GEOLOGY 11 FOSSILS AND THEIR GEOLOGIC RANGE Perhaps the most important value of fossils to geology is the means they provide for differentiating and recognizing units of rock deposited during particular increments of geologic time. Because life has evolved, and thus has changed through time, fossils serve as distinctive markers in a vertical succession of sedimentary rocks. Correlation of similar fossil types to determine time equivalence of rock units is called Biostratigraphic correlation or biostratigraphy. In order to use fossils for biostratigraphic purposes, geologists must know the total time span or geologic range during which a specific group of organisms lived. In this laboratory exercise you will construct a geologic range chart for some of the more significant fossil groups used in biostratigraphy. On the lab tables you will find trays of fossils that are grouped according to the period in which they lived. Each fossil is accompanied by a number that indicates its relative abundance and biostratigraphic importance during the interval. These numbers indicate whether a fossil is: 1) rare 2) common 3) abundant and diverse 4) a major index fossil for that period Use last weeks lab sketches and your fossil identification handout to identify all the fossils in each tray. Your final product will be a completed range chart for the major fossil groups and a diagram showing the realtive abundance of each group through time (see attached). If a fossil does not exist during a specific interval of time (it will not be present in some of the trays) leave the entry blank. QUESTIONS: 1) What fossil groups went extinct at the very end of the Paleozoic (not during but at the end )? 2) What trend in abundance do the Trilobites show through the Paleozoic? What could have caused this trend? Think about where they lived and their possible relationship to other organisms. 3) What is the principle difference between Paleozoic reefs and Mesozoic/Cenozoic reefs in the geologic record? What sort of coral whould be characteristic of each? 4) Give at least one example where a newly opened niche allowed the rapid radiation of another group of organisms. 5) Graptolites were planktonic animals, and they are commonly found in Ordovician shales, a time of extremely high sea level. Generally high sea level results in widespread areas of shollow-marine deposition. How might sea level, shale and the graptolites be related? 6) Consider the relative abundance of different organisms. Why is it usually easier to date shallow marine strata of Cretaceous age versus similar strata of Triassic age? 7) Considering predator-prey relationships, what factors could be responsible for the general decline of types of fossils and relative abundances of fossils from the Devonian onward?.