Jurassic Period

When was it?
We currently estimate that the Jurassic Period began 201.3 Million Years ago, and ended 145 Million Years Ago.

How do we "know" that's when it was?
We interpret the age and order of rocks and fossils in Earth history by using two very different methods:


 * We look at rocks and fossils to interpret what age of the Geologic Time Scale during which they probably formed. This is commonly called "Relative Dating", and generates a "Relative Geologic Age".
 * These features are common in Jurassic rocks - you may find some while exploring Utah or other places!
 * Sea shell fossils:
 * ammonites with an extremely fancy ammonitic suture.
 * Plant Fossils:
 * pollen ...
 * We also measure the number of certain rare isotopes (atoms that form the same element, but have different numbers of neutrons) inside minerals of a rock, and this helps us estimate a number-of-years age for the rock. We call this "Chronometry" (time measure), or "Absolute Dating", which generates an "Absolute Age".
 * These isotope groups are useful for studying rocks of Jurassic age:
 * Argon-Argon
 * Uranium-Lead
 * Uranium-Lead

What were the continents like?
Pangea was still together at the start of the Jurassic Period, but it started to rift (break) apart at the beginning of the Jurassic. By the end of the Jurassic Period, Pangea was officially split up into a bunch of different continents. These continents continued to move apart, until they look a lot more recognizable on a map of the Cretaceous world.

What kinds of ecosystems lives on Earth during the Jurassic Period?

 * At sea:
 * Creepy drifting ammonoids and giant water monsters
 * gentle meadows and hills of sea sponges
 * coral reefs
 * a few disasters
 * On land:
 * forests and sedge, but no grass or flowers!
 * Plenty of volcanic island arcs (think: Aleutian archipelago - the long tail running west along Alaska; or the islands that create the nations of Japan or Thailand)