Cenozoic vertebrate tunnels in southern Brazil

This work updates the present knowledge about the tunnels excavated by Cenozoic vertebrates in the four southernmost states of Brazil and discusses whether the producers of the different kinds of tunnels can be identifi ed and how. At a regional scale, tunnels are common in the states of Rio Grande do Sul and Santa Catarina, but rare in the states of Paraná and São Paulo. A few isolated occurrences are known from some other states, but data is still too scarce to allow any conclusion. Only ~30% of these tunnels are not entirely clogged with sediments and can be entered for investigation. The diameters of the tunnels range in three intervals of ~0.8, ~1.3 and > 2.0 m and lengths of individual tunnels may reach up to 100 m. Often, several clogged and/or open tunnels show up on the same location, suggesting that the tunnels form systems. The interconnected tunnels and chambers of such systems sum tunnel lengths of several hundred meters. On the walls of ~20% of the tunnels, three different groups of structures can be found: (i) inorganic marks such as grooves produced by running water and anthropogenic traces such as Indian rock art, post-colonial tool marks and vandalism (scratches); (ii) scratches from the paleovertebrates that dug the tunnel; and (iii) traces from re-occupying animals, extinct or not. Some tunnels host several thousand traces, especially digging scratches. Biogenic action produced by plant roots and inorganic processes, represented by running water inside the tunnels, produces a characteristic set of clogging and destruction features. The roof of the tunnel often collapses and the broken material is washed inside the tunnels. On the surface, this process results in a series of aligned craters and holes while the tunnel is clogged with sediments. The producers, considering the South American Megafauna during the Cenozoic, possibly were giant armadillos and ground sloths. The present wetness of the tunnels suggests that they were excavated during a drier climate than today, mainly for shelter. Ongoing investigations aim to clarify the questions that remain such as ventilation of the tunnel systems and the origin and interpretation of the surface structures on the walls.