Index Fossils and Their Significance

Index Fossils

• Index fossils are widely distributed fossils with a limited time span.
• They are used to determine the age of rocks and establish relationships between rock layers.
• "Fossil" literally means anything dug from the earth.
• Fossils are remnants of plants or animals, or related items, providing evidence of past life.
• Fossil taxa are unique sets of fossils that characterize rock layers.
• William Smith, a British engineer, recognised the use of fossils to correlate rock layers across vast distances.
• Index fossils, or guide fossils, are restricted to specific geologic horizons.
• Some index fossils represent animal groups.

Animal Index Fossils

• Ammonites: Molluscs with coiled shells; Perisphinctes tiziani (Jurassic) and Trophites subbullatus (Triassic) are examples.
• Gastropods: Molluscs with asymmetrical, spirally-twisted shells; Nerinea trinodosa (Jurassic) is an example.
• Pelecypods: Bivalve shelled molluscs with a tongue-shaped foot; Venericardia planicosta (Tertiary) is an example.
• Brachiopods: Bilaterally symmetrical animals with bivalve shells; Billingsella corrugata (Cambrian) and Dictyoclostus americanus (Carboniferous) are examples.
• Crinoids: Marine invertebrates with cup-shaped bodies and feathery tentacles; Cactocrinus multibrachiatus (Carboniferous) is an example.
• Trilobites: Extinct marine arthropods with three lobes; Paradoxides pinus and Bathyurus extans are examples from the Cambrian and Ordovician periods, respectively.

Characteristics of Index Fossils

• An organism must have lived for only a short period of Earth's history.
• Numerous fossils of the organism must be found in various rock layers.
• The fossils must be found over a wide geographical area.
• The organism's characteristics must be unique.