The Chlorophyceae encompass the widest range of morphologies in the green algae. The colonial Volvocaceae (Volvocales) and their unicellular relative Chlamydomonas reinhardtii (Chlamydomonaceae) have frequently been used as a model in studies addressing the evolutionary pathways leading from unicellularity to multicellularity, including a division of labor within the algal thallus (Kirk, 1998, 1999). Molecular evidence suggests a minimum age of 400-500 Ma for a few Chlamydomonas species (Van den Hoek et al., 1988).
Nevertheless, the fossil record of Chlamydomonaceae is virtually nonexistent, and that of Volvocaceae is meager, perhaps because colonies (coenobia) disintegrate almost immediately upon death (Tappan, 1980). Fossil unicellular algae suggestive of Chlamydomonas are preserved in Cenomanian (Late Cretaceous) amber from southern Germany (Schonborn et al., 1999). The algal cells are thick-walled, oval in lateral view, up to 10 pm long and display a single, cup-shaped chloroplast, which is characteristic of extant Chlamydomonas; flagella are not recognizable.
A rare microfossil that has been interpreted as a volvo-cacean alga is Eovolvox silesiensis from the Devonian of Poland (Kazmierczak, 1975, 1981). This fossil consists of hollow spherules with a surface layer composed of closely spaced, ovoid, pyriform, or spindle-shaped isomorphic cells. It is similar in basic structure to Symphysosphaera radialis from the Lower Cambrian of China (Yin, 1992). Affinities with the Volvocaceae have also been suggested for a few other Paleozoic and Mesozoic microfossils (reviewed in Kazmierczak, 1981). Few of these records, however, can be regarded as unequivocal (Kirk, 1998). The most biologically interesting fossil with possible affinities to the Volvocales is the endophyte Lageniastrum macrosporae (Lageniastraceae) from the Lower Carboniferous (Visean) of France (Renault, 1896a; Krings et al., 2005a). This alga occurs inside lycopsid megaspores in the form of dome-shaped, three-dimensional colonies composed of up to 500 lens- to pear-shaped cells arranged in a single layer and bounded by a transparent membrane. Lageniastrum macrosporae colonies display a striking similarity in organization to certain extant species of Volvox , including the presence of radiating protoplasmic strands that interconnect adjacent cells in the colony (FIG. 4.5).
Several groups of chlorophycean green microalgae have been important geologically. Some are responsible for the formation of certain coals and also may have contributed to the formation of petroleum in oil shales (Wolf and Cox, 1981). These deposits contain irregularly shaped, yellow bodies that were formed by the hydrocarbon-producing alga Botryococcus braunii. This taxon, the only member in the family Botryococcaceae, is a living planktonic colonial green alga in the order Tetrasporales that is known from both temperate and tropical climates throughout the world. The fossil colonies consist of pear-shaped cells arranged in radial rows and surrounded by a mucilage layer which has been described as a cuticularized layer. In the fossils, the yellow bodies are thought to represent paraffins and fatty acids secreted by the cells and bound together by the mucilage-like
substance of the sheath. Botryococcus colonies are known from every geologic period beginning in the Ordovician (Martin-Closas, 2003) and extending to the present day, and they can be the dominant plankton of certain freshwater ecosystems (Batten and Grenfell, 1996), for example, in the Rotliegend (Late Pennsylvanian-Early Permian) of the Saar-Nahe Basin in Germany (Clausing, 1999). Fossil Botryococcus has been shown to represent valuable proxy indicator for certain paleoenvironments (Guy-Olsen, 1998). Modern Botryococcus colonies live in standing bodies of both fresh and brackish water (Colbath and Grenfell, 1995). The alga has recently attracted attention in biotechnology since it represents an unusually rich, renewable source of hydrocarbons and other chemicals (Banerjee et al., 2002).
Chlorophycean green algae belonging to the order Chlorococcales have been reported from the Messel Oil Shale (middle Eocene) of Germany (Goth et al., 1988). These kerogen-rich sediments have a very high percentage of organic matter made up of unicellular algae that are morphologically identical to the modern species Tetraedron minimum (Chlorococcaceae). The rectangular unicells range from 5 to 20 pm long. Geochemical analyses of these cells indicate the presence of an insoluble, non-hydrolyzable, highly aliphatic biopolymer that is believed to represent a significant precursor in the formation of n-alkanes in crude oils. Laminar concentrations of fossil T. minimum have been described from lower Miocene lacustrine sediments near Hausen in the Rhön Mountains, central Germany (Goth and Schiller, 1994).
The genus Pediastrum (Hydrodictyaceae) consists of disk-shaped coenobia or colonies composed of a variable number of cells (Gray, 1960). Cells are arranged in a concentric pattern, with each cell of the outer ring containing one to three spines. The geological range of Pediastrum remains uncertain. Stanevich et al. (2007) suggested that the Neoproterozoic acritarch Dictyotidium minor from the Chencha Formation of eastern Siberia is structurally similar to the extant P. borya-num, and thus may represent an early relative of that genus. Fossils that appear similar to Pediastrum have also been found in Silurian (Deflandrastrum; Combaz, 1962) and Triassic rocks (Plaesiodictyon (FIG. 4.6); Brenner and Foster, 1994; Wood and Benson, 2000). The first unequivocal representatives of Pediastrum come from the Early Cretaceous of Britain and North America (Batten, 1996); others have been reported from the Upper Cretaceous-Neogene of southern South America (reviewed in Zamaloa and Tell, 2005). The genus is also known from the Miocene of Oregon and the Eocene of southern Sumatra. The Miocene Pediastrum fossils from southern South America are 8- to 32-celled coenobia
with the marginal cells each bearing two spines (Tell and Zamaloa, 2004). Today the genus lives exclusively in freshwater, and its presence in Cretaceous marine rocks suggests a difference in the physiologic tolerance of the taxon, or that the genus is not as good a proxy record for freshwater environments as has been suggested (Evitt, 1963a).
Another extant chlorococcalean green alga that is also known from the fossil record is Scenedesmus. It consists of cylindrical cells with rounded or pointed ends that are joined laterally into 4- to 16-celled coenobia. Two fossil species ( S. hanleyi and S. tschudyi) have been reported from the Upper Cretaceous and lower Paleocene of Colorado and Mexico (Fleming, 1989). Coenobia of S. tschudyi consist of four or eight cells, with the terminal cells possessing elongate extensions, whereas coenobia of S. hanleyi have four oval cells. Both Pediastrum and Scenedesmus are green algae almost exclusively restricted to freshwater habitats and important non-marine paleoecological indicators of the presence of lacustrine environments (but see above).
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