The basic unit (so to speak) of biostratigraphy is the index fossil. An index fossil is defined as one which: had a wide geographic distribution during its lifetime, was readily preserved, is easily identified, and had a short vertical range (i.e. a fast evolving organism). It is difficult for organisms to fulfill all these requirements because organisms with widespread geographic distribution were typically not fast evoloving.

An example of specimens of a typical palynology study.
Source: Agashe, Shripad N., (1997) pp271

Good index fossils include graptolites, conodonts, and ammonites. The best index fossils include the foraminifera (calcareous microfossils-see previous page), nanofossils, spores, and diatoms. Microfossils are generally better index fossils primarily because of their abundance, for example one cubed centimeter of sediment may contain upto 2000 specimens. This multitude of specimens improves the resolution of your study and allows you to state your conclusions with more confidence.