Predators and Prey in our Devonian Seas. It is here we see the first tetrapods — land-living vertebrates — appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids. In the oceans, brachiopods flourished. Crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common... and a mighty one-ton eating machine that ruled our ancient seas.