Sponges and invertebrates dominated the seas during the Paleozoic Era. This era is said to have ran about 542 million to 251 million years ago. This was the time when the supercontinent broke and formed new continents. This era is most famous for the "Cambrian Explosion" where life exploded on Earth that started the evolution of arthropods and chordates.
Life mostly flourished in the seas during the Paleozoic Era. This was proven by invertebrate-fossil records found. The most famous was the armored arthropod known as the trilobite. This organism scuttled around the seafloor about 270 million years ago.