During the Paleozoic Era, there were multi-cellular organisms like trilobites, mollusks, jawless fish, seaweeds and finally, jawed fish, sharks, plants and early amphibians and reptiles. At the beginning, there was an increase in diversity of multi-celled animals, while the end of the era was marked by the largest mass extinction in history.
During the Cambrian period of the Paleozoic Era, bottom scavenger trilobites were dominant. Archaeocyathids were spongelike and lived in reefs. Mollusks, snails and simple algae also existed. The oldest known animal with a vertebra was the jawless fish.
In the Ordovician period, there were bottom-dwelling marine animals like brachiopods. During the Devonian period, sharks and bony fish evolved. The first vascular plants had no roots or leaves. Ferns also grew. The fir land animals were invertebrates, such as scorpions, millipedes and spiders. Amphibians evolved from lobe-finned fish. Cockroaches, dragonflies and reptiles also evolved.