No lugar errado, na hora errada: icnofósseis de metazoários no Paleoproterozóico de Minas Gerais?

Abstract

WRONG TIME, WRONG PLACE: METAZOAN ICHNOFOSSILS IN THE PALAEOPROTEROZOIC OF MINAS GERAIS STATE? Two sedimentary structures comparable to metazoan ichnofossils are described in Palaeoproterozoic rocks of the Cercadinho Formation, Minas Supergroup of the Quadrilatero Ferrifero region, Minas Gerais state, Brazil. The structures are parabolic perturbations in the lamination, perpendicular to the bedding of metasandstones with tabular, tangential and parallel cross-laminations. The first one, similar to an Arenicolites-type burrowing, reaches 1.4 cm in width and about 4.5 cm in high, with sharp walls and disordered internal lamination; the second is comparable to an escape ichnite, up to 1.7 cm in width and 6.7 cm in high. The age of the Cercadinho Formation is well constrained between 2.1 and 2.4 Ga by a series of geochronological and stratigraphical data. A hypothesis that these structures are in fact of metazoan affinities in Palaeoproterozoic rocks is incongruent with the oldest records of metazoan fossils. Another possibility, more likely, is that the structures are not organic-related, and its genesis is so far unknown. Preliminary laboratory experiments, however, advocate the difficulty to generate similar structures by abiotic processes.