Estudio de huellas de afinidad mamaliana de la Formación Yacoraite, Valle del Tronco, provincia de Salta, y sus implicancias paleobiológicas

Abstract

In this Final Degree Project, small footprints collected from rocks of the Yacoraite Formation (Maastrichtian-Danian), from Quebrada Sunchales Sur, south of the Valle del Tonco, in the Salta Province, Argentina, were analyzed. The geological formation is characterized by an abundant and diverse fossil record, both corporeal and ichnological, in addition to levels with microbialites, stromatolites, and evidence of sedimentary structures induced by microbial activity (ESIAM, the acronym in Spanish). The paleoenvironmental context is a carbonate basin, with shallow waters of epicontinental platforms, with the presence of large lake systems. The footprints are preserved as negative epichnia in six wackestones-packstones slabs, housed in the Department of Paleontology, Facultad de Ciencias Naturales, Universidad Nacional de Salta, and with a request for collection numbers in process. Two petrographic thin-sections were made in a slab, indicating the presence of various siliciclastic, carbonatic, and bioclast sedimentary components forming a mixed calcarenite. Some of the slabs show evidence of the development of ESIAM, such as chips, deformed fragments, wrinkled surface, and sedimentary rims associated with the edges of the footprints. All these components were interpreted as corresponding to a mixed environment, in an epicontinental zone, with both terrigenous and marine influence due to intermittent changes. There are at least twelve small footprints, some partially preserved. Their general morphology is between circular and semicircular to irregular. Footprints are larger than, but similar in shape. The impressions of the digits are not clearly distinguished, at least preserving four, arranged parallel to each other, with no claw or digital pad traces. The spatial arrangement is asymmetrical with respect to the midline, with the imprints of the hands located internally and posterior to those of the feet, and corresponding to a jumping or hopping gait with transition to a galloping gait. These characteristics are consistent with the diagnosis of the ichnogenus Brasilichnium, although due to the fragmentary condition and preservational quality of the study material, it has been left in open nomenclature with an uncertain assignment, as Brasilichnium cf. saltatorium. The producer of these footprints is possibly a small mammal, capable of jumping, probably fast, and belonging to one of the representative groups for the southern hemisphere. However, due to different biases in the fossil record, it was not possible to assign it to a particular group. The tracks from Valle del Tonco correspond to the first record of mammalian origin for the Yacoraite Formation and the first from the Maastrichtian age, recording the ichnogenus Brasilichnium from the upper Upper Triassic and extending to the Upper Cretaceous (upper Maastrichtian). In addition, knowledge of the paleobiology of mammals for this period is expanded, providing information on their functionality and locomotion.