Paleobiology de uma nova microflürula silicificada do Grupo Bambuí (Proterozoico Superior) da região de Unaí, MG

Abstract

A moderately well-preserved and abundant assemblage of coccoidal, bacilliform, and filamentous organic-walled microfossils is here described from black chert colleted near the top of a 30 m-thick exposure of predominantly microbially laminated dolostone from the lower part of the Upper Proterozoic Bambui Group, 12,5 km NW of Unai, Minas Gerais, south-central Brazil. Of the 11 morphotypes described in this study, small colonies of coccoidal cells (3 species of Myxococcoides; Glenobotrydion aenigmatis Schopf; Form A) are most abundant, followed by tubular filaments (Siphonophyous sp.; 2 species of Eomycetopsis), and, finally, rare, solitary, large (20-40 µm) coccoidal forms (Form B; Form C) and a single colony of bacilliform cells (Eosynechococcus moorei Hofmann). The assemblage is dominated by apparently planktonic, small-celled (rarely exceeding 13 µm), colonial coccoidal forms, although filamentous forms may have played a significant role in the local benthos, as suggested by faintly preserved, palimpsestic (or "ghost") filamentous fabrics and by the presence of poorly preserved tubes of Siphonophycus up to 32 µm in diameter. A single, large pair of thickwalled coccoidal cells (Form B) is here interpreted as a possible acritarch of as yet undetermined biostratigraphic value. The generally small, simple microfossils of this assemblage are similar to elements of the five other known Bambui assemblages and apparently rather typical of many other Late Proterozoic microfloras.