The Paleozoic Era saw changes to Earth's landscapes, life forms and atmosphere over the course of several million years. On land, at sea, below the surface and in the skies, change occurred throughout the six distinct periods comprising the Paleozoic Era. The early Paleozoic periods introduced many plant species, while later periods brought invertebrates, dinosaurs and birds.
The first and oldest Paleozoic period, called the Cambrian period, existed between 542 and 488 million years ago. This period introduced aquatic plant species and small, uni-cellular organisms including fungi and bacteria. These organisms lived primarily in marine environments. Over 35 types of animals emerged during the Cambrian period, appearing in basic forms. These organisms evolved further during the successive Ordovician period. The Ordovician era featured expansion in biodiversity and complexity of life forms. During the middle Paleozoic periods, organisms appeared on land in addition to water. Leafless vascular plants, insects, arthropods and other invertebrates emerged during the Silurian and early Devonian periods; later in the Devonian era, amphibians and reptiles lived on land and at sea. In addition to flora and fauna, Earth's lands changed during the Paleozoic Era too. Shallow seas covered large surface areas of existing continents. Landmasses featured rocky surfaces, made primarily of sedimentary rocks. Mountain ranges appeared during this time, forming from incessant action of tectonic plates.