Sponges or Porifera are among the most primitive and simplest of metazoan organisms and have a long geologic record, based largely on mineralized skeletal elements termed spicules. Spicules, then, are of interest to micropaleontologists, and their study and interpretation provide opportunities for wide-based research by researchers ranging from student beginners to professionals and for research throughout the geologic column and across the country. Much of the detailed geologic record of spicules is yet to be discovered.

Polycystine radiolaria that produce siliceous tests are known to range from Cambrian to Holocene. They have proven to be enormously useful in providing age control for siliceous marine sequences of Middle Devonian and younger ages, particularly for cherts and shales that are commonly devoid of other biostratigraphically useful fossils. The utility of radiolarian biostratigraphy became widely recognized in the 1970s and 1980s when it was applied in dating deformed marine siliceous sequences in orogenic belts around the world, most notably in Cordilleran North America and other areas along the Pacific rim (e.g., Jones and Murchey, 1986; Aitchison and Murchey, 1992; Ichikawa et al., 1990).

Mesozoic radiolarians generally range in size from 100 to 400 μm. All were planktonic and exclusively marine. Radiolarians possess an exceptional geological record and have been abundant since the Cambrian; they were widely distributed in the Triassic to Cretaceous seas.

Radiolarians are marine zooplankton possessing a tough, central capsular membrane that divides the cytoplasm into intracapsular (containing the nucleus, organelles, and food reserves) and extracapsular (with food-gathering rhizopodia and digestive vacuoles) portions (Figure 1). They bear two kinds of pseudopodia, the axopodia and filopodia. The axopodia extend radially through the ectoplasm and capsular membrane to the interior of the endoplasm. The axopodia are inserted into a special structure, the axoplast (Figure 1). The development of the axoplast and its complex is of fundamental importance in radiolarian taxonomy. For a detailed description of radiolarian cytology, biology, and reproduction, see Anderson (1983).

The Cretaceous record of the diatoms presented here outlines our understanding of their early morphological development. Recent documentation of well preserved Lower Cretaceous (Aptian/Albian) diatom assemblages provides a window into the early history of the diatoms and serves as a base for comparing their subsequent morphological changes. A brief review of diatom biology and morphology is provided to introduce this paper and that of Barron and Baldauf (this volume). Additional background information on the diatoms can be found in the works of Schrader and Schuette (1981), Tappan (1980), Bach and Burkhardt (1984), Barron (1985a, 1993), Fenner (1985), Ricard (1987), Bradbury (1988), Round et al. (1990), and Picket-Heaps et al. (1990).

Diatoms, golden brown algae, are present in most aqueous environments. Within the marine environment marine diatoms occupy the photic zone and represent the lowest level of the marine food chain. Diatoms are either planktonic or benthic and possess an external siliceous skeleton or frustule, that is boxlike in structure. The size of diatom frustules ranges from less than 1 μm to more than 1,000 μm, but most frustules range in size from 10 to 100 μm. Diatoms are present in the geological record from at least the Cretaceous (Harwood and Nikolaev, this volume) and have numerous advantages for biostratigraphic correlation and paleoenvironmental reconstruction of marine sedimentary sequences. This chapter summarizes the current state of marine diatom biostratigraphy for the Cenozoic and provides examples of how marine diatoms are used in paleoenvironmental reconstructions. No attempt is made to illustrate the various diatom taxa discussed; the reader is referred to published references such as the syntheses of Fenner (1985) and Barron (1985).

Diatoms are golden brown algae (class Bacillariophyceae) whose cellular contents are enclosed between two valves or shells of silica. They are classified into groups with radial symmetry (centric diatoms) and axial symmetry (pennate diatoms). The latter are subdivided as raphid and araphid diatoms according to the presence or absence of raphes (slit-like structures) that allow diatoms to move along firm surfaces. Many centric and some araphid diatoms are planktonic, maintained by turbulence in the limnetic region of a lake, whereas raphid diatoms live on the lake bottom or are attached to objects in the illuminated zone.

Chrysophytes are a diverse group of algae, commonly known as the golden-brown algae (Bold and Wynne, 1978), and consist of two classes, the Chrysophyceae and Synurophyceae (Andersen, 1987). Over 800 species have been described (Kristiansen, 1990; Kristiansen and Takahashi, 1982; Preisig, 1995), and it is estimated that more than 1,000 species exist. Chrysophytes are distinguished from other algae on the basis of chloroplast type and structure, photosynthetic pigments, storage product, flagellar apparatus, and especially the production of a siliceous resting stage, called the stomatocyst, statospore, or cyst (Hibberd, 1976; Kristiansen, 1990; Kristiansen and Takahashi, 1982).

The silicoflagellates are single-celled plankton that have a delicate siliceous skeleton made up of tubular rod-shaped elements. The group is both photosynthetic and autotrophic and has thus been claimed by botanists and zoologists. All silicoflagellates are included in the order Dictyochales, which is now generally considered to be in the division Chrysophyta, golden algae, on the basis of pigmentation and the structure of the chloroplasts and mitochondria.

General.—Ebridians, in many ways, are “neglected step-children” in the family of siliceous microplankton. Reduced to only three living species and never particularly abundant througout their history, ebridians do show up in marine, diatom-bearing sediments from practically every region of the globe.