Silurian organisms lived during the Silurian period, which lasted between 443.7 and 416 million years ago. The Silurian is the shortest geologic period aside from the Neogene, with a total length of just 27.7 million years. In contrast, the Cretaceous period lasted 80 million years. The Silurian period occurred immediately after the Ordovician and just prior to the Devonian. The beginning of the Silurian is defined by a series of extinction events, so severe that they wiped out 60% of all animals, the second largest mass extinction in history.
The most famous organisms of the Silurian are the numerous fishes that evolved during this time, some of which had distinctive shielded heads (placoderms and relatives), the eurypterids (sea scorpions), marine predators which extended up to 2 m (6.5 ft) in length, and the earliest well-developed terrestrial flora and fauna, including vascular plants and terrestrial arthropods such as millipedes, mites, harvestmen, springtails, and spiders.
The Silurian was the beginning of a warm period which continued to last for over a hundred million years. Continental glaciers slowly retreated and disappeared by the middle of the period, opening up vast warm, shallow seas for diverse fishes, echidnoderms (starfish, sea lillies, and relatives), nautiloids, trilobites, mollusks, brachiopods, eurypterids, and crustaceans. The marine fauna throughout the Silurian was more in a period of diversification than any fundamental innovations.
Much of the evolutionary action of the Silurian, from our casual perspective, took place on the land. Although simple inch-tall, non-vascular plants similar to liverworts and mosses had existed during the Ordovician, the first true vascular plants appeared during the second half of the Silurian, setting the stage for the first extensive land colonization by plants. Vascular plants contain specialized tissues for moving around water and nutrients, allowing plants to grow much taller than they could otherwise. The first vascular plants, such as Cooksonia, were only a few inches tall, but they formed the basis for miniature ecosystems which included the aforementioned menagerie of early terrestrial arthropods. The earliest known fossil of a terrestrial animal is that of a millipede dated to 428 million years ago.
Michael is a longtime WiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.