All too often books written for a popular audience include animals such as mammoths, mastodons, plesiosaurs, ichthyosaurs, and the sail-backed Dimetrodon.Geologists say it probably once belonged to a woolly mammoth or a mastodon.In lower latitudes, mastodons and elephants, giant deer and ox, beavers, dogs and cats, and other familiar species existed in the forests and grasslands.