Paleogeography: From the Late Silurian through Early Devonian, continental positions changed little. But by the Middle-Late Devonian, continental movement increased, resulting in numerous mountain-building events. By 390 million years ago, North America and Europe collided to form a large continent, called Euramerica, which sat near the equator. The supercontinent Gondwana and the newly joined Euramerica were surrounded by subduction zones on all sides. Most of the continental landmasses were bunched up, and a vast ocean covered the rest of the planet. North American Paleogeography: Most of North America was covered by a shallow sea throughout the Devonian, although some small landmasses were exposed around the continent. On its southern margin (now the East Coast), the collision with Avalonia begun in the Late Silurian continued into the Devonian and produced an extensive chain of mountains. The rivers that drained these mountains deposited abundant sand and mud over parts of present-day Canada and the Northeast from Maine to Tennessee. Paleontology: The warm, tropical seas were teeming with brachiopods, trilobites, crinoids, ammonites, and tabulate and rugose corals. The Devonian is often referred to as “the Age of Fishes.” Many new kinds of fish appeared. Placoderms, armored fish with bony plates in their mouths instead of teeth, reached their greatest diversity. Armored jawless fishes became abundant, as did early Chondrichthyes (sharks and rays). The first ray-finned and lobe-finned fishes also appeared. Many of these taxa died out around the end of the Devonian Period for reasons that are still not well understood. Two major animal groups colonized the land during the Devonian. The first tetrapods (four-limbed, land-living vertebrates) appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids.