DEVONIAN TIME PERIOD

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"The Age of Fishes"


CLIMATE AND CONTINENTS
During the Devonian Period, North America and Europe were formed together to make the supercontinent Euramerica which caused the formation of the Appalachian Mountains. South America, Africa, Antarctica, India and Australia formed Gondwana. Both large land masses were formed on the equator. The two continents moved toward each other throughout the Devonian Period. The waterway between the two continents covered a subduction zone. A subduction zone is an area where one plate is moving underneath the other. The two continents eventually formed the supercontinent Pangea. 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 the leaves. Ferns and the first trees began to cover the land when plants started to move further inland.

THE AGE OF FISHES
The Devonian Period is famous for the thousands of species of fish that developed in Devonian seas.They had no jaws and the support structure was made of cartilage. This material doesn’t fossilize well, so the earliest fossils were of fish whose outside skin was protected by scales and plates made of boney tissue. 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. There were many more interesting fish and other animals that developed duing this period.
MASS EXTINCTION ENDS THE DEVONIAN PERIOD
Species had begun to branch out and include both land and water habitats. The Devonian Period ended with a mass extinction. The Devonian extinction hurt the water habits much more than those on land. The sponges and corals were the most affected. No major reef building happened again for thousands of years.


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SILURIAN TIME PERIOD


443 million years ago


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CLIMATE AND CONTINENTS

The Silurian witnessed a relative stabilization of the earth's general climate which followed the harsh conditions the earth saw in earlier periods such as the Devonian. One change was the melting of the glaciers which led to the substantial rise of the sea levels.
The Silurian world consisted of a north polar ocean and a south polar super continent which was called Gondwana with a ring of about six continents. A large portion of the Rodinian landmass had separated and those fragments migrated toward the equatorial region. Most of these fragments were eventually assembled by a series of plate collisions into the super-continents of Laurussia and Laurasia.
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PLANTS AND ANIMALS
The Silurian Age saw plants and animals emerge on land. But first, there was regrouping from following the climax of Ordovician period. Fish also became diversifying and extending there feeding options further than vacuuming off the sea bed. Coral reefs made their first appearance during this time and marked the rapid evolution of jaw-less fish as well as the first freshwater fish.


STRATIGRAPHY
Silurian's stratigraphy is subdivided into four which is from oldest to youngest. It contains the Llandovery, the Wenlock, the Ludlow, and the Pridoli. Each of these "epochs" is distinguished from the others by the appearance of new species of graptolites. Graptolites are a group of extinct aquatic animals that put in their first appeared in the Cambrien period.

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SOURCES


http://www.palaeos.com/Paleozoic/Silurian/Silurian.htm
http://science.nationalgeographic.com/science/prehistoric-world/silurian/
http://science.nationalgeographic.com/science/prehistoric-world/devonian/
http://www.palaeos.com/Paleozoic/Devonian/Devonian.htm
http://www.fossils-facts-and-finds.com/devonian_period.html