This Chapter considers the preserved, known record of Proterozoic and selected Early Cambrian microfossils: the microbiology of the middle eon of earth history. The evolutionary changes (evidenced morphologically) that took place during the Proterozoic were somewhat transitional between those of the preceding Archean (Section 1.5) and succeeding Phanerozoic Eons. The Early Proterozoic record is dominated by simple bacterial and cyanobacterial prokaryotes, some of which exhibit a significant degree of morphological complexity by about 2 Ga (Section 5.4); by the Late Proterozoic, various types of eukaryotic phytoplankters had arisen, including “giant” sphaeromorph and acanthomorph acritarchs as well as the enigmatic melanocyrillids. The evolutionary fabric of the Proterozoic is a complex one, and holds the key to the evolution of significant grades in microbiological organization. Here we attempt to dissect that fabric so that we can study it with critical and (we hope) open eyes.

Although the amount of information available for the task is less than one might prefer, it nevertheless is immense, even overwhelming; included in this mass of data are many uncritical reports of microfossils that must be filtered out before meaningful interpretations can be made. We might compare this dataset to that available for recent reviews of the Archean (Schopf and Walter 1983) and Early Proterozoic (Hofmann and Schopf 1983) microbiotas. The Archean compilation included 43 categories of microfossils and microfossil-like objects from 28 geologic units; two of these categories were accepted as representing true microfossils.