They have a beautiful coiled shell, often strikingly ornamented with external ribbing. Individual ammonite species survived for relatively brief intervals, often a million years or less. Their rapid evolution , abundance , and widespread distribution makes them one of the most valuable index fossils , especially for Triassic, Jurassic, and Cretaceous periods of the Mesozoic . Most ammonites are preserved as internal molds and suture lines show up clearly. Microfossils A fossil too small to be studied without the aid of a microscope.
Microfossils are extremely important tools in biostratigraphy and paleoceanography .
When conditions are not suited for survival, dinoflagellates encase themselves in a protective armor of tough organic matter, forming what is called a cyst . It is this cyst that forms dinoflagellate fossils. Important primary producers. Important index fossil for Mesozoic to Recent . They are important primary producers in the modern oceans.