Geologic Time Geologic time scale spans 4.5 billion years. We can focus on the last ~500 million years due to the study of fossils. Fossils are the remains or traces of prehistoric life. Fossils have been used to divide geologic time into eons, eras, periods, and epochs.

Fossils What is a fossil? Fossils are the remains or traces of organisms found in sedimentary rocks. What conditions are necessary for fossils to form? Rapid Burial If the organism is quickly buried by fine-grained sediment it is protected from being eaten by scavengers or decomposed by bacteria (i.e. little to no oxygen). Presence of Hard Body Parts Fossils of organisms that contained hard parts are abundant in the fossil record, but only rare traces of soft tissue organisms are seen as fossils. Soft-bodied organisms could get buried by volcanic ash. Low Oxygen Environment In a low oxygen environment, there is less bacteria and as a result, decomposition is slower.

Fossils and their Importance in Geology: What is the importance of fossils to geologist? What does a fossil indicate? Fossils indicate the age of sedimentary rocks. Approximate age of the rock can be determined if we know when a life form existed on Earth. E.g. Olenellus Trilobites indicate Cambrian Period of Paleozoic Era (540-490 Ma). Fossils indicate the environments in which rocks formed. For example, fossils of coral indicate a warm tropical environment. 3. Fossils are used to correlate (match up) rocks. FOSSIL CORRELATION!

Fossils and their Importance in Geology: What is the importance of fossils to geologist? what does a fossil indicate? Fossils provide the basis by which the subdivisions of the Geologic Time Scale are made. Division of the Geologic Time Scale is marked by some significant event in the evolution of Earth. Examples: - The extinction of trilobites marked the end of the Paleozoic Era (248 Ma). - The extinction of dinosaurs marked the end of the Mesozoic Era (65 Ma). Fossils can also indicate evolutionary pathways. Fossil evidence show the progression (evolution) of life forms with time. Invertebrates → Fish → Land Plants → amphibians → reptiles → Mammals → Birds → Humans

Methods of Fossilization Fossils are preserved in the rock record in several ways! 1) Petrification (or Petrifaction) By Replacement 2) Carbonization 3) Mold and Cast Preservation (Intact) Ice, Mummification, and Amber Traces (Indirect Evidence) Tracks, Burrows/Tunnels, Eggs, gastrolites (stomach stones), and Coprolites (feces).

For The Exam You are expected to know how they form Not their characteristics

Petrification Occurs when the small internal cavities and pores of the original structure are filled with precipitated mineral matter. Formation: cell walls and solid material are removed It is replaced by mineral material carried by ground water. The process is called replacement. Sometimes internal details and structures are retained.

Carbonization Formation: Fine sediment encloses delicate matter such as leaves (e.g. ferns) in a oxygen-poor environment. As time passes, pressure squeezes out the liquid and gaseous components of the organism leaving behind a thin residue of carbon.

Mold and Cast Often preserve a replica of a plant or animal in sedimentary rocks. Formation: An organism is buried in sediment and then dissolved by groundwater leaving a hollow depression or an impression called a mold. The mold shows only the original EXTERNAL SHAPE and SURFACE MARKINGS of the organism. It does not reveal the internal structure! When minerals or sediment fills the hollow depression or impression it forms a cast.

Preservation Original remains can be preserved in ice or in amber (hardened tree sap). Formation: Either ice or amber covers the organism quickly after dying – or alive It is protected from decay (oxygen-free environment) and from pressures that would cause it to be crushed. The entire organism has been preserved; even the soft parts, which usually decay and disappear. Examples: (1) Woolly Mammoths preserved in ice in Alaska and Siberia. (2) Insects preserved in tree sap (amber). Cane in Jurassic Park.

Trace Fossils Show traces left in the rock by an organism. Formation: Not the organism itself Imprints or wastes left by the organism Examples include: Tracks - animal footprints made in soft sediment. The sediment later turns into sedimentary rock. Burrows/Tunnels - Animal trails made in soft sediment. The sediment later turns into sedimentary rock. Coprolites - Fossil dung (feces). Gastrolites – Stomach stones.

Information From Trace Fossils A measurement of the depth and surface area of a footprint when coupled with the supposed soil characteristics in which it was made can give a reasonably accurate measure of the creature’s body mass.

Core Lab #2 “Estimating Dinosaur Size and Speed from Trackways”