Cretaceous (Maastrichtian) chelonian burrows preserved in floodplain deposits in the Bauru Basin of Brazil: Evidence for the fossorial origin of turtle shells

Burrowing behavior is an important adaptation of animals that live in arid and semi-arid conditions. In this paper, we describe examples of vertebrate burrows from the Upper Cretaceous (Maastrichtian) Adamantina Formation of the Bauru Basin, Brazil, most likely produced by turtles. The Adamantina Formation preserves abundant and diverse turtle body fossils such as Bauruemys elegans (Testudines: Pleurodira); however turtle burrows have not been previously documented. The newly reported burrows are preserved in fluvial sandstone facies and exposed in sections that partially preserve their three-dimensional geometry. Burrows are simple J-shaped tunnels with a cross-sectional shape that is semicircular (half-dome) with a flattened floor. Such burrows show a partially preserved entrance with an inclined ramp angle (22°), and grooves and ridges up to 1 cm in width preserved along the burrows walls and floor. The architecture and sedimentary facies of the host sandstone body, together with the occurrence of Taenidium barretti and the absence of rhizoliths, suggest that the burrow was excavated by scratch digging into an exposed point bar of a meandering river channel. Based on burrow morphology, dimensions, as well as ridges,and grooves in the walls and floor, we propose that burrows were formed by a chelonian (such as a freshwater turtle) during aestivation. We highlight that these are first examples of turtle burrows reported from the Cretaceous, and their occurrence reinforces the hypothesis that the original function of turtle shells was as an adaptation to fossorial behavior.