Rare Girvanella Form of Proterozoic Stromatolites

Description: Precambrian stromatolite is the oldest of all fossils, and with much labor (cutting and polishing), it is most beautiful. The banding that commonly appears in stromatolite is a record of the growth patterns of colonies of microorganisms, principally photosynthetic prokaryotes (e.g, cyanobacteria). The colors that are often expressed are the result of the interaction of biological and sedimentary processes, together with subsequent chemistry and mineral exchange.

This particular specimen exhibits a rare structure for stromatolite, the so-called Girvanella-form comprising an irregular, unsymmetrical spherical nodule. Note the cross section of the nodule contained within the specimen. Such forms are more commonly found in Cambrian and younger stromatolite, when bacterial colonies were subject to predation, such as from various members of Domain Eucarya. In the case of this Lower Proterozoic stromatolite, the Girvanella structure could be the result of a new bacterial colony emerging subsequent to a violent disruption of the reef system. The wavelike banding is evocative of such a stormy sea.

Coming from the Chocolay Group in an area known as the Chocolay Hills located southeast of Ishpeming, Michigan, it dates to the Lower Proterozoic, some 2.2 to 2.4 Billion years old. This stromatolitic carbonate has been hand-polished to a mirror finish on one side which exhibits intricate banding and a spectrum of reddish-brown colors that are at once both vivid and awesome.