Microfossils from the Middle Precambrian McArthur Group, Northern Territory, Australia

Abstract

Microfossils have been detected in several formations of the McArthur Group (about 1600 m.y. old), in petrological thin sections and in macerations. Some of these occur associated with a lead-zinc ore body; others have been found in well-preserved dolomites and cherts. The variety of microfossils observed is considerable, and their state of preservation is good. Large numbers of singlecelled, colonial and multicellular organisms occur; in the latter, at least one clear case of cell differentiation can be demonstrated. Some of the organisms are morphologically comparable with blue-green algae such as the Chroococaceae, but, unlike the well-known Bitter Springs microflora, the assemblage is notably poor in filamentous algae. The filaments that do occur are not septate, and may represent discarded blue-green algal sheaths. Many of the microorganisms are extremely small in size, and in some cases, colonial structures composed of large numbers of 1 μm diameter cells are present, that may represent bacterial remains. Most of the microfossils occur in stromatolitic cherts, but the lead-zinc orebody from which some were obtained is a fine-grained dolomitic shale.

Stratigraphically, this new assemblage occurs in sediments of age intermediate between the well-known Gunflint Chert assemblage and the equally well-known Bitter Springs flora. The level of organization of the microfossils represents a great advance on that of the Gunflint Chert microfossils, in that demonstrably colonial and large multicellular microorganisms occur, as well as cells of a relatively large size. No convincing evidence for the presence of nuclei or nuclear membranes has yet been found in McArthur Group microorganisms, but the large size and organizational complexity of some of the structures suggests that the origin of the eukaryotic cell may occur rather earlier in geologic times than previous indications have suggested.