Why is devonian age of fishes

During our visit to the Earth of million years ago, we must forget all that is familiar to us and put ourselves in the place of a space traveler who is arriving at a new planet for the first time Could this really be the Earth, this planet with unrecognizable continents and a northern hemisphere covered by an unending ocean?

The six continents and two large oceans we know so well do not even exist yet. And the climate seems hotter than usual; in fact, there is no ice at the North Pole! Forests are few and scattered in the tropical regions, and do not contain many different types of trees. This caused the formation of the Appalachian Mountain Range. The other large land mass was Gondwana. These two large land masses lay close to one another near the equator. The two continents were moving toward each other throughout the Devonian Period.

The waterway between the two continents covered a subduction zone. This is an area where one plate is moving underneath the other. Eventually this would mean that the two continents would collide to form the supercontinent Pangea in the Permian Period.

That event is more than 64 million years later. Laying so close to the equator meant that the climate of the Devonian was warm. The warm temperatures made life on land particularly good for the plants. They developed vascular tissues to carry water and food through roots and leaves. The most important development was the seed. Now plants were not dependent on the presence of water for reproduction and they could move further inland.

Ferns and the first trees began to cover the land. Even a primitive vertebrate, the tetrapod or four-footed vertebrate, developed the ability to live outside the water and move on land. The Devonian is known as the Age of Fishes. It is famous for the thousands of species of fish that developed in Devonian seas.

We know this because of the fish fossils found in Devonian rocks. When fish first started to develop, they had no jaws and the support structure was made of cartilage. These fish were called Ostracoderms.

Fish with Jaws The next development was the fish with jaws, gills and paired fins. The Placoderms were the first fish to have all three of these characteristics. The largest of the Placoderms was the Dunkleosteus. It was a huge predator in the Devonian seas. It could be as long as 10 meters. Instead of teeth, it had large boney plates that stuck down in the front of its mouth opening. The powerful jaws were deadly to other fish, sharks and even other Dunkleosteus.

Ancient Sharks Sharks, or Chondricthyes , developed during the Devonian also. Sharks are thought to be descendents of the large Placoderms, but they lost the ability to form the boney armor on the outside of the body and were unable to form bones on the inside also. Their body is supported by cartilage. Because of the skeletons of cartilage, very little fossil evidence is available.

They did leave behind their 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 fossil creature from the Devonian discovered more recently has been hailed as a vital link between fish and the first vertebrates to walk on land.

Found in the Canadian Arctic in , 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. 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 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. All rights reserved.