Life in ancient intracratonic seas after a mass extinction: trace fossils from the Ordovician-Silurian Vila Maria Formation, Paraná basin, Brazil

Most Phanerozoic mass extinctions record an ichnodiversity reduction, a decrease in bioturbation intensity, small burrow sizes, and reduced burrow penetration. Studies of trace fossils of the Late Ordovician Mass Extinction (LOME) and its subsequent faunal recovery are scarce. The intracratonic Paraná Basin of southern Brazil hosts deposits of the Rio Ivaí Group (Sandbian-Aeronian) that records the entire Hirnantian glaciation (Iapó Formation) and the global sea-level rise during the latest Hirnantian-early Silurian (Vila Maria Formation; VMF). The age of VMF is well constrained to Hirnantian-Rhuddanian by the presence of palynomorphs and Rb/Sr dating. Although the occurrence of trace fossils is known since the original descriptions of the section, no comprehensive study has been published yet. The goal of this contribution is to report the presence of trace fossils from the VMF and their implications regarding the faunal recovery after the LOME. Our study focuses on the Córrego da Aldeia section, which comprises a ca. 23 m-thick succession. The outcrop is heavily covered by vegetation and is of difficult access. The studied succession of the VMF comprises a thickening- and coarsening-upward succession, consisting of black shale and siltstone in the lower interval (11 m thick), and siltstone and very fine- to coarse-grained sandstone in the upper interval (12 m thick). The lower interval contains dispersed bivalves, gastropods, inarticulate brachiopods, and ostracods, in places forming pavements, as well as locally small Planolites, showing a very low bioturbation index. Absence of wave-produced structures suggests deposition below storm wave base (i.e. shelf). The upper interval displays parallel, planar cross stratification and trough-cross stratification, and is thought to record deposition in offshore to nearshore settings. In cross section, trace fossils are hard to visualize. Fortunately, several loose sandstone blocks coming from the upper interval were accessible at a waterfall, showing several bioturbated surfaces. In addition to Arthrophycus alleghaniensis previously recorded in this formation, sandstone beds are host to a variety of poorly preserved trace fossils including Diplocraterion, Cruziana, Palaeophycus, Planolites, and Skolithos, as well as arthropod scratch imprints and undetermined meniscate burrows. The paucity of trace fossils in the lower interval is consistent with observations worldwide regarding the impact of the LOME, mostly because of the formation of anoxic waters in the shelf. However, the presence of bioturbated deposits in the overlying offshore to nearshore deposits may be regarded as evidence of an incipient recovery. Discriminating between facies and evolutionary controls on trace-fossil distribution is of paramount importance to assess the impact of mass extinctions on the infauna.