Recent paleontological work tends to show that the development of armor was an ancient and perhaps universal development in early vertebrate stocks. Armor was presumably developed in relation to defense; and eurypterids were the only potential enemies for the early fresh-water dwelling vertebrates. The two groups are often found together; no food supplies for eurypterids other than fishes are discoverable; the decline of the eurypterids is synchronous with the development among fishes of greater speed, larger size, and marine migration, all factors which would remove them from the range of the eurypterids. Besides armor, other vertebrate features, such as the development of fast swimming, may have been influenced by this eurypterid "menace.".