Microbial mediation in invertebrate trace fossil preservation in Sousa Basin (Early Cretaceous), Brazil

Well-preserved invertebrate and vertebrate trace fossils commonly occur in reddish, fine to very fine-grained sandstone and mudstone bedsets of the Antenor Navarro and Sousa formations in the Sousa Basin, referring to an Early Cretaceous pull-apart basin in northeastern Brazil, related to the South Atlantic opening. Palaeoenvironments are understood as oxidizing terrestrial (alluvial fans and meandering rivers with extensive floodplains or perennial flooded areas), under hot and humid palaeoclimates punctuated by drought episodes. The ichnodiversity is low but trace fossils are abundant in the invertebrate ichnoassemblages analyzed in this paper. The ichnoassemblages are composed mainly of shallow endostratal traces. Repichnia and Fodinichnia behaviours are represented by Aulichnites isp., Palaeophycus isp., ?Arenicolites isp., Planolites isp., Taenidium isp. and Phycodes isp. and are a taphonomic challenge in terms of their production and preservation, particularly considering its palaeoenvironment. Field observations and collected samples reveal common presence of microbially induced sedimentary structures associated to these traces. Biofilms and thin microbial mats seem to produce the adequate conditions of nutrient supply, moisture and substrate biostabilization to allow trace makers' survival and taphonomically enhance the preservation of the invertebrate trace fossils in an unstable and even sometimes inhospitable environment.

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