![Jurassic fossil zones and zonal fossils. These six ammonite species never overlap in …[Credits : Based on Arkell in James R. Beerbower, Search for the Past: An Introduction to Paleontology, …]](http://media-2.web.britannica.com/eb-media/88/788-004-20DFD806.gif)
Jurassic fossil zones and zonal fossils. These six ammonite species never overlap in stratigraphic distribution and therefore provide a guide for correlation wherever they occur. The ranges of hypothetical species (indicated by letters) within the Jurassic are also shown.
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