The Devonian period is a time in Earth's history during the Paleozoic era, which is part of the larger Phanerozoic eon. It lasted 60.7 million years, starting at the end of the Silurian period, 419.62 million years ago, and ending with the start of the Carboniferous period, 358.86 million years ago. This was the fourth time period in both the Paleozoic and the Phanerozoic. It is named after Devon, a region in South West England, where rocks from this time were first studied.

During the Devonian, plants began to grow on land for the first time in a major way. Free-spore plants, called pteridophytes, spread across dry land, forming large forests that later became coal. By the middle of the Devonian, many plants developed leaves and true roots. By the end of the period, the first seed-bearing plants, called pteridospermatophytes, appeared. This process of rapid plant growth and spread, which started in the Silurian, is called the Silurian-Devonian Terrestrial Revolution. Early land animals, such as arthropods like myriapods, arachnids, and hexapods, also became common during this time, having started to move onto land as early as the Ordovician period.

Fish, especially those with jaws, became very diverse during the Devonian, which is why this time is often called the Age of Fishes. Armored fish called placoderms dominated most aquatic environments. In the oceans, cartilaginous fish, such as early sharks, became more common than in earlier periods. Tetrapodomorphs, which are ancestors of all four-legged vertebrates, began to develop stronger and more muscular fins that gradually evolved into limbs. However, these animals did not fully adapt to life on land until much later, during the Late Carboniferous.

Ammonites, a type of mollusk, first appeared during the Devonian. Trilobites, brachiopods, and coral reefs were also common during this time. A major extinction event, called the Late Devonian extinction, began about 375 million years ago and severely reduced marine life, killing most reef systems, jawless fish, placoderms, and nearly all trilobites except a few species. A second extinction event, the end-Devonian extinction, occurred around 359 million years ago and further disrupted ecosystems, ending the last calcite sponge reefs and placoderms.

During the Devonian, the supercontinent Gondwana was located in the southern part of the Earth, while the continent of Siberia was to the north and Laurussia was to the east. Major geological events included the closing of the Rheic Ocean, the separation of South China from Gondwana, and the expansion of the Paleo-Tethys Ocean. Mountain-building events, such as the Acadian Orogeny in North America and the start of the Variscan Orogeny in Europe, occurred as Laurussia and Gondwana moved closer together. These events happened before the formation of the supercontinent Pangaea in the Late Paleozoic.

History

The Devonian Period is named after Devon, a county in southwestern England. In the 1830s, a disagreement arose about the age and structure of rocks found in the area. This debate, called the Great Devonian Controversy, involved scientists Roderick Murchison, Adam Sedgwick, and Henry De la Beche. Murchison and Sedgwick won the debate and created the name "Devonian System" for the period.

The rock layers that mark the beginning and end of the Devonian Period are clearly identified, but the exact dates are not certain. According to the International Commission on Stratigraphy, the Devonian Period lasted from the end of the Silurian Period, 419.62 million years ago, to the start of the Carboniferous Period, 358.86 million years ago. In North America, this time is also considered the beginning of the Mississippian subperiod of the Carboniferous.

In 19th-century writings, the Devonian Period was sometimes called the "Old Red Age." This name comes from the red and brown rock layers in Great Britain known as the Old Red Sandstone, where early fossils were discovered. Another common name for the period is the "Age of the Fishes," which refers to the development of many important fish groups during this time. Older studies of the Anglo-Welsh region divided the Devonian Period into stages called Downtonian, Dittonian, Breconian, and Farlovian. The last three are classified as part of the Devonian.

Some sources incorrectly describe the Devonian Period as a "greenhouse age." This idea comes from early discoveries in Western Europe and eastern North America, which were near the Equator as part of the supercontinent Euramerica. These regions had warm, tropical climates with reefs, suggesting high temperatures and humidity. However, the Devonian climate varied greatly across different times and regions. For example, during the Early Devonian, dry conditions were common in areas such as Siberia, Australia, North America, and China. Meanwhile, Africa and South America had warm temperate climates. By the Late Devonian, dry conditions were less widespread, and temperate climates became more common worldwide.

Subdivisions

The Devonian Period is divided into three main parts: Early, Middle, and Late. The rocks from these times are called the Lower, Middle, and Upper parts of the Devonian System.

The Early Devonian lasted from 419.62 to 393.47 million years ago. It began with the Lochkovian Stage (419.62 to 413.02 million years ago), followed by the Pragian Stage (413.02 to 410.62 million years ago), and then the Emsian Stage, which ended when the Middle Devonian began at 393.47 million years ago. During this time, the first ammonoids appeared. These simple creatures evolved from bactritoid nautiloids. These early ammonoids belonged to the order Agoniatitida. Over time, this group developed into new orders, such as Goniatitida and Clymeniida. These cephalopod mollusks became the main marine animals until the start of the Mesozoic Era.

The Middle Devonian had two parts: the Eifelian Stage, followed by the Givetian Stage, which began at 387.95 million years ago. During this time, jawless agnathan fish became less common in freshwater and marine environments. This decline happened partly because of major environmental changes and partly because of increased competition, predation, and diversity among jawed fish. The shallow, warm, low-oxygen waters of Devonian inland lakes, surrounded by early plants, helped some fish develop traits like strong lungs and the ability to move briefly onto land.

The Late Devonian began with the Frasnian Stage (382.31 to 372.15 million years ago), when the first forests appeared on land. The first tetrapods (four-legged vertebrates) appeared in the Famennian Stage, which started and ended with extinction events. The Devonian Period ended at 358.86 million years ago.

Climate

The Devonian Period was generally warm, though glaciers may have existed during its early and middle stages. The difference in temperature between the equator and the poles was smaller than it is today. Weather conditions were often dry, especially near the equator, where it was the driest. Studies of ancient sea temperatures using certain fossils suggest an average of 30 °C (86 °F) in the Early Devonian. Average surface temperatures during the Early Devonian were about 16 °C. Carbon dioxide levels decreased significantly throughout the Devonian. Newly formed forests absorbed carbon from the air, which was later buried in sediments. This process may have caused a cooling of about 5 °C (9 °F) during the Mid-Devonian. By the Late Devonian, temperatures rose again to levels similar to those in the Early Devonian. Although carbon dioxide levels did not increase, warmer temperatures led to more continental weathering. Evidence such as plant distribution supports the idea of warming during this time. Climate conditions influenced the types of organisms that built reefs. Microbes were the main reef-forming organisms during warm periods, while corals and stromatoporoid sponges became more dominant during cooler times. The warming at the end of the Devonian may have contributed to the extinction of stromatoporoids. As the Devonian ended, Earth quickly cooled, entering an icehouse climate that marked the start of the Late Paleozoic icehouse.

Paleogeography

During the Devonian Period, Earth had many continents and ocean basins of different sizes. The largest continent, Gondwana, was entirely in the Southern Hemisphere. It included modern-day South America, Africa, Australia, Antarctica, India, and parts of North America and Asia. The second-largest continent, Laurussia, was northwest of Gondwana and included much of modern-day North America and Europe. Smaller continents, microcontinents, and terranes were also present east of Laurussia and north of Gondwana, corresponding to parts of Europe and Asia. The Devonian Period was a time of major tectonic activity as Laurussia and Gondwana moved closer together.

Sea levels were high worldwide, and much of Earth’s land was covered by shallow seas where tropical reef organisms lived. The vast "world ocean," Panthalassa, covered much of the Northern Hemisphere and areas east of Gondwana and west of Laurussia. Other smaller oceans included the Paleo-Tethys Ocean and the Rheic Ocean.

By the early Devonian, Laurussia (also called Euramerica) had formed when the continents Laurentia (modern-day North America) and Baltica (modern-day northern and eastern Europe) collided. This collision caused mountain ranges to form along the southeastern coast of Laurussia. In present-day eastern North America, the Acadian Orogeny continued to raise the Appalachian Mountains. Further east, the same collision helped form the Caledonian Mountains of Great Britain and Scandinavia. Later in the Devonian, orogenic collapse led to granite intrusions in Scotland.

Most of Laurussia was located south of the equator but moved northward during the Devonian and rotated counterclockwise toward its modern position. While parts like Greenland and Ellesmere Island had tropical conditions, much of Laurussia was in the dry zone near the Tropic of Capricorn, where red sandstone formed due to drought conditions. This red sandstone earned Laurussia the name "the Old Red Continent." Much of western Laurussia (modern-day North America) was covered by subtropical inland seas with diverse marine life. Devonian marine deposits are common in the midwestern and northeastern United States, and reefs extended along the southeast coast of Laurussia, now corresponding to southern England, Belgium, and other parts of Europe.

In the Early and Middle Devonian, the west coast of Laurussia had a passive margin with coastal waters, river deltas, and estuaries found today in Idaho and Nevada. In the Late Devonian, a volcanic island arc collided with the continental shelf, starting the Antler orogeny, which continued into the Carboniferous Period. Mountain-building events also occurred in the far northeastern part of Laurussia, with deformed mountain remnants still visible on Ellesmere Island and Svalbard. Many Devonian collisions in Laurussia created mountain ranges and foreland basins, which often contain fossils.

Gondwana was the largest continent on Earth, entirely south of the equator. Its northeastern part (now Australia) reached tropical latitudes, while its southwestern part (now South America) was near the South Pole, with Brazil close to the South Pole. The northwestern edge of Gondwana was an active margin where smaller landmasses and island arcs, such as Chilenia, Cuyania, and Chaitenia (now parts of Chile and Patagonia), collided. These events caused volcanic activity and plutons, but by the Late Devonian, much of South America was covered by shallow seas. These polar seas hosted the Malvinokaffric Realm, a unique brachiopod fauna that extended eastward to areas now equivalent to South Africa and Antarctica.

The northern edge of Gondwana was mostly a passive margin with marine deposits in regions like northwest Africa and Tibet. The eastern margin was also active, with mountain-building events and granite and kimberlite intrusions in areas now equivalent to eastern Australia, Tasmania, and Antarctica.

Several island microcontinents (which later formed modern-day Asia) stretched north of Gondwana, separated by the Paleo-Tethys Ocean. The eastern branch of the Paleo-Tethys Ocean opened during the Devonian when South China and Annamia (a terrane equivalent to Indochina) detached from Gondwana. These regions remained close to Gondwana, with Devonian fossils more similar to Australian species than to Asian ones. Other Asian terranes, like Sibumasu (western Indochina), Tibet, and Cimmerian blocks, remained attached to Gondwana.

North China and the Tarim Block (now northwestern China) drifted northward, moving over older oceanic crust. Further west were the Turkestan Ocean and microcontinents like Kazakhstania, Siberia, and Amuria. Kazakhstania was volcanically active during the Devonian as it absorbed smaller island arcs. Siberia, the largest landmass in the Northern Hemisphere, was initially inverted relative to its modern position but later moved northward and rotated clockwise. It approached Laurussia but remained separated by the Ural Ocean. Late Devonian rifting and mantle plumes caused flood basalts in Siberia, including the Vilyuy Traps, which may have contributed to the Late Devonian Mass Extinction. Volcanic activity also affected Amuria (now Manchuria and Mongolia).

Life

During the Devonian Period, sea levels were generally high. Marine life was dominated by conodonts, bryozoans, many types of brachiopods, hederellids, microconchids, and corals. Lily-like crinoids, though not plants, were common, and trilobites were still found in many areas. Bivalves became widespread in deep water and outer shelf environments. The first ammonites appeared around 400 million years ago during the early Devonian. Bactritoids also appeared in the Early Devonian, and some scientists believe their increase in numbers may have been linked to lower oxygen levels in deeper water. Among vertebrates, jawless armored fish declined, while jawed fish increased in both the ocean and freshwater. Armored placoderms were common early in the Devonian but became extinct in the Late Devonian, possibly due to competition with other fish. Early cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes) also became diverse and played important roles in Devonian seas. The first widespread genus of cartilaginous fish, Cladoselache, appeared during the Devonian. The large variety of fish during this time led to the Devonian being called "The Age of Fishes" in popular culture.

The Devonian saw a major increase in the diversity of free-swimming marine animals, driven by the abundance of plankton and competition in underwater habitats. This growth has been called the Devonian Nekton Revolution by some scientists. However, other scientists have questioned this idea, noting that a 2018 study found that while the proportion of nektonic life increased when the Devonian began, it decreased later in the Devonian, especially during the Pragian stage. The study suggested that the overall diversity of nektonic life during the Devonian was not much higher than in other periods and was actually greater during the Silurian to Lochkovian and Carboniferous to Permian intervals. Instead, the researchers believe the increase in nekton diversity was part of a longer trend across the entire Paleozoic era.

A now-dry barrier reef in the Kimberley Basin of northwest Australia once stretched 350 kilometers (220 miles), surrounding a Devonian continent. Reefs are typically built by organisms that produce carbonate structures near sea level. Modern reefs are mainly made by corals and calcareous algae, but Devonian reefs were either built by cyanobacteria (microbial reefs) or by coral-like stromatoporoids and tabulate and rugose corals (coral-stromatoporoid reefs). Microbial reefs were more common during the warm early and late Devonian, while coral-stromatoporoid reefs were more common during the cooler middle Devonian.

By the Devonian, life was well established on land. The moss forests and bacterial and algal mats of the Silurian were joined by early plants with roots, which helped create stable soils and supported arthropods like mites, scorpions, trigonotarbids, and myriapods. The earliest known terrestrial crustaceans, Peracarida, also appeared in the Devonian. The largest land organism at the start of the Devonian was Prototaxites, a mysterious organism that may have been a large fungus, a rolled liverwort mat, or another unknown life form. It grew over 8 meters (26 feet) tall and towered over the low vegetation. The first known insect fossils appeared around 416 million years ago in the Early Devonian. Evidence of the earliest tetrapods (four-legged vertebrates) includes trace fossils in shallow lagoon environments during the Middle Devonian, though these traces have been debated and may instead represent fish feeding marks.

Many Early Devonian plants lacked true roots or leaves but had vascular tissue. Some, like Drepanophycus, spread through spores and vegetative growth. Early plants such as Cooksonia were short, leafless, and had spore-producing structures at their ends. Fossils of Armoricaphyton chateaupannense, about 400 million years old, show the oldest known plants with woody tissue. By the Middle Devonian, shrub-like forests of lycophytes, horsetails, ferns, and progymnosperms appeared. Most of these plants had true roots and leaves and grew quite tall. The earliest known trees, including lycopods, cladoxylopsids, and progymnosperms like Archaeopteris, appeared in the Middle Devonian. These plants could grow tall on land because they produced lignin, a substance that gave them strength, improved their vascular systems, and protected them from disease and herbivores. During the Eifelian stage, cladoxylopsid trees formed the first forests in Earth's history. By the end of the Devonian, the first seed-forming plants appeared. This rapid growth of plant life is known as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution.

The spread of plants on land acted as a carbon sink, possibly reducing atmospheric carbon dioxide levels. This may have cooled the climate and led to a major extinction event, known as the Late Devonian extinction.

Early arthropods co-evolved with the growing diversity of terrestrial plants. The relationship between insects and seed plants, which became a key part of modern ecosystems, began in the Late Devonian. The development of soils and root systems likely changed erosion and sediment patterns. The rapid evolution of terrestrial ecosystems with many animals created opportunities for vertebrates to move onto land. By the end of the Devonian, arthropods were firmly established on land.

Late Devonian extinction

The Late Devonian extinction is not one event, but several extinction events that happened at three different times: the Givetian-Frasnian boundary, the Frasnian-Famennian boundary, and the Devonian-Carboniferous boundary. These events are grouped together as one of the "Big Five" major extinction events in Earth's history. This extinction mainly impacted marine life, especially shallow warm-water organisms, but had less effect on cool-water organisms. The group most affected by this event was the reef-builders that formed large Devonian reef systems.

Marine groups such as brachiopods, trilobites, ammonites, and acritarchs were severely affected. About 96% of vertebrates, including conodonts and bony fishes, disappeared, and all ostracoderms and placoderms went extinct. Land plants and freshwater species, like early tetrapods, were not greatly affected by the Late Devonian extinction. However, some scientists believe the extinction nearly wiped out tetrapods.

The causes of the Late Devonian extinctions are still unknown, and all ideas remain theories. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact may have caused the extinction events. While there were collisions during the Late Devonian, such as the Alamo bolide impact, there is little evidence to support the existence of a large crater from that time.