On the interpretation and classification of Precambrian organic-walled microfossils

Petrographic approach to the study of organic microfossils from the Irati Subgroup (Permian, Paraná Basin, Brazil)

“…Principal among these are microfossils here interpreted as cyanobacteria on the basis of their cell size, morphology, colonial arrangement, apparent patterns of cellular division, and presence of extracellular sheaths, and still others as chlorophycean microalgae by analogy with modern and fossil analogues. Certain yellowish-brown organic remains, resembling indistinctly defined coccoidal or colonial microfossils commonly (and often indiscriminately) attributed to Botryococcus, were also recognized but are here referred to here as cryptarchs, a term of convenience proposed by Diver and Peat (1979) for unornamented, simple coccoidal, colonial or filamentous organic-walled microfossils of imprecise biological affinity (equivalent, respectively, to what other authors have called sphaeromorph, synaplomorph and nematomorph acritarchs) (Figs. 4e9).…”

Primary endosymbiosis events date to the later Proterozoic with cross-calibrated phylogenetic dating of duplicated ATPase proteins

“…General similarity to an extant group is an insufficient basis for using a fossil as a date calibration: Characters must place the fossil in the crown group rather than a stem group [which is sometimes an insufficiently appreciated distinction (8)] to constrain the date of the last common ancestor of the crown group (7). However, microfossils typically have a very small number of diagnosable characters (9), thus running the risk of misclassification, especially as a result of homoplasy.…”

The Distribution and Evolution of Microbial Life in the Late Proterozoic Era

“…Many Proterozoic fossils that have been assigned to the cyanobacteria have close morphologilc counterparts among living photosynthetic or other bacteria. Recognizing this fact, Diver & Peat (22) have advocated an artificial system for the classification of Precambrian microfossils in which taxa are grouped mor phologically without regard for inferred biological affinities.…”