Index Fossils PDF

## Index Fossils ### Key Concepts Index fossils are: - Commonly found - Widely distributed - Limited in time span. They are used for: - Determining the age of organic rocks and other fossil assemblages - Establishing relationships between rock units. The word "fossil" literally means anything d...

## Index Fossils ### Key Concepts Index fossils are: - Commonly found - Widely distributed - Limited in time span. They are used for: - Determining the age of organic rocks and other fossil assemblages - Establishing relationships between rock units. The word "fossil" literally means anything dug out of the earth. Fossils are: - Vestiges of plants or animals preserved in strata of earth - Evidence of their presence in the geologic past. In a strict sense, fossils include: - Remains of organisms or their parts - Anything connected with an organism proving its existence (trace fossil). ### History and Usage In 1796, William Smith (a British civil engineer) first noted that rock was characterized by unique sets of fossil taxa. By noting the presence of fossils, it became possible to: - Correlate rock units of varying lithologies across vast distances - Establish time horizons in lithologically uniform diachronous rock units. Certain floral and faunal remains are often found to be restricted to a particular geologic horizon. Such fossils are known as: - **Index fossils** - **Guide fossils.** ### Index Fossils An index fossil is: - An abundant and easily identifiable fossil - Has a wide geographic distribution - Has a short geologic range. **Geologic range** refers to the total length of geologic time that an organism was, or has been, present on earth. ### Animal Index Fossils - **Ammonites:** - Group of fossils molluscs possessing exogastrically coiled shells. - _Perisphinctes tiziani_ is a Jurassic index fossil. - _Trophites subbullatus_ is a Triassic index fossil. - **Gastropods:** - Another group of molluscs - Have asymmetrical bodies with usually dextral, spirally twisted shells. - _Nerinea trinodosa_ is a Jurassic index fossil. - **Pelecypods:** - Molluscs enclosed with bivalved shells and a ventrally located tongue-shaped foot. - _Venericardia planicosta_ is a Tertiary index fossil. - **Brachiopods:** - Bilaterally symmetrical coelomate animals with bivalved shells. - _Billingsella corrugata_ is a Cambrian index fossil. - _Dictyoclostus americanus_ is a Carboniferous (Pennsylvanian) fossil brachiopod. - **Crinoid:** - Marine invertebrate - Usually possess a cup-shaped body and five or more feathery tentacles - _Cactocrinus multibrachiatus_ of Carboniferous (Mississippian) is a good example of index fossil. - **Trilobites:** - Extinct marine arthropods - Have more or less oval bodies divided into three lobes by two longitudinal furrows - _Paradoxides pinus_ is a Cambrian index fossil. - _Bathyurus extans_ is an Ordovician index fossil. ### Characteristics of Index Fossils 1. An organism must have lived only during a short part of Earth's history. 2. Many fossils of the organisms must be found in rock layers. 3. The fossil must be found over a wide area of Earth. 4. The organism must be unique.