Skolithos Dominated Piperock in Nonmarine Environments: An Example from the Triassic Caturrita Formation, Southern Brazil

The Triassic red beds of the Caturrita Formation (Paraná Basin, southern Brazil) contain abundant Skolithos, common Taenidium, and rare Arenicolites in low-angle, trough cross-bedded fine-grained sandstones interpreted as crevasse-splay deposits, forming a Skolithos dominated composite piperock and providing easily mappable horizons. Skolithos cf. serratus is the dominant trace fossil, being closely spaced (∼ 160 burrows/m2), slightly inclined, and having sharp, irregular boundaries with delicate scratch ornament. The average burrow length is approximately 0.2 m, but some may extend as deep as 0.4 m into the substrate. They crosscut a previous monospecific Taenidium barretti ichnocoenosis, formed by vertical meniscate burrows, which is superimposed on a previous Skolithos linearis ichnocoenosis, formed by abundant S. linearis, subordinate horizontal T. barretti, and rare Arenicolites isp. Colonization was firstly generated in softgrounds, maybe under subaqueous conditions, and reveals gradual substrate desiccation, the S. cf. serratus being produced in firmgrounds. The analyzed specimens of Skolithos resemble modern tiger beetle and midge larva burrows, while Taenidium is similar to modern oligochaete burrows. The dense concentration of vertical burrows reveals opportunistic colonization and suggests important environmental (climatic?) changes opening the colonization window to insects in floodplains. This highlights the stratigraphic significance of the colonized surfaces, which represent nondepositional hiatuses.