The Prymnesiophyta, also known as Haptophyta, is a group of autotrophic, planktonic uninucleate flagellates characterized by the presence of a haptonema (a filamentous, microtubule-supported appendage) that lies between two smooth, approximately equal flagella (Lee, 1999; Andersen, 2004). The group includes at least 500 extant and many more fossil species.
Geologically important members of the Prymnesiophyta are certain calcareous nannofossils termed coccolitho-phores or coccolithophorids (FIG. 4.42 ). Surrounding the living cell of these organisms are small (20 pm in diameter), calcified scales termed coccoliths which demonstrate a complex morphology and structure. Coccolithophores and coccoliths are valuable biostratigraphic markers, as well as indicators of paleoclimate (Wise, 1988). The living counterparts of these unicellular organisms are included within the Coccolithophorales (Jordan and Chamberlain, 1997), a group that is principally marine, and currently makes up ~45% of the total phytoplankton in middle latitudes. Coccolithophores have a significant impact on their environment since they are the major primary producers that convert dissolved COc in the ocean to calcium carbonate (CaCO3) (Rost and Riebesell,
2004; Baumann et al., 2005) and therefore influence global biogeochemical cycles. Moreover, they have the ability to increase the albedo of the Earth by reflecting light from their coccolith-covered surfaces and by producing dimethylsulfide, a gas that contributes to the formation of aerosols which enhance cloud formation in the atmosphere (Brand, 1994). Coccolithophores live in the upper 200 m of the water column and, therefore, fossil assemblages can be used to delineate former oceanographic and climatic conditions of the surface waters. Coccoliths almost always occur in the fossil record as isolated entities; rarely are the scales found attached to the nannoplankton that produced them. Because of the small size of the scales, electron microscopy has become an important research tool to study coccoliths. Based on the structure and morphology of the scales, several artificial families have been established (Bown and Young, 1997; Young and Bown, 1997a, b). Coccoliths are commonly divided into two major groups: heterococcoliths are constructed of crystal elements that differ in size and shape, whereas the crystal elements making up holococcoliths are essentially identical in size and shape (Siesser and Winter, 1994). In addition, a third category of similar status, nannoliths, occur as fossils and are most commonly defined as calcareous nannofossils of uncertain affinity, but probably related to the coccolithophores (Young and Bown, 1997b; Young et al., 1997). Nannoliths first occur in the Carnian (Late Triassic); coccoliths appear somewhat later, during the Norian, and are particularly abundant in the younger Mesozoic and Cenozoic. Coccolithophores appear to have been at their zenith during the Late Cretaceous (Perch-Nielsen, 1985; Bown et al., 2004), where their coccoliths often form thick deposits; the famous White Cliffs of Dover, England, consist largely of coccoliths. Their amazing abundance during the Cretaceous has been attributed to the chemistry of seawater at the time—a low Mg/Ca ratio and high Ca concentration (Stanley, 2006). In contrast, modern seawa-ter has low Ca concentration and a high Mg/Ca ratio, which apparently limits coccolithophore population growth today.
Several ideas have been advanced regarding the function of the scales in coccolithophores (Young, 1994). One suggests that the scales function to shield the cell from excessive light, although perhaps the more popular corollary argues that the convexo-concave surface of the scales actually focuses light into the cell. Their small size, along with their abundance in younger Mesozoic and Cenozoic rocks and typically restricted stratigraphic range, has made coc-coliths important index fossils and biostratigraphic markers. An analysis of their distribution through time indicates that a major extinction event occurred during the latest Cretaceous, followed by a recovery and radiation during the early Paleocene. This recovery, however, did not result in species richness similar to that seen in the Late Cretaceous (Bown et al., 2004).
The discoasters are a distinct type of calcareous nannofos-sil believed to be related to the coccolithophores; in some treatments, they are included within the nannoliths (Bown et al., 2004). The tiny skeletons of discoasters appear as stars or rosettes, between 10 and 35 pm in diameter. They lack modern analogs but were a conspicuous component of the nannoplankton during most of the Cenozoic (Aubry, 1984). Discoasters became extinct at the end of the Pliocene (Chepstow-Lusty and Chapman, 1995; Kitaeva et al., 1997). They differ from coccoliths in being composed of tubular forms of calcite, whereas the coccoliths are formed of rhom-bohedral and hexagonal calcite crystals.
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