Devonian

The Devonian was a long time period in Earth's history, lasting about 60 million years from about 419 million years ago to 359 million years ago. It was part of a time called the Paleozoic era. This period got its name from a place called Devon in southwestern England, where scientists first studied rocks from this time.

During the Devonian, many important changes happened in the world of living things. Plants began to spread out onto the land, and the first animals like spiders and insects also started living on land. The oceans were full of many kinds of fish, and this time is often called the "Age of Fishes" because of how many different fish lived then.

Some of the fish in the Devonian oceans started to change in ways that would later lead to animals with four limbs, like amphibians and eventually reptiles, birds, and mammals. However, a big change happened toward the end of the Devonian when many kinds of sea creatures, including some fish and coral reefs, disappeared in events known as extinctions.

History

The Devonian Period is named after Devon, a county in southwestern England. In the 1830s, scientists had a big debate about the age and structure of rocks in that area. They decided to call this time in Earth's history the Devonian Period.

During this period, many types of fish evolved and developed. The climate varied a lot — some places were dry and others were warm and humid. These changes happened in different parts of the world and at different times during the Devonian.

Subdivisions

The Devonian Period is divided into three parts: Early, Middle, and Late. Rocks from each part are called Lower, Middle, and Upper Devonian.

The Early Devonian lasted from 419.62 to 393.47 million years ago. It began with the Lochkovian Stage, followed by the Pragian, and then the Emsian. During this time, the first ammonoids appeared. These were simple sea creatures that came from nautiloids.

The Middle Devonian had two parts: the Eifelian and the Givetian. In these times, some jawless fish started to disappear because of changes in their environment and because new fish with jaws were becoming more common. Some of these early fish began to develop lungs and could move out of water for short times.

The Late Devonian started with the Frasnian, from 382.31 to 372.15 million years ago. During this time, the first forests appeared on land. The Famennian followed, when the first tetrapods, or four-legged animals, appeared in fossils. This part ended the Devonian Period at 358.86 million years ago.

Climate

The Devonian was a warm time, but there might have been some glaciers early on. The weather was very dry, especially near the equator. The oceans were warm, with temperatures around 30 °C (86 °F) early in this period. As forests grew, they took carbon from the air, which may have cooled the Earth a little. Later, it got warmer again before cooling sharply at the end of the Devonian, starting a colder time in Earth's history.

CO₂ levels dropped steeply throughout the Devonian Period. The newly evolved forests drew carbon out of the atmosphere, which were then buried into sediments. This may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO₂ concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to a Late Devonian warming. The climate would have affected the dominant organisms in reefs; microbes would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids. At the terminus of the Devonian, Earth rapidly cooled into an icehouse, marking the beginning of the Late Paleozoic icehouse.

Paleogeography

The Devonian world had many continents and oceans of different sizes. The largest continent, Gondwana, was located entirely in the Southern Hemisphere. It corresponds to modern-day South America, Africa, Australia, Antarctica, and India, as well as parts of North America and Asia. The second-largest continent, Laurussia, was northwest of Gondwana and corresponds to much of modern-day North America and Europe.

Sea levels were high, and much of the land was covered by shallow seas. The large ocean, Panthalassa, occupied much of the Northern Hemisphere and areas east of Gondwana and west of Laurussia. Other smaller oceans included the Paleo-Tethys Ocean and Rheic Ocean.

Laurussia

By the early Devonian, the continent Laurussia was fully formed from the collision of Laurentia (modern-day North America) and Baltica (northern and eastern Europe). This collision created mountain ranges along the southeastern coast. In present-day eastern North America, this raised the Appalachian Mountains. Further east, it also raised the Caledonian Mountains in Great Britain and Scandinavia.

Most of Laurussia was south of the equator but moved northwards during the Devonian. While the northern parts had tropical conditions, most of the continent was dry, forming the Old Red Sandstone beds. For much of the Devonian, western Laurussia was covered by inland seas with many reefs and marine life. Devonian marine deposits are common in the midwestern and northeastern United States.

Gondwana

Gondwana was the largest continent, completely south of the equator. Its northeastern part reached tropical latitudes, while the southwestern part was near the South Pole. The northwestern edge was active, with many smaller land masses and island arcs joining it. These included Chilenia, Cuyania, and Chaitenia, which now form parts of Chile and Patagonia.

The northern rim of Gondwana had many marine deposits. The eastern margin was also active, with mountain building and intrusions affecting areas now equivalent to eastern Australia, Tasmania, and Antarctica.

Asian terranes

Several island microcontinents stretched north of Gondwana. They were separated by the Paleo-Tethys Ocean. The eastern part of this ocean began to open in the Devonian. South China and Annamia (part of Indochina) detached from Gondwana but stayed close enough that their fossils were similar to Australian species.

North China and the Tarim Block moved northwards. Further west were Kazakhstania, Siberia, and Amuria. Siberia was the largest landmass in the Northern Hemisphere, moving northwards and twisting during the Devonian.

Closure of the Rheic Ocean

The Rheic Ocean separated Laurussia and Gondwana and shrank during the Devonian as the continents moved closer. This led to the combination of marine faunas from both continents. The closure of the ocean began in the Devonian and continued into the Carboniferous, eventually leading to the formation of Pangaea. The collision of terranes with Laurussia started the Variscan Orogeny, a major mountain-building event.

Life

Marine biota

Sea levels were high during the Devonian. The oceans were full of different sea creatures. Common animals included conodonts, bryozoans, many kinds of brachiopods, strange hederellids, microconchids, and corals. Flower-like crinoids (which are animals, even though they look like flowers) were very common, and trilobites were still around. Bivalves became common in deeper water. The first ammonites appeared around 400 million years ago. Early jawed fish appeared, and some fish had armor. This time is often called "The Age of Fishes" because of all the different fish species.

Reefs

There was once a huge reef in what is now Australia. Reefs are built by organisms that can make structures near the sea's surface. In the Devonian, reefs were made by different kinds of life, including types of cyanobacteria and coral-like stromatoporoids.

Terrestrial biota

Life was beginning to spread onto land during the Devonian. Early plants created the first stable soils. Small creatures like mites, scorpions, and early insects lived on land. One of the largest land creatures was Prototaxites, which may have been a giant fungus or plant.

The greening of land

Early land plants did not have true roots or leaves, but some had simple structures. By the Middle Devonian, taller plants with roots and leaves appeared, including early trees. These plants helped create soils and changed the land.

Animals and the first soils

As plants grew, small land animals like arthropods lived among them. This helped change the land and made it possible for the first vertebrates to live on land.

Gallery

• Dunkleosteus, one of the largest armoured fish, lived during the Late Devonian

• Titanichthys, a large fish from the Famennian of the Cleveland Shale of Ohio

• Lower jaw of Eastmanosteus from the Middle Devonian of Wisconsin

• Tooth of the lobe-finned fish Onychodus from the Middle Devonian of Wisconsin

• Bothriolepis, a type of fish that lived from the Mid to Late Devonian

• Shark-like Cladoselache, and other fish like Eusthenopteron and Bothriolepis

• Melocrinites nodosus spinosus, a spiny crinoid from the Middle Devonian of Wisconsin

• An enrolled trilobite from the Devonian of Ohio

• The coral Aulopora from the Middle Devonian of Ohio

• Tropidoleptus carinatus, a brachiopod from the Middle Devonian of New York

• Pleurodictyum americanum, a coral from the Middle Devonian of New York

• A hederelloid from the Devonian of Michigan

• A Devonian brachiopod with hederelloids

• Prototaxites, a large possible fungus from the Devonian

Late Devonian extinction

Main article: Late Devonian extinction

The Late Devonian extinction was not one single event but a series of smaller events happening over time. It is considered one of the biggest losses of life in Earth's history. This extinction mainly affected sea creatures, especially those that lived in warm, shallow waters, like the builders of large underwater reefs.

Many groups of sea animals were greatly reduced, including brachiopods, trilobites, and ammonites. About 96% of certain sea vertebrates, such as conodonts and bony fishes, disappeared. However, land plants and creatures that lived in fresh water, including the early ancestors of four-legged animals, were not as strongly affected.

Scientists are still not sure what caused these extinctions. One idea from 1969 suggested that an asteroid impact might have been responsible, but there is little evidence to support this theory.