Organizational Language, Activity 8.2 2. Read the sentences, paying special attention to the underlined organizational language. Then check ( √ ) the correct summary. a) Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the odds against preservation very high. However, the chances of escaping complete destruction are vastly improved if the organism happens to have a mineralized skeleton and dies in a place where it can be quickly buried by sediment. Some circumstances greatly increase the chances that a fossil will form . Many fossils are attacked by different forces, but a few of them are not destroyed .
b. The empty shell of a snail or clam may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time . Indeed, unaltered shells of marine invertebrates are known from deposits over 100 million years old. In many marine creatures, however, the skeleton is composed of a mineral variety of calcium carbonate called aragonite . Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively unstable, and in time changes to the more stable calcite . Shells made from aragonite can last for millions of years . Some shells do not change for long periods of time, but shells made from aragonite change to another mineral . c. Many other processes may alter the shell of a clam or snail and enhance its chances for preservation . Water containing dissolved silica, calcium carbonate, or iron may circulate through the enclosing sediment and be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vessels and nerves. In such cases, the original composition of the bone or shell remains, but the fossil is made harder and more durable. Fossils are harder and stronger when water adds minerals inside the shell’s structure . Some clam and snail shells remain even after they are altered by many different processes.
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