Perhaps two of the most important groups of fossils are the prokaryotes and protists, both single-celled organisms. They are not spectacular fossils and so may be less interesting to students than the more complicated metazoans and metaphytes, yet these two groups not only dominate life on Earth today, but they contribute enormously to our understanding of Earth and life history. Prokaryotes dominated the fossil record of Earth from 3.5 to nearly .5 billion years ago (Knoll, 1985). For the last 1.5 billion years, protists have been an important element in marine and probably other ecosystems.

In terms of biochemical and intracellular organization, living systems can be divided into two major “Superkingdoms,” eukaryotes and prokaryotes.

Eukaryotes, comprising the more advanced and later evolving Superkingdom, include unicellular or multicellular organisms (viz., members of the Kingdoms Protista, Fungi, Plantae and Animalia) characterized by nucleus-, mitochondrion-, and (in plants and some protists) chloroplast-containing cells that are capable typically of mitotic cell division. Paleontologic evidence indicates that the eukaryotic cell originated during the Middle Proterozoic, probably about 1.4 to 1.5 Ga ago (Schopf and Oehler, 1976).

Prokaryotes, comprising the more primitive and earlier evolving Superkingdom, include microbial microorganisms (viz., members of the Kingdom Monera: bacteria, cyanobacteria, archaebacteria, and prochlorophytes) characterized by cells that lack membrane-bound nuclei, mitochondria, chloroplasts, and similar organelles and that reproduce by non-mitotic and non-meiotic division. Some authors (e.g., Woese and Fox, 1977) subdivide prokaryotes (monerans) into two kingdoms, the Kingdom Archaebacteriae (including methanogenic, extremely halophilic and some thermoacidophilic bacteria) and the Kingdom Eubacteriae (including all non-archaebacterial prokaryotes), based on the chemistry of their cell walls, membranes, transfer RNA's and RNA polymerase subunits. Paleontologic evidence indicates that prokaryotes originated early in Earth history - the group was extant, morphologically varied and evidently physiologically advanced at least as early as 3.3 to 3.5 Ga ago (Schopf and Packer, 1987).

Dinoflagellates are single-celled organisms of the Division Pyrrhophyta. Most people, although not familiar with dinoflagellates, are familiar with their effects. Bioluminescent dinoflagellates cause a sparkling of the sea at night as the waves break, and certain dinoflagellates may produce blooms called “red tides,” which poison marine life or which cause toxins to accumulate in shellfish, poisoning those who eat them. The Old Testament (Exodus 7:20–21) and the writings of the ancient Greeks refer to red water and the killing of fish. The Red Sea may have gotten its name from dinoflagellate blooms.

In 1934 the Geological Society of America published its second Memoir: “Paleozoic Plankton of North America,” by Rudolf Ruedemann. This work was concerned primarily with invertebrates, especially the enigmatic and now-extinct graptolites. The primary producers of that era were virtually unknown; accordingly, the author devoted only a few pages to description and discussion of seaweeds. And although Ruedemann (1934, p. 25) recognized that “the microscopic algae must be considered as the very basis” of Paleozoic life, he submitted that “obviously it will be difficult to find their remains in the Paleozoic rocks.” O.M.B. Bulman (1964, p. 467), another graptolite expert, condensed his discussion on the primary producers to two paragraphs in “Lower Palaeozoic Plankton,” his Anniversary Address as President of the Geological Society of London; but at least he acknowledged the “universal occurrence” of fossil phytoplankters.

Calcareous nannoplankton are minute plant-like protists that live in the marine photic zone. There has been an exponential increase in nannoplankton studies in the last two decades, largely owing to the recognition of the usefulness of this group for biostratigraphic and paleoenvironmental interpretations.

Diatoms are unicellular golden brown algae, that are characterized by an external box-like skeleton (or frustule) of opaline silica. Diatom frustules are commonly very intricate and varied, and the patterns and types of ornamentation of the frustule form the basis of nearly all diatom taxonomy. The size of diatom frustules ranges from less than one μm (micro-meter) to over 1000 μm, but most frustules occur in the 10–100 μm size range. Thus, diatoms are comparable in size to dinoflagellates but generally larger than calcareous nannofossils. Unlike calcareous nannoplankton and dinoflagellates, however, the vegetative cell of diatoms lacks flagella, although many diatoms produce flagellated gametes during their brief period of sexual reproduction. Consequently, planktonic diatoms are subject to passive current-related dispersal in the surface layers of the ocean. Adaptations that promote flotation include frustule shapes and processes which increase the ratio of surface area to volume, formation of colonies, and storage of fats or oils in the cell which reduce its overall specific gravity.

The silicoflagellates, ebridians and archaeomonads are three groups of quite dissimilar organisms. However, they are sometimes discussed collectively because each group has a relatively small siliceous skeleton, and is relatively less common, and much less understood, than the diatoms or radiolarians. The silicoflagellates are the most abundant of the three groups. They usually make up one or two percent of the siliceous component of marine sediments (Riedel, 1959), and can be viewed as the least abundant of the major plankton groups. The ebridians and archaeomonads, when found at all, are generally only a minor constituent of the sediment.

Foraminifera are testate protozoans that exhibit a bewildering variety of test composition and morphology (Figure 1). They inhabit all marine environments from the intertidal zone to the deep ocean floor, they range from the poles to the tropics, and they can be planktonic or benthonic. Their abundance in sediment samples may reach tens of thousands per square meter and their diversity in tropical environments may exceed 60 or 70 species in a sample of 300 individuals. A geological range stretching from the very earliest Cambrian through to the present day is characterized by varying patterns of evolution and extinction that have proven to be of considerable biological and geological significance.

Radiolaria are marine holoplanktonic animal-like protists belonging to the Superclass Actinopodea of the Subphylum Sarcodina. When biologists speak of radiolarians they usually mean the subgroup Acantharia that are common in nearshore waters and are sometimes involved in plankton blooms. When paleontologists speak of radiolarians they usually mean the subgroups preserved in the fossil record; the Polycystina (or polycystine) encompassing Spumellaria and Nassellaria, which possess solid opaline skeletal structures, and the Phaeodaria (or phaeodarians), which possess hollow skeletal structures of an admixture of silica and organic matter.

In an essay published in 1977, S.J. Gould argued that three major questions have dominated paleontological thinking for more than a century. Does the history of life have direction? What is the motor of evolutionary change? And, what is the tempo of change? Certainly, these “eternal metaphors” have figured prominently in the research of invertebrate paleontologists during the past decade. Temporal pattern has been sought in changing morphologies within lineages, the changing structure and composition of communities, trends in taxonomic diversity, and even the occurrence of mass extinctions. Drifting continents, changing climates and oceanic circulation patterns, biological interactions, and extraterrestrial influences have all been championed as significant determinants of evolutionary change. The punctuation/gradualism debate has generated an impressive body of stratigraphic and morphological data, if not an unambiguous resolution of the issue.