Silurian Period—443.8 to 419.2 MYA

fossil coral
Silurian age fossil corals, Great Basin National Park, Nevada.

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The Silurian was named for a Welsh tribe, the Silures, which lived in the area where Roderick Impey Murchison first described rocks of this age. Murchison studied the complex geology of western Wales in the 1830s and carefully documented the abundant fossils present in the Silurian strata.

Significant Silurian events

During the Silurian Period Earth underwent considerable changes, which had important repercussions for the environment and life. In particular, Earth’s atmosphere developed an effective ozone layer concentration. Generally speaking, the climate stabilized, ending the previous pattern of climatic fluctuations that periodically caused continental glaciations; glacial melting contributed to a substantial rise in global sea level.

Possibly the most remarkable biological event during the Silurian was the evolution and diversification of fish. Not only does this time period mark the wide and rapid spread of jawless fish, but also the appearances of both the first known freshwater fish and the first fish with jaws.

Learn more about events in the Silurian Period

By the end of the Silurian Period, a new group of fish had appeared both in freshwater and oceans—the so-called jawed fish. These fish had scales and abundant sharp, spiny fins. Their jaws indicate that they must have been effective predators. The development of jaws was an important step in the evolution of fish and indeed for all vertebrates. It is also an interesting example of an often-encountered feature of evolution: alteration of an existing structure (body part) for a new function. Most paleontologists believe that the jaws of fish evolved from the cartilaginous gill supports that existed in the heads of jawless fish. They were in the right place anatomically, and with a minimum of change could function as simple jaws.
The ozone layer was a “hot topic” in the media in the 1990s and is an interesting aspect of the movement of life onto land. The importance of the upper atmospheric ozone layer for life on Earth is that ozone molecules (O_3) absorb short-wavelength (ultraviolet) radiation from the sun. Without this O_3 shield, most life on land would be severely affected, if not killed outright, by intense ultraviolet radiation. Life in the oceans is much less sensitive because even a relatively thin covering of water provides an effective shield, blocking the damaging radiation (Macdougall 1996).

During the Silurian Period, the atmosphere came to contain the optimum concentration of oxygen for creating ozone. The maximum ozone production in the upper atmosphere occurs when the oxygen content is approximately 10% of today’s level. At this concentration of atmospheric oxygen, the ozone layer provides the most effective protection against lethal ultraviolet radiation. This is approximately where it stood during the Silurian Period.
Most oxygen in the atmosphere exists as molecules with two atoms (O_2), but a fraction exists as ozone—a molecule with three atoms (O_3). Ozone is produced by a chemical reaction in the presence of sunlight. Close to Earth’s surface, in the troposphere, ozone is toxic. It is a dangerous constituent of smog, which is formed near ground level by the interaction of sunlight with pollutants, particularly nitrogen oxides. It can severely damage and even kill plants and is an irritant and health hazard to humans. At a distance, however, ozone is a lifesaver. Its concentration in the upper stratosphere forms a shield around Earth, preventing most of the incoming ultraviolet solar radiation from reaching the troposphere and Earth’s surface.

In 1985 scientists taking regular measurements of ozone concentrations over Antarctica announced that the amount of ozone in the springtime atmosphere had fallen by 40% in the period 1977–1984. Other sources, including satellite data, confirmed these findings. In the Antarctic spring, a hole the size of the United States and about 6 miles (10 km) thick appears in the ozone layer.

Ozone is broken apart mainly by reaction with chlorine monoxide (Cl_O. One molecule of chlorine monoxide can destroy many thousands of molecules of ozone. Research on the breakdown of the ozone layer in the late 1980s and early 1990s, which concentrated on the role of human-manufactured pollutants, discovered that chlorofluorocarbons used in refrigerators, aerosol cans, and the manufacture of insulating plastic foam are responsible for the decline in concentrations of ozone. Chlorofluorocarbons were originally considered the ideal industrial chemical because they are extremely stable and nontoxic and do not break down in the troposphere. Because they are lightweight gases, however, they eventually rise into the stratosphere when they encounter intense ultraviolet energy and breakdown, releasing chlorine.

Source: Bradshaw and Weaver (1991).

Visit—Silurian Parks

Every park contains some slice of geologic time. Below, we highlight selected parks associated with the Silurian Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Paleozoic

Part of a series of articles titled Geologic Time Periods in the Paleozoic Era.

Chesapeake & Ohio Canal National Historical Park, Chickasaw National Recreation Area, Cumberland Gap National Historical Park, Death Valley National Park, Delaware Water Gap National Recreation Area, Katahdin Woods and Waters National Monument more »

Last updated: April 28, 2023