Devonian Subdivisions Devonian Palaeogeography Devonian Fauna

The Devonian is a major division of the geologic timescale that extends from the end of the Silurian period (360 million years before the present (BP)) to the beginning of the Mississippian subperiod of the Carboniferous (408.5 million years BP). The Devonian period is named for England's Devonshire area where Devonian outcrops are common.

As with most older geologic periods, the rock beds that define period's the start and end are well identified, but the exact dates are uncertain by 5-15 million years.

1 Devonian subdivisions

The Devonian is usually broken into lower, middle, and upper subdivisions. The Faunal stages from youngest to oldest are:

2 Devonian palaeogeography

The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together. Near the equator, Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountain s and forming the Caledonides in Britain and ScandinaviaScandinavia is the cultural and historic region of the Scandinavian Peninsula. The Scandinavian countries are Norway, Sweden and Denmark, which mutually recognize each other as parts of Scandinavia. The collective label "Scandinavia" reflects the cultural. The southern continentDymaxion map by Buckminster Fuller shows land mass with minimal distortion as only one continuous continent A continent (from the Latin continere for "to hold together") is a large continuous mass of land on the planet Earth. There is no single standard fs remained tied together in the supercontinentA supercontinent is a mass of land comprising more than one continent. Since the definition of continent is arbitrary, the definition of supercontinent is also arbitrary (as is the definition of a subcontinent), but the term refers to a landmass containin of Gondwana. The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas, where tropical reefIn nautical parlance, a reef is a rock, sandbar, or other feature beneath the surface of the water, but shallow enough to be a hazard to ships; see also shoal''. Many reefs result from abiotic processes—deposition of sands, wave erosion planning down rock organisms lived. A deep, enormous oceanOcean covers almost three quarters (71%) of the surface of the Earth. This global, interconnected body of salt water is divided by the continents and larger archipelagos into five oceans as follows: Arctic Ocean Atlantic Ocean Indian Ocean Pacific Ocean S covered the rest of the planetA planet (from the Greek , planetes or "wanderers") is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3.

The Devonian has been referred to as the "greenhouse age". FossilFor other uses of the term, see Fossil (disambiguation Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. The totality of fossils and their placement in rock formations and sedimentary layers is known as the fo signatures of widespread reefs indicate that the climateThe climate is the weather averaged over a long period of time. A descriptive saying is that "climate is what you expect, weather is what you get". The IPCC glossary definition is: : Climate in a narrow sense is usually defined as the “average weather”, o was mild and warm, as well as moderately humid. The Devonian has also been referred to as the "Age of Fish".

3 Devonian fauna

3.1 Marine biota

Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods and corals. Lily-like crinoids were abundant, and trilobites were still fairly common, but less diverse than in earlier periods. The ostracoderms were joined in the mid-Devonian by the first jawed fishes, the great armored placoderms, as well as the first sharks and ray-finned fish. The first shark, the Cladoselache, appeared in the oceans during the Devonion period. They became abundant and diverse. In the late Devonian the lobe-finned fish appeared, giving rise to the first tetrapods.

3.2 Devonian reefs

A great barrier reef, now left high and dry in the Kimberley Basin of northwest Australia, once extended a thousand kilometers, fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant frameworks close to sea level. The main contributors of the Devonian reefs were unlike modern reefs, which are constructed mainly by corals and calcareous algae. They were composed of calcareous algae and coral-like stromatoporoids , and tabulate and rugose corals, in that order of importance.

3.3 Terrestrial biota

By the Devonian Period, life was well underway in its colonization of the land. The bacterial and algal mats were joined early in the period by primitive plants that created the first recognizable soils and harbored some arthropods like mites, scorpions and myriapod s. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.

By the late Devonian, forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-like ancestral fern Archaeopteris, grew as a large tree with true wood. These are the oldest known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the late Devonian.

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event.

Also in the Devonian, both vertebrates and arthropods were solidly established on the land.

4 See also

Phacops rana - a Devonian trilobite.

5 External links

Geologic timescale

<Hide>

Home > Devonian