Devonian
From EvoWiki
| Devonian |
|---|
| 416-359 mya |
| Era: Paleozoic |
| Preceded by Silurian |
| Followed by Carboniferous |
The Devonian the fourth geologic period of the Paleozoic era. The Devonian period occurred between 416-359 million years ago. The Devonian period is named after a large county in England known as Devon, where rocks from this period were first studied.
During the Devonian period the first Tetrapods (vertebrates having four leg-like appendages) descended from primitive lobe-finned fish such as Tiktaalik roseae that possessed structures similar to limbs and began to walk on land around 365 million years ago during the late Devonian. Also the first true wingless insect Rhyniognatha hirsti appeared in the middle Devonian (although insects would not become common until the Carboniferous period) and began to colonise dry land. Early seed-bearing plants evolved in the Devonian in response to dry conditions but however as a result of their versatility seed-bearing plants soon spread out to more moist terrestrial habitats and formed the worlds first forests (circa 380 million years ago.) Many important events happened in the oceans as well such as the evolution of the ammonite mollusks who would become so abundant that they featured as important members of marine habitats until the end of the Cretaceous period and the Mesozoic Era around 65 million years ago when they went extinct with the dinosaurs and 50% of the worlds fauna. Trilobites were still common during the Devonian although they became less diverse in the Devonian period then previous periods due to the abundance of mobile swimming predators such as sharks and predatory bony fish (Osteichthyes) such as Dunkleosteus. Large coral reefs continued to be common during the Devonian period.
Three continents the supercontinent Gondwana (in the south), Siberia (in the north), and the minor supercontinent of Euramerica (in between) encompassed most of the worlds landmasses during the Devonian period.
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Subdivisions
The Devonian period is usually divided into three epochs the Early Devonian the Middle Devonian and the Late Devonian witch correspond to systems of rocks that are referred to as belonging to the to the lower, middle, and upper parts of the Devonian System. The epochs and faunal stages of the Devonian period are listed below in order of youngest to oldest.
Epochs
Early Devonian
The Early Devonian (from 416.0 to 397.5 million years ago) is the first epoch of the Devonian period. In the Early Devonian the now-vanished Acadian Mountains appeared as a result of the collision between the microcontinent of Avalonia and the northeast tip of the continent Euramerica (Also known as Laurussia or the Old Red Continent). In the southeastern portion of Euramerica collisions between tectonic plates occurred near the western margins of the Russian Platform and orogenes (episodes of mountain building) happened in the Urals and Scythian regions. The East-European Platform experienced tectonic inversion and uplift. Transtensional basins formed within the Pechora basin. The Early Devonian saw the very start of placoderm diversity, with members from all of the known orders, save for Phyllolepida and Ptyctodontida, present. Jawless fishes (e.g. Cephalaspis and Pteraspis) and eurypterids were common in Alluvial enviorments during the Early Devonian period however osteichthyan (bony) fish were rare in Alluvial environments during the early Devonian period and were more common in the seas. Vascular plants during the Early Devonian were still usually small, reproduced by using spores and were limited to the shore and lowland habitats like their Silurian counterparts. Terrestrial Arthropods were well established during the Early Devonian due to the development of vascular plants which created habitats and provided food for many types of arthropods such as spiders, myriapods (predatory centipedes and herbivorous millipedes) and primitive wingless insects.
Middle Devonian
The Middle Devonian is the second epoch of the Devonian period. It occurred around 397.5 million years ago to 385.3 million years ago and includes the Eifelian/Southwood, Givetian, and the Cazenovia/Cazenovian Faunal stages. During the Middle Devonian epoch the Class of armored, bony jawless fishes known as the ostracoderms which were common in the previous Silurian period began to decline in diversity and began to be replaced by the jawed fish who were more efficient at acquiring food. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop essential characteristics such as well developed lungs, ability to crawl out of the water and onto the land for short periods of time, possibly in search of food which would be developed by the tetrapods later in the Late Devonian which are descendants of these early fish. Also on dry land the first lycopods, sphenopsids (horsetails), earlyferns, and a group called the progymnosperms (ancestral to higher or seed plants) also appeared during the middle Devonian epoch. A diverse collection of spiders, mites, myriapods and collembolids lived alongside Middle Devonian flora.
Late Devonian
The Late Devonian is the final Epoch of the Devonian period it occurred between 385.3 million years ago and 359 million years ago. It encompasses the Famennian and the Frasnian Faunal stages of the Devonian period. The first true tetrapods evolved from primitive lobe finned fish (Sarcopterygii) that resembled tetrapods such as Tiktaalik and Panderichthys around 365 million years ago and the first trees (Wattieza) also appeared during the Late Devonian epoch circa 385 million years ago. The late Paleozoic and early Mesozoic supercontinent Pangaea began to coalesce during the Late Devonian from smaller continents and Microcontinents. The worlds earliest thick coals formed during the Late Devonian period from the worlds first tropical rainforests which were located in the southern portion of China and the Canadian Arctic. Also glaciers began to form at the south pole during the late Devonian as a result of the global cooling caused by the increase of oxygen in the atmosphere. Many significant changes occurred in the Devonian environment around the end of the late Devonian period (The Frasnian Faunal stage) such as a large drop in sea level, algal blooms which resulted in anoxia (or the deficiency of oxygen in the oceans) and global cooling which resulted from the increase of oxygen in the atmosphere which resulted from the expansion of primitive forests during the end of the Devonian period, all of these environmental changes caused 75% of life on earth to go extinct during the end of the period in the Late Devonian Extinction Event which was one of the worst extinction events in Geological History.
Faunal stages
Late (most recent)
- Famennian/Chautauquan/Canadaway/Conneaut/Conneautan/Conewango/Conewangan
- Frasnian/Senecan/Sonyea/Sonyean/West Falls
Middle
- Cazenovia/Cazenovian
- Givetian/Erian/Senecan/Tioughniogan/Tioughnioga/Taghanic/Taghanican/Genesee/Geneseean
- Eifelian/Southwood
Early (oldest)
Life in the Devonian
Terrestrial Life
During the Devonian period multicellular life forms such as wingless insects (by the Early Devonian around 396-407 million years ago), tetropods (by the Late Devonian around 365 million years ago), mites (by the Early Devonian), scorpions (by the Early Devonian) and plants (although they have been living in terrestrial habitats since the Ordovician period around 474 million years ago they became much more complex in the Devonian than in previous periods) were colonising terrestrial ecosystems. Although the Devonian is usually known for fish evolution and the evolution of primitive tetropods such as Acanthostega gunnari plants were probably the most affected by the radical changes in the multicellular life of the Devonian period as they underwent a massive evolutionary radiation known as the Devonian Explosion which resulted in the appearance of many new genera of plants and by the Middle Devonian and Late Devonian (around 380 million years ago) the first true forests and the first trees (from the genus Wattieza). The Devonian was the first period in earth history that recognizable soils were present. These newly formed soils provided a hospitable habitat for life which hosted a variety of organisms such as mites, scorpions and myriapods. (Note: Arthropods have existed in terrestrial habitats much earlier than the Devonian period and are known even from Cambrian fossils such as Climactichnites and Protichnites.) Early Devonian plants did not posses leaves and root systems like their Middle Devonian, late Devonian and modern counterparts and usually only grew to a few centimeters tall but however they still managed to spread across terrestrial habitats and become common in wet, moist ecosystems close to fresh water. By contrast Late Devonian plants formed large forests at the equator composed of early trees such as Archaeopteris and the giant cladoxylopsid trees which usually grew up to lengths of 8 meters (26 feet) 10 meters (32.80 feet). The first seed-forming plants also evolved during the end of the Devonian period originally to adapt to dry conditions but due to their versatility they became common even in moist habitats and formed large forests. Primitive arthropods (such as Wingless insects and arachnids) also reacted to the change in terrestrial flora during the middle and late Devonian by evolving to co-dependence with the terrestrial flora.
Also a large 8 meeter (26 foot) long fungus known as Prototaxites inhabited both Late Devonian forests and Silurian period (And Early Devonian) beaches.
Marine life
Devonian sea levels were generally higher than the sea levels of the previous Silurian and Ordovician periods. However many Silurian fauna such as bryozoa, diverse and abundant brachiopods, the enigmatic hederelloids, and corals continued to be common throughout much of the Devonian period and often dominated aquatic ecosystems much as they did in the previous Silurian period. Trilobites were still somewhat diverse during the Devonian period but were less common than in the previous Cambrian, Ordovician and Silurian periods due to the abundance of mobile swimming predators such as sharks and predatory bony fish (Osteichthyes) such as Dunkleosteus as most Trilobites were slow-moving bottom feeders that were susceptible to being attacked by fast, swimming predators such as Sharks and jawed carnivorous bony fish. Primitive jawless fish known as the ostracoderms thrived during the Early Devonian but were being replaced and becoming less diverse during the Middle Devonian and the Late Devonian due to the diversification of jawed fish who were more efficient herbivores and predators. Although primitive sharks had already been in existence since the Ordovician period the first truly successful species of shark Cladoselache evolved and became successful in the Devonian oceans.
Devonian paleogeography
During the Devonian period the shape of the earth's landscape changed as a result of the immense tectonic activity created by the continents Laurasia and Gondwanaland the large supercontient which included most of the earths southern landmasses were drawing closer to each other.
The Late Paleozoic and early Mesozoic supercontient Pangaea began to form in the Devonian period from the tectonic plates containing North America and Europe. The formation of Pangaea contributed to the rise of the Appalachian Mountains and completing the formation of the Caledonian Mountains (the Caledonian orogeny) which had been underway since the Silurian period.
The Silurian continents Laurentia and Baltica merged during the Devonian to form the continent Euramerica (or Laurussia). The collision caused the new continent to move towards the natural dry zone along the Tropic of Capricorn as a result.
A large, deep ocean known as Panthalassa (from Greek meaning all oceans or universal ocean) encompassed most of the worlds oceans although many smaller ocean's such as the Paleo-Tethys, Proto-Tethys, Rheic Ocean, and Ural Ocean also existed during this time period.
Sea levels were high during the Devonian period which allowed much of the worlds dry land to become submerged under shallow seas where coral reefs flourished.
Rocks from Devonian strata are gas and oil producers in some areas.
Late Devonian Extinction Events
This extinction event occurred about 364 to 359 million years ago. It is clear that there was a decrease in biodiversity during the Late Devonian Extinction Event but scientists are unsure how long the event took place with estimates ranging from 15 million years to 500,000 years. The Late Devonian Extinction Event mostly affected marine organisms, Terrestrial organisms were not severely affected by the Late Devonian Extinction Event. Tetrapods began to colonize land just before this period around 365 million years ago. The beginning of the formation of the supercontinent Pangaea lead to the reduction of shallow-water marine environments, leading to the extinction of all placoderms, all trilobite groups except for Proetida, the last armored jawless fishes, many ammonites, and numerous other groups but however many scientists believe that the beginning of the formation of Pangaea was not the only cause of the Late Devonian Extinction Event and that the Late Devonian Extinction Event had multiple causes including a meteorite impact and global cooling caused by the expansion of primitive forests.
See also
External links
- Wikipedia [1]
- Palaeos [2]
- Devonian Times [3]
- The Dry Dredgers, an active group of amateur paleontologists in the Cincinnati area
| Paleozoic era | |||||
|---|---|---|---|---|---|
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian |
