Clou Din A

the presence of Precambrian shelly fossils is, of course, dependent on where one places the boundary between the Precambrian and Cambrian in the global sedimentary record. Most geologists and paleontologists agree that the Ediacaran fauna should be considered Precambrian and that "typical" Cambrian fossils such as trilobites and brachiopods should be considered Cambrian. Using the presence of Ediacaran fossils as indicating Precambrian, there is only one genus of shelly fossils that seems to be unequivocally Precambrian in age. This is Cloudina (figure 4.2), named for Preston Cloud (Germs 1972). Cloudina was first described from the Nama Group of Namibia, the same sedimentary sequence that produced the first finds of Ediacaran soft-bodied fossils just after the turn of the century. Clou-dina may prove useful for identifying Precambrian rocks in other regions, as this fossil is distinctive and appears to occur in widely separated regions of the southern continents that were once part of the supercontinent Gondwana (to be discussed further in chapter 6).

FIGURE 4.2. The earliest known calcium carbonate shelly fossil, Cloudina. It is from the Precambrian Nama Group of Namibia. Length of specimen 2 cm.

Zaine and Fairchild (1987) have recently illustrated a specimen of Cloudina from Brazil that shows the distinctive "cone-in-cone" wall structure of this creature's shell.

Cloudina is a tubular fossil up to a few centimeters in length. Its shell was originally calcium carbonate in composition. Calcium carbonate shell comes in two varieties or mineral types, calcite and aragonite. Calcite is a geologically common mineral and is the major constituent of limestone. Shelly organisms commonly secrete calcite to construct shells. Aragonite, chemically the same as calcite, differs radically in its crystal structure, and is less common than calcite in ancient sediments, primarily because it is less chemically stable. Buried aragonite, when subjected to the changes in acidity and pressure that occur when rocks become cemented, may recrys-tallize as calcite or dissolve altogether and return to water as carbonate ions. Aragonite is also commonly used by marine animals to fabricate shells. Some mollusks, such as the edible mussel [Mytilus], form a shell that has alternate layers of calcite and aragonite.

Cloudina fossils are found in platy limestone beds within the Nama Group, sandwiched between quartzite (a tightly cemented variety of sandstone) beds that have the impressions of soft-bodied fossils such as Pteridinium. Cloudina tubes are often preserved bent or kinked, suggesting that in life they may have been slightly flexible. Lowenstam (1980) feels that they were only partly mineralized, and that they were not rigid, unflexing tubular skeletons like some of the Cambrian shelly fossils that were to follow. Cloudina can be seen easily with the naked eye (some tubes are four or more centimeters long). The best way to study the structure of fossils like Cloudina is with thin sections, that is, with thin slices of fossilifer-ous rock ground flat on a lapidary wheel and mounted to glass slides so that light passes through. Thin sections allow study of shell microstructure with the aid of a transmitted light microscope. Shell structure in Cloudina consists of tube wall layers arranged in cone-in-cone fashion, like stacks of paper cups with their bottoms torn out. This wall construction led Glaessner (1976) to conclude that Cloudina is the fossil of a tube-dwelling annelid worm.

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