Phanerozoic/Devonian period

The Devonian is a period within the Paleozoic era in geologic time. It spans from 416ma to 359ma. It follows the Silurian period, and precedes the Mississippian.

The Devonian system was proposed by Sedgwick and Murchison in 1839, after finding differences between fossils in rocks between the carboniferous and the underlying Silurian, found near Devonshire, England, for which the system is named.

During the Devonian period, land-walking tetrapods evolved from fish. The evolution of the seed allowed plants to move further from the ocean, allowing them to grow in fully dry places and form forests.

Geology

NA and Europe collided forming a large continent called Euramerica, also known as Laurasia, which caused the formation of the Appalachian mountain range. Euramerica was formed by the collision of the Laruasian and Baltic cratons.

Another large mass was called Gondwana, or Gondwanaland; made up of South America, Africa, Antarctica, India and Australia, it included Florida and most of Southern Europe for a time. This large land formation was named after the formations of the Gondwana district of India, displaying many geologic similarities. Gondwanaland had a lifespan of around 520 million years, it existed as nearly 650 million years ago, and only started breaking up about 130 million years ago.

Euramerica and Gondwana lay close to each other and were near the equator. These two continents would eventually collide to form Pangea in the Permian period, the event is more than 64 million years after the Devonian.

Climate

Because the land masses were near the equator in the Devonian, the climate was warm. This made life on land good for plants. Plants developed vascular tissues to carry food and water into the roots and leaves, perhaps the most important development was that of the seed. Because of the seed the plants no longer relied on the presence of water to reproduce, they were able to move further inland. Ferns and trees began to grow and cover the land.

There was a wide variation in temperature on different continents in the Devonian. A southern cool zone begins to form, while land northward experienced an increase in temperature. It seems that it isn't simply planetary cooling that's occurring during the Devonian, but rather changes in temperature due to a drop in sea levels combined with the effects of the continents coming together to form the supercontinent Pangea.

Life

Red Hill is a fossil elite located in Pennsylvania. The geology tells us it was formed 361 million years ago during the Late Devonian, and it amassed a variety of animal and place fossils. Most of the vertebrate fossils were recovered from the shallow channel margin. In floodplain ponds, both vertebrates and invertebrate fossils were found.

Fish

Devonian is called the age of fishes, rather appropriately. There were very significant evolutionary changes in all of the major fish lineages during this period. Lobe and ray fin fishes first appear during the Devonian, the love-fins diversify rapidly, reaching their peak during the Devonian and carboniferous periods. The first shark fossils come from the early Devonian and they increase in variety throughout the period

Vertebrates

At least 13 vertebrate taxa were discovered at Red Hill, making it the most diverse gathering of vertebrate remains associated with Late Devonian tetrapods. From the shallow channel margin one of the most common is the Turrisaspis Elektor. Phyllolepis, Ageleodus, and Gyranthus are also very common lobe-fin fishes collect at Red Hill. The size of these ranged from 3m in length all the way to less than 10cm. In the floodplain ponds, other vertebrate taxa were collected a well; one was a specimen of the ray-fish, Limnomis Delaneyi.

Invertebrates

Invertebrates were also found in both areas of Red Hill. The quiet waters of the flood plain were favorable conditions to preserving delicate invertebrates. Here we find a Gigantocharinus Szatmaryi, which resembles a modern spider, and an Orsadesmus Rubecollus, which is early version of the millipede. In the shallow channel margin, they have found unidentified Arthropods that resemble millipedes but are much larger, about 7cm long, as well as scorpion fragments.

Plants / Fauna

Early vegetation consisted of low-lying plants near the oceans edge. By the late Devonian, they extended upstream forming floodplain forests, consisting of large trees. Terrestrial plants had started forming faster, coal was being formed and natural fires could be sustained.

Development of Soil

A much overlooked importance of the Devonian is the development of soils that took place during it. Changes to weather and soil formation during the period had great effects on nearly all environments. Weatherin g is a process that breaks bigger rocks into smaller fragments and eventually into soil. Vascular plants during the period increased the weathering by reducing erosion, both physically and chemically, and increased the amount of the time the rocks can be weathered. The process of slowing down the speed of weathering increases the amount of finer particles, such as clay, in the soil. An increase of soil during the period is associated with the development of plant rooting systems; root traces become much more frequent during the Devonian. One major benefit of these enhanced soils is that flooding was much less dangerous to plants than it was before, because the soil could absorb large quantities of storm water.

The First Forests

Vegetation of the Silurian and Early Devonian was short and typically only found near the waters edge. However by the middle Devonian, there’s evidence of taller forms evolved independently. One of the most famous Middle Devonian sites is the Gelboa Forest, located in eastern New York. The vegetation at this site reached heights up to 9m tall, but it didn’t have some important characteristics of a modern forest, perhaps most important, lacking deeply penetrating roots to stabilize stream banks and enhance soil formation.

In the late Devonian, vegetation that resembled modern forests first started occurring because of the appearance of the Archaeopteris. This genus has been recorded from nearly all known Devonian land masses, from tropical to sub-polar. They were very large trees, 20m or more in height, and possessed webbed leaves to provide shade. Unlike the vegetation in the Gelboa forest, these trees had a deeper root system allowing them to appear in drier floodplains and costal lowlands.

These forests produced a very large amount of organic matter due to leaves being shed and falling. This organic matter may have had a great effect on aquatic systems; it is likely that, through flooding, the organic matter found its way into streams which stabilize stream habitats.

Extinction event

The Devonian is one of the “big five” extinction events, however the timing and duration of the late Devonian mass extinction is subject to great debate and several interpretations.

For example, one proposed cause is a prolonged marine biotic crisis lasting about 20-25 million years, punctuated by nearly 10 extinction events. The two largest extinction events are the Kellwasser (from middle Devonian to carboniferous) and Hangeberg Event (the Devonian-Carboniferous boundary). Others recognize that there were several extinction events, but regard the Kellwasser as the Late Devonian extinction. The Kellwasser event accounted for 20% of all animal families and about 80% of all animal species, while the Hangeberg event accounts for 16% of all marine families.

Another theory is called The Devonian Plant Hypothesis, proposed by Thomas Algeo, Rober Berner, J. Barry Manard and Stephen Scheckler in 1995. It attributes the extinctions in tropical oceans to terrestrial plants. It is argued that black shales in the seas of North America and Eurasia were produced by organic matter imported from vegetated landscapes. Terrestrial plants contributed to accelerated soil formation which resulted in silicate weathering. This process removes Co2 from the atmosphere, which is believed to have contributed to global cooling. A short period of glaciations occurred at the end of the Devonian, associated with the Hangeberg event.

References

1. "Fossils-facts-and-finds". <http://www.fossils-facts-and-finds.com/devonian_period.html>. (May 15, 2010)
2. Kazlev, Alan. 5 April 2002 "Paleos". <http://www.palaeos.com/Earth/Geography/Gondwana.htm>. (May 15, 2010)
3. Levin, Harold. The Earth Through Time 8th edition. Wiley & Sons, 2006.
4. Murphy, Dennis. 2006. "Devonian Times". <http://www.devoniantimes.org/>. (May 15, 2010)
5. Speer, Brian.1 Feb 1998 "Devonian". UCMP Berkeley. <http://www.ucmp.berkeley.edu/devonian/devonian.html>. (May 15, 2010)