416 Million to 359 Million Years Ago
When the Devonian period dawned about 416 million years ago the planet was changing its appearance. The great supercontinent of Gondwana was headed steadily northward, away from the South Pole, and a second supercontinent began to form that straddled the Equator. Known as Euramerica, or Laurussia, it was created by the coming together of parts of North America, northern Europe, Russia, and Greenland.
Red-colored sediments, generated when North America collided with Europe, give the Devonian its name, as these distinguishing rocks were first studied in Devon, England.
The Devonian, part of the Paleozoic era, is otherwise known as the Age of Fishes, as it spawned a remarkable variety of fish. The most formidable of them were the armored placoderms, a group that first appeared during the Silurian with powerful jaws lined with bladelike plates that acted as teeth. Early placoderms fed on mollusks and other invertebrates, but later species developed into ferocious, fish-slicing monsters measuring up to 33 feet (10 meters) long. Other types of bone-plated fish that lacked jaws developed a range of bizarre forms. Fossil specimens include species with horseshoe-shaped heads and others that looked like rounded shields.
Shark Ancestors
Despite their heavy protection, these primitive fishes weren't built to last. The Devonian ancestors of fishes living today belonged to two main nonarmored groups. The cartilaginous fish, so-called because cartilage formed their skeletons, later gave rise to sharks and rays. They had small, rough scales, fixed fins, and sharp, replaceable teeth. The second group, the bony fish, were covered in scales and had maneuverable fins and gas-filled swim bladders for controlling their buoyancy. Most modern fishes are bony fish.
The bony fish included lobefins. Named after the thick, fleshy base to their fins, lobefins are credited with the giant evolutionary stride that led to the amphibians, making lobefins the ancestors of all four-limbed land vertebrates, including dinosaurs and mammals. The fossils of these remarkable animals come from the red rocks of Devon. Some lobefins are still around today, such as the famous "living fossil" fish, the coelacanth.
A recently discovered fossil creature from the Devonian has been hailed as a vital link between fish and the first vertebrates to walk on land. Found in the Canadian Arctic in 2004, Tiktaalik had a crocodile-like head and strong, bony fins that scientists think it used like legs to move in shallow waters or even on land. The fish showed other characteristics of terrestrial animals, including ribs, a neck, and nostrils on its snout for breathing air.
The first amphibians breathed through simple lungs and their skin. They may have spent most of their lives in the water, leaving it only to escape the attentions of predatory fish.
The first ammonoids also arose during the Devonian. Related to octopuses and squid, these marine animals survived until the end of the Cretaceous period, 65 million years ago.
Plant Proliferation
Plants began spreading beyond the wetlands during the Devonian, with new types developing that could survive on dry land. Toward the end of the Devonian the first forests arose as stemmed plants evolved strong, woody structures capable of supporting raised branches and leaves. Some Devonian trees are known to have grown 100 feet (30 meters) tall. By the end of the period the first ferns, horsetails, and seed plants had also appeared.
The new life burgeoning on land apparently escaped the worst effects of the mass extinction that ended the Devonian. The main victims were marine creatures, with up to 70 percent of species wiped out. Reef-building communities almost completely disappeared. Theories put forward to explain this extinction include global cooling due to the re-glaciation of Gondwana, or reduced atmospheric levels of the greenhouse gas carbon dioxide because of the foresting of the continents. A major asteroid impact has also been suggested.
