The following material focuses on key fossil taxa present in the Trenton Limestone, their distribution or occurrence within the overall succession of Upper Ordovician strata, and their role in the establishment of the Upper Ordovician time scale.
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Fossil taxa used in biochronologic investigations rarely satisfy all aspects of the ideal index fossil.
That is, they often violate one or more of the following rules: 1), must have a widespread distribution (fossils tend to be limited to a small region or are found only in a particular depositional environment as opposed to globally); 2), must show rapid evolution (fossils change rapidly in preservable morphology so that distinctive identifiable species are easily recognized); 3), must be present in substantial numbers (so that fossils can be observed by the biostratigrapher); and 4), fossils should be robust mineralogically (so that depositional and diagenetic processes do not remove the fossils from the rock record).
Biostratigraphy is a sub-discipline of sedimentary geology that relies on the physical zonation of biota, both in time and space, in order to establish the relative stratigraphic position (i.e.
older, younger, same age) of sedimentary rocks between different geographic localities.
Both graptolites and conodonts are very useful for establishing the relative age of many Paleozoic rocks including those of the Trenton Group.
In fact, some of the first biostratigraphic studies of the Trenton Limestones and equivalent Utica Group black shales led to the recognition and establishment of the North American Time Scale for Upper Ordovician time, based partly on graptolite biozonation.If a given taxa is both wide-ranging and evolutionarily short-lived, and if it is robust enough to be preserved in the fossil record, then the taxa is often referred to as an index fossil.An index fossil identified in the rock record would constrain the age of the rock within which it is contained to a very specific interval of time when the organism lived.Although the basic rules of biostratigraphic zonation were established in the late 18th to early 19th centuries in Europe (ultimately resulting in the development of the Relative Geologic Time Scale), the implementation of biostratigraphic techniques was in use in the United States during the early to mid-1800's.Some of the first geological surveys to be completed in the United States included those of the New York State Geological Survey.The diagram to the below, modified from Holland (2003), represents a compendium of chronostratigraphic data for the Middle to Upper Ordovician of the eastern United States.