SYSTEMATICS AND BIOSTRATIGRAPHIC IMPLICATIONS OF A NEW NOTOUNGULATE ASSEMBLAGE (MAMMALIA, PAN-PERISSODACTYLA) FROM THE INDIA MUERTA FORMATION (LATE MIOCENE), NORTHWESTERN ARGENTINA

The fossils of the India Muerta Formation (Neogene, Tucumán Province, Northwestern Argentina) include several vertebrate groups, mainly metatherian, xenarthran, and notoungulate mammals. Nevertheless, these remains have been scarcely mentioned in the literature, being mostly noted in faunistic lists, without a focus on taxonomic or morphological aspects. Additionally, most of the biostratigraphic or paleobiogeographic considerations, which are based merely on lithostratigraphic inferences, have suggested a correlation mainly with the Andalhuala (Late Miocene–Pliocene) and the Corral Quemado (Pliocene) formations, both units corresponding to the Santa María sedimentary basin of the Calchaquí valleys of Northwestern Argentina. Here, we present a study of a notoungulate assemblage recently recovered from levels of the India Muerta Formation, clarifying the geological context and correlating fossil levels with western outcrops. Our study identifies remains grouped systematically as two toxodontids, one mesotheriid, and four hegetotheriids, some of which represent the first records for the unit and/or the region. The sedimentological analysis suggests that this fauna developed in a paleoenvironment corresponding to a complex of braided to meandering fluvial systems. Based on these new data, the fossiliferous levels of the India Muerta Formation reinforce a Late Miocene (Tortonian) age. Consequently, our chronological proposal leads to a closer correlation between the India Muerta Formation and the Las Arcas and Chiquimil formations, which immediately underlie the Andalhuala Formation in western valleys. These inferences agree with previous radioisotopic analyses and, hence, the evidence yielded by the presence of these newly documented notoungulates stands as our most reliable basis for stratigraphic correlation.