Paleozoic Era is classified into 6 periods and the creatures existing in that period are listed as:

1. Cambrian Period: During the Cambrian Period, multicellular life experienced its greatest adaptive radiation in what is called the Cambrian Explosion. Many species were fossilized during this period because many organisms evolved hard body parts, including shells and outer skeletons. Landmasses moved in ways that created vast shallow marine habitats. Jawless fishes first appeared. The Cambrian ended with a large mass extinction in which nearly 30 percent of all animal groups died. Example: Elrathia
2. Ordovician Period: Oceans flooded large land areas, creating more shallow marine habitats. Animal groups that survived the Cambrian extinction experienced dramatic adaptive radiations. These radiations generated great diversity in major animal phyla. Invertebrates dominated the seas. Early vertebrates evolved bony coverings. Example: Pleurocysities
3. Silurian Period: During the Silurian Period, land areas rose, draining shallow seas and creating moist tropical habitats. Jawless fishes underwent an extensive adiation, and the first fish with true jaws appeared. The first multicellular land plants evolved from aquatic ancestors. Arthropods became the first animals to live on land. Examples: Sea Lily Fossil and Cephalaspis
4. Devonian Period: During the Devonian Period, invertebrates and vertebrates thrived in the seas. Fishes evolved jaws, bony skeletons, and scales. Sharks began their adaptive radiation. Certain groups of fishes evolved leglike fins, and some of these evolved into the first amphibians. Some land plants, such as ferns, adapted to drier areas. Insects began to radiate on land.
5. Carboniferous Period: During the Carboniferous Period, mountain building created a wide range of habitats, from swampy lowlands to drier upland areas. Giant ferns, club mosses, and horsetails formed vast swampy forests. Amphibians, insects, and land plants experienced major adaptive radiations. Winged insects evolved into many forms, including huge dragonflies and cockroaches. For early vertebrates, insects were food; for plants, insects were predators. The first reptiles evolved from ancient amphibians.
6. Permian Period: During the Permian Period, invertebrates, vertebrates, and land plants continued to expand over Earth’s continents. Reptiles experienced the first of several major adaptive radiations, which produced the ancestors of modern reptiles, dinosaurs, and mammals. The Permian Period ended with the biggest mass extinction of all time. More than 50 percent of terrestrial animal families and more than 95 percent of marine species became extinct. Example: Crinoid