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

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.

see also Geologic Time Scale.

Nancy Weaver

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.

Devonian period

In geologic time , the Devonian Period, the fourth period of the Paleozoic Era , covers the time roughly 410 million years ago (mya) until 360 mya.

The Devonian Period spans three epochs. The Early Devonian Epoch is the most ancient, followed in sequence by the Middle Devonian Epoch, and the Late Devonian Epoch.

The Early Devonian Epoch is divided chronologically (from the most ancient to the most recent) into the Gedinnian, Siegenian, and Emsian stages. The Middle Devonian Epoch is divided chronologically (from the most ancient to the most recent) into the Eifelian and Givetian stages. The Late Devonian Epoch is divided chronologically (from the most ancient to the most recent) into the Frasnian and Famennian stages.

In terms of paleogeography (the study of the evolution of the continents from supercontinents and the establishment of geologic features), the Devonian Period featured continued cleavage of supercontinent landmass and fusion of plates into the supercontinent Laurasia and eventually the supercontinent Pangaea.

Differentiated by fossil remains and continental movements, the Silurian Period preceded the Devonian Period. The Devonian is followed in geologic time by the Carboniferous Period (360 mya to 286 mya). In many modern geological texts, especially those in the United States, the time of Carboniferous Period covered by two alternate geologic periods, the Mississippian Period (360 mya to 325 mya) and the Pennsylvanian Period (325 mya to 286 mya). A mass extinction marks the end of the Devonian Period. In accord with a mass extinction, many fossils dated to the Devonian Period are not found in Carboniferous Period (i.e., alternatively, Mississippian Period and Pennsylvanian Period) formations.

The Devonian Period marked a geologically active period. The North American and European continents—with more tropical climates due to more equatorial positions—drifted together. As a result, the two continents share a similar fossil record for the Devonian Period. Similar fossil finds dating to the Devonian Period are found in Germany, Canada, and the United States.

The fossil record indicates that it was during the Devonian Period (also termed the "Age of Fishes" because of the appearance of sharks and bony fishes) that amphibians and more terrestrial (land based) vertebrates evolved. Seed plants also appeared, continuing a diversification and development of botanical species, especially vascular plants. By the end of the Devonian Period, the first forests appeared.

There were a number of major impacts from large meteorites that date to the Devonian Period. Similar to the K-T event , many scientists argue that these impacts could have provided the environmental stresses that eliminated approximately 25% of Devonian Period species. Impact craters dating to the Devonian Period have been identified in modern China, Canada, Russia, and Sweden.

See also Archean; Cambrian Period; Cenozoic Era; Cretaceous Period; Dating methods; Eocene Epoch; Evolution, evidence of; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Tertiary Period; Triassic Period

Devonian The fourth of the six periods of the Palaeozoic Era and the first of the Upper Palaeozoic Sub-era. It began about 408.5 Ma ago and ended about 362.5 Ma ago. In Europe there are both marine and continental facies present, the latter being commonly known as the Old Red Sandstone. Although originally described from the type area in Devon, the marine Devonian is subdivided stratigraphically into stages established in the exceptionally fossiliferous deposits of the Ardennes in Belgium. These stages are the Gedinnian (408.5–401 Ma), Siegennian (401–394 Ma), and Emsian (394–387 Ma) of the Lower Devonian, the Eifelian (387–380 Ma) and Givetian (380–374 Ma) of the Middle Devonian, and the Frasnian (374–367 Ma) and Famennian (367–362.5 Ma) of the Upper Devonian. The subdivision of the marine deposits is based on lithologies and the presence of an abundant invertebrate fauna including goniatites and spiriferid brachiopods. The continental Old Red Sandstone deposits contain a fauna of jawless fish and plants belonging to the primitive psilophyte group. As a result of the Caledonian orogeny of late Silurian times, much of the British Isles was covered with continental red-bed facies.

Devonian The fourth of the six periods of the Palaeozoic Era, from about 408–360 Ma ago, and the first period of the Upper Palaeozoic Sub-era. In Europe there are both marine and continental facies present, the latter being commonly known as the Old Red Sandstone. Although originally described from the type area in Devon, the marine Devonian is subdivided stratigraphically into stages established in the exceptionally fossiliferous deposits of the Ardennes in Belgium. These stages are the Gedinnian (408–401 Ma ago), Siegennian (401–394 Ma ago), and Emsian (394–387 Ma ago) of the Lower Devonian, the Eifelian (also called Couvinian, 387–380 Ma ago) and Givetian (380–374 Ma ago) of the Middle Devonian, and the Frasnian (374–367 Ma ago) and Famennian (367–360 Ma ago) of the Upper Devonian. The subdivision of the marine deposits is based on lithologies and the presence of an abundant invertebrate fauna including goniatites (Ammonoidea) and spiriferid brachiopods (Spiriferida). The continental Old Red Sandstone deposits contain a fauna of jawless fish and plants belonging to the primitive psilophyte group. As a result of the Caledonian orogeny of late Silurian times, much of the British Isles was covered with continental red-bed facies. Fossils of vascular plants are abundant in Devonian beds; the Rhynie chert flora (Middle Devonian) consists of well preserved psilophytes (see PSILOPHYTALES).

Devonian The fourth of the six periods of the Palaeozoic Era, about 416–359.2 Ma ago, and the first period of the Upper Palaeozoic Sub-Era. In Europe there are both marine and continental facies present, the latter being commonly known as the Old Red Sandstone. Although originally described from the type area in Devon, the marine Devonian is subdivided stratigraphically into stages established in the exceptionally fossiliferous deposits of the Ardennes in Belgium. These stages are the Gedinnian (416–411.2 Ma ago), Siegennian (411.2–400 Ma ago), and Emsian (400–390 Ma ago) of the Lower Devonian, the Eifelian (also called Couvinian, 390–380 Ma ago) and Givetian (380–375 Ma ago) of the Middle Devonian, and the Frasnian (375–370 Ma ago) and Famennian (370–359.2 Ma ago) of the Upper Devonian. The subdivision of the marine deposits is based on lithologies and the presence of an abundant invertebrate fauna including goniatites (Ammonoidea) and spiriferid brachiopods (Spiriferida). The continental Old Red Sandstone deposits contain a fauna of jawless fish and plants belonging to the primitive psilophyte group. As a result of the Caledonian orogeny ( mountain-building episode) of late Silurian times, much of the British Isles was covered with continental red-bed facies.