The Silurian was the third geological period of the Paleozoic era, extending from approximately 443.8 million to 419.2 million years ago. It is divided into four epochs: the Pridoli, the Ludlow, the Wenlock, and the Llandovery (Figure 1). The Silurian is named after the Silures, a Welsh border tribe, by Roderick Murchison (British geologist) in 1839.
Following the Ordovician-Silurian extinction event (Figure 2) and the major glaciations at the end of the Ordovician, biodiversity made a rapid recovery and many new species developed. Fishes species, both fresh and saltwater, exploded in number(Figure 3). The first jawed fish appeared, marking an important evolutionary step for later vertebrates. Life on land is definitely established by this time, with early relatives of spiders and centipedes appearing. Vascular plants first clearly appeared in the Silurian, which are the predecessors of many of today's modern plants (Figure 4).
The Silurian was marked by a dynamic, constantly changing climate and strong fluctuations in sea level. The melting Ordovician glaciers led to sea level rising and many shallow seas developing. Shallow seas are more easily warmed than deep seas, encouraging the development of many species. Tectonic plate movement was not as drastic as in previous periods but mountain-building events and the establishment of more varied marine habitats were occurring.
Figure 2. Ordovician strata overlying darker Silurian strata.
Figure 3. Silurian fish artist rendition.
Figure 4. Cooksonia, an early vascular plant.