The Ordovician-Silurian period refers to a significant geological time frame that spans from approximately 485 to 419 million years ago. It is divided into two main epochs: the Ordovician, which is known for a diverse marine life, and the Silurian, which saw the first appearance of terrestrial plants and early land animals. This period is marked by major changes in sea levels and climate. During the Ordovician, the Earth was largely covered by shallow seas, hosting a variety of organisms such as trilobites, brachiopods, and graptolites. The Silurian period followed, characterized by the stabilization of the climate and the evolution of more complex ecosystems, including the emergence of vascular plants and early arthropods on land.