indicates a close relationship with Indalecia (Litopterna?, Indaleciidae), a genus previously recorded for the Lower Lumbrera Formation (?Ypresian; Salta Province, northwestern Argentina). Given that the Indaleciidae and the representatives of Amilnedwardsidae and Notonychopidae were considered related in the context of the order Notopterna, a comparison was made between PVL 5901 and these taxa, confirming its resemblance with Indalecia. However, PVL 5901 differs from Indalecia grandensis (type and only species of the genus) by the presence of a cuspule on the base of the ectoflexus, the absence of a small fossette mesial to the distolabial fossette, and a conspicuous entoflexus. Hence, we refer PVL 5901 to Indalecia sp. This is the first common faunal element between the Río Loro and Lower Lumbrera formations. Nevertheless, the marked taxonomic discrepancies between the Río Loro vertebrate assemblage and other early Paleogene units contrast with this single common record, and a partial temporal correspondence between the Río Loro and Lower Lumbrera formations is not supported here. Instead, this record indicates a more extended temporal and geographical distribution for the genus Indalecia in northwestern Argentina.