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17 Feb

On land, a relatively smaller extinction of diapsids and synapsids cleared the way for other forms to dominate, and led to what has been called the "Age of Dinosaurs." Also, the great forests of fern-like plants shifted to gymnosperms, plants with their offspring enclosed within seeds.

Modern conifers, the most familiar gymnosperms of today, first appear in the fossil record of the Permian.

The current stratigraphy divides the Permian into three series or epochs: the Cisuralian (299 to 270.6 mya), Guadalupian (270.6 to 260.4 mya), and Lopingian (260.4 to 251 mya).* Find out more about how these periods of time are defined.

Permian shales, sandstones, siltstones, limestones, sands, marls, and dolostones were deposited as a result of sea-level fluctuations.

These fluctuation cycles can be seen in the rock layers.

Relatively few sites lend themselves to direct radioactive dating, so the age of intermediate strata is often estimated.

There are indications that the climate of the Earth shifted during the Permian, with decreasing glaciation as the interiors of continents became drier.

Until the later 1990s, there was little consensus on the order of strata in the late Permian.

Many of the continents of today in somewhat intact form met in Pangea (only Asia was broken up at the time), which stretched from the northern to the southern pole.

The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.

For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.

By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.

From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.