Silurian organisms lived between 443.7 and 416 million years ago, during the Silurian period. Apart from the Neogene, the Silurian is the shortest geologic period, lasting only 27.7 million years. The Cretaceous period, on the other hand, lasted 80 million years. The Silurian period began shortly after the Ordovician and ended just before the Devonian. The beginning of the Silurian is marked by a series of mass extinction events that wiped out 60% of all animals, making it the second largest mass extinction in history.
The Silurian is best known for its numerous fishes, some of which had distinctive shielded heads (placoderms and relatives), eurypterids (sea scorpions), marine predators that grew up to 2 m (6.5 ft) in length, and the earliest well-developed terrestrial flora and fauna, which included vascular plants and terrestrial arthropods such as millipedes, mites, harvestmen, springtails,
The Silurian was the start of a long period of warm weather that lasted over a hundred million years. By the middle of the period, continental glaciers had retreated and vanished, revealing vast warm, shallow seas teeming with fish, echidnoderms (starfish, sea lillies, and relatives), nautiloids, trilobites, mollusks, brachiopods, eurypterids, and crustaceans. Rather than any fundamental innovations, the Silurian saw a period of diversification in the marine fauna.
From our vantage point, much of the Silurian’s evolutionary action took place on land. Although simple inch-tall non-vascular plants like liverworts and mosses existed during the Ordovician, the first true vascular plants appeared during the Silurian’s second half, paving the way for the first widespread plant colonization of land. Vascular plants have specialized tissues that allow them to move water and nutrients around, allowing them to grow much taller than they could otherwise. Even though the earliest vascular plants, such as Cooksonia, were only a few inches tall, they provided the foundation for miniature ecosystems that included the aforementioned menagerie of early terrestrial arthropods. A millipede fossil from 428 million years ago is the oldest known terrestrial animal fossil.