Análise morfológica e paleoecológica de icnofósseis atribuídos a tetrápodes não-arcossauros da Formação Botucatu (Cretáceo Inferior)

Abstract

Abstract Ichnofossils, studied by Paleoichnology, are structures produced as a result of behavioral and physiological activities of organisms of the past. Fossil tracks (footprints), one of the best-known types of ichnofossils (bioturbations), are produced by the interaction of the animal's autopods with the substrate, the latter being deformed in the process. The shape of the autopods, the conditions of the substrate and the locomotion behavior of the trackmaker directly influence the morphology of the tracks. During the beginning of the Cretaceous, a huge sandy paleodesert covered a large area of the Brazilian territory, giving rise later to the eolian sandstones of the Botucatu Formation. Its main fossiliferous content are the tracks and excavations produced by the animals (vertebrates and invertebrates) that lived in the paleodesert, being the region of the Municipality of Araraquara (São Paulo State) the one that presents the greatest quantity and diversity of ichnofossils. The present work had as main objective the morphological and paleoecological analysis of ichnofossils with affinity to non-archosaurian tetrapods from the Botucatu Formation. The descriptions of the tracks and trackways were carried out based on parameters established in the scientific literature, such as width and length of the tracks, oblique pace, stride, pace angulation, trackway width and rotation of the autopods. Several specimens of tracks with affinity to larger mammaliaformes have been described and analyzed, and a new ichnotaxon, Aracoaraichnium leonardii, has been defined. It was possible to determine that the trackmaker of these larger tracks belonged to a different species than the trackmaker of the Brasilichnium tracks, and not an advanced ontogenetic stage. In this way, the trackmaker of A. leonardii represents a new component in the ecological structure of the Botucatu paleodesert. It is inferred that the trackmakers of this ichnotaxon could feed on invertebrates or plant material, depending on the availability of resources, being potential competitors of the trackmakers of Brasilichnium. Tracks with affinity to Lepidosauria (lizards) were also described and analyzed for the first time. Such tracks are pentadactyl, with curved digits. The digit I is directed to the inside of the trackway and opposite the digit V, which is in a posterolateral position. Marks attributed to drag and contact of the tail on the substrate were identified. The icnotaxonomic attribution was tentatively made to the icnogenus Rhyncosauroides, and future analyzes based on new, better preserved specimens may elucidate this issue. From the analysis of the trackways, it was possible to identify three distinct locomotion behaviors, being the typical cursorial gait, a gait where the footprints are rotated in the same direction, diverging from the trackway’s midline, and one where the producer makes stops. The recognition of a new trackmaker with a lizard-like affinity increases the complexity of the ecological web of the Botucatu Formation and contributed to the understanding of the distribution and niche occupation of these animals in the Lower Cretaceous.