310 million years ago, gastropods, including drilling univalves, periwinkles, and snails, were present in the ancient seas, while bivalve gastropods like mussels, clams, oysters, and scallops have remained almost unchanged. [1] 290 million years ago, gastropods, brachiopods, sponges, and reef-building corals continued to increase, with warm seas bathing the polar coasts. [2]