The dominant marine/aquatic animals during the Devonian included the placoderm fishes, and important marine invertebrates included trilobites, nautiloids and the first ammonit … es. The first land animals were arthropods, including scorpions, spider-like and mite-like arachnids, primitive insects, and myriapods. The Devonian Period, from the Paleozoic Era, was also known as the Age of the Fishes. 2  At th…. 3  Domain Archaea: a New Discovery Organisms from the domain archaea are microscopic organisms that were just discovered in the 1970s.