Devonian
The Devonian period, from 437 to 408 million years ago, was named for the English county where it was first identified. It has sometimes been called the Age of Fishes. Spectacular fish fossils abound in the massive Old Red Sandstone sediments that covered a large portion of Laurasia, the super-continent that would later split apart to form Europe, Greenland, and North America. These fossils indicate that a vast radiation (or divergence) in size and function was taking place among Devonian vertebrates. The jawless Agnathans had multiplied into many groups distributed around the world by the late Silurian (438 mya). Then, in the Devonian, came the fish, which developed jaws and were such successful competitors that the Agnathans were reduced almost to extinction, with only the lampreys and hagfish as their descendants.
Vast schools of eight-to-ten-inch spined fishes, the Acanthodians, swam in the mid-deep waters (beyond the continental shelf). Some were toothless, but many had razor-sharp teeth and devoured huge quantities of the bony fish, which also swam in great numbers in the clear warm seas. The bony fish included the ray fin, the lungfish, and the fleshy, lobe-finned ancestors of amphibians. Enormous placoderms, up to thirty feet in length, dominated the oceans with their armored bodies and tooth-lined, hinged jaws. Early sharks arose, possibly from placoderms, whom they would replace as the
| Era | Period | Epoch | Million Years Before Present |
| Paleozoic | Permian | | 286 |
| Pennsylvanian | | 320 |
| Missipian | | 360 |
| Devonian | | 408 |
| Silurian | | 438 |
| Ordovician | | 505 |
| Cambrian | | 570 |
reigning predators of the deep by the end of the Devonian period. And vast coral reefs, some hundreds of miles long, transformed the shallow waters into virtual metropolises swarming with marine life at all levels.
Even more exciting than the proliferation of sea fauna was the evolutionary step toward dry land. Fish began exploring up the brackish estu-aries into freshwater, followed by ravenous six-foot sea scorpions, the fearsome Eurypterids. The lobe-finned fish ventured into shallower and shallower water, eventually developing the rudimentary lungs that would allow them to breathe air. Next, their explorations on the muddy shores encouraged innovations in skeletons and fins that allowed them to support their weight in the stronger pull of gravity of the new environment. Gradually, the lower paired fins developed into the four limbs of amphibians. The most complete fossil of an early tetrapod (four-limbed) amphibian comes from the tropical swamps of Devonian Greenland. Ichthyostega was a lumbering, forty-inch carnivore, the ancestor of all existing land vertebrates.
Yet another major innovation occurred in the Devonian. As the earliest plants and invertebrates made their way onto land, they formed cooperative communities that make possible life as it exists today. Preserved in the Rhynie Cherts of Scotland are perfect slices of pondside life from the period. The minerals (silicon) in the water formed fossil images of the plants. These fossils show the first plants that grew and decomposed to form the first humus-rich soils on the Earth. Living amongst them were the earliest terrestrial arthropods : scorpions, mites, and spider-like arachnids. These tiny animals are responsible for breaking down organic material and releasing the nutrients back into the soil. Without this decomposition activity there could be no larger plants and therefore no land animals. This 400-million-year-old partnership between microscopic plants and animals is a fundamental feature of life as we know it.
Geologic Time Scale.
Bibliography
Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.
Friday, Adrian, and David S. Ingram, eds. The Cambridge Encyclopedia of Life Sciences. London: Cambridge University Press, 1985.
Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.
McLoughlan, John C. Synapsida: A New Look Into the Origin of Mammals. New York: Viking Press, 1980.
Steele, Rodney, and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New York: McGraw Hill, 1979.
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