In the paleontology of Old Earth, approximately 444 to 416 million years ago; the geologic period of the Paleozoic Era between the end of the Ordovician and the beginning of the Devonian. More generally and informally, any gardenworld or hab space that seems similar in the array of plant and animal species might be referred to as 'Silurian'.
During the Silurian the planet was in a greenhouse stage; relatively warm, with extensive epicontinental seas. Climates were relatively stable, though the weather itself included some very strong storms. The atmosphere contained less than three quarters of the oxygen than in the modern age but the carbon dioxide levels were 16 times higher. The primary continents were Euramerica, at the equator, consisting of what would later be Europe and North America, a small northern landmass consisting of the future Siberia, and Gondwana, consisting of most of the other later land masses, in the southern hemisphere. A large ocean dominated the northern hemisphere. The first bony fish with movable jaws appeared, and there were many varieties of eurypterids (sea scorpions), some of them several metres long. Trilobites, brachiopods, and molluscs were abundant. On land, there were no plants with deep roots, but there were 'forests' of mosses and simple vascular plants, especially along bodies of water, and there were spider-like arachnids and myriapods as well as many smaller arthropods on land (though as yet no insects).
There are a number of extant replicas of portions of Silurian landscapes scattered across the Terragen Sphere, filled with lazurogened species of varying degrees of authenticity, but there are no known full planetary models. Lazurogens from this period are not as common as those from later times, in part because the Silurian does not sport as many spectacular species and in part because of the relative difficulty of creating authentic replicas from the available data.