Impact of Late Ordovician glaciation in trace fossil assemblages in the Gondwana: A case study on the Serra Grande Group (Parnaíba Basin, NE Brazil)

Sara C. Memória¹Renata Guimarães Netto^2*Daniel Sedorko³Luiz Saturnino de Andrade⁴Bernardo Vazquez García¹

• ¹Universidade do Vale do Rio dos Sinos, São Leopoldo, Brazil
• ²University of the Rio dos Sinos Valley, São Leopoldo, Brazil
• ³Universidade Federal do Rio de Janeiro Museu Nacional, Rio de Janeiro, Brazil
• ⁴Universidade de Brasilia Instituto de Geociencias, Brasília, Brazil

The Hirnantian Ice Age had a significant impact on marine invertebrate biota at the end of the Ordovician, due to drastic cooling and falling sea levels. The Parnaíba Basin (NE Brazil) was located closer to the South Pole during the Hirnantian and exhibits a significant ichnofaunal turnover in the Serra Grande Group deposits, which represent the filling of the basin during the Late Ordovician to Early Silurian. The benthic biota inhabiting the Parnaíba Basin during the Hirnantian is represented by the glacial paleoichnocenosis, preserved in the diamictites of the upper portion of the Ipú Formation. The local occurrence and low ichnodiversity of the glacial assemblage are indicators of harsh conditions during the Hirnantian cooling. Resting/dwelling traces of large burrowing sea anemones (Conichnus isp.) and burrows of potential commensal or scavenger organisms (Palaeophycus tubularis) are the main components of the ichnofossil assemblage, suggesting severe restrictions on the occupation of ecological space by endobenthic organisms. In contrast, the postglacial deposits of the Tianguá and Jaicós formations exhibit greater ichnodiversity, as evidenced by the 27 ichnotaxa and some unidentified ichnofossils that compose distinct suites preserved in delta plain, tidal flats, prodelta, and delta front settings. The glacial paleoichnocenosis of the Serra Grande Group is convergent with the few Hirnantian ichnofauna found in Gondwanan deposits, whereas the Llandovery post-glacial paleoichnocenosis is similar to those found worldwide since the early Llandovery. Despite the abrupt decrease in ichnodiversity in Gondwana terranes throughout the Hirnantian, the ichnodisparity remains high, showing that the main ecological niches remained active and occupied by tolerant species. The preponderance of plug-shaped burrows, likely produced by soft-bodied cnidarians in Gondwana’s Hirnantian ichnofauna supports the idea that these organisms were able to tolerate the significant physicochemical variations generated by glaciation in shallow seas. In contrast, the postglacial deposits exhibit greater ichnodiversity, indicating that the ichnofauna was significant renewed as a result of the worldwide eustatic sea-level rise during the Llandovery stage.