Precambrian carbonaceous megafossils.

Abstract

The development of organisms of megascopic size and the related acquisition of the nucleus in cells are landmark evolutionary events. Based on the presently known paleontologic record of megafossils of probable algal affinities, these events occurred by Mid-Proterozoic time, well before the development of Ediacaran metazoans. While the first accounts of Precambrian carbonaceous fossils date back to the middle of the 19th century, relatively few new finds were made during the next hundred years. Only in the past 20 years, and particularly in the last 6, have such reports become more frequent. The fossils are generally preserved as millimetric compressions on bedding planes. They include circular, elliptical, ovate, and angulate films, and rectilinear, curvilinear, spiralform, and twisted ribbons. Most of them are unornamented, except for variably oriented folds and markings that are best regarded as compactional artifacts; some Late Proterozoic types exhibit branching and striate or granular structures with presumed biologic function. More than two dozen genera and over 40 species have now been formally erected, but these include synonyms. Among the megascopic compressions are some that can be interpreted as megascopic colonies of procaryotes. Others, such as the distinct and cosmopolitan Chuaria and Tawuia, are regarded as probable algal megafossils with some value (as a biota) for broad intercontinental correlation of Middle to Late Proterozoic sequences. -Author