The Devonian Period is the fourth period of the Paleozoic Era. This period started about 419 million years ago and ended 358.9 million years ago. The Devonian Period is also sometimes called the “Age of Fishes” because many types of fish (both marine and freshwater) appeared during this period.
The Devonian Period was a remarkable time in earth’s geologic history. It marked the beginning of proper life (both flora and fauna) on land. Major climatic and geographical changes also occurred during this time. Other events include a mass extinction that caused many marine organisms to die off and reef-building communities to almost become a memory. Despite this, terrestrial life thrived, with many plants and arthropods becoming even more diversified.
Devonian Period — Timeline
The Devonian Period, which lasted between 419.2 million years and 358.9 million years ago, was named after Devon, a country in the southwestern area of England. The controversial red-colored rock deposits found in this area puzzled geologists for years. The scientists were undecided about the structure of this rock and its time relationship with other rock deposits. This argument gave rise to the classification of the Devonian Period.
The exact age and timeline of the Devonian remain controversial to date. The Devonian Period as an interval on the geologic time scale, follows the Silurian Period and precedes the Carboniferous Period in the Paleozoic Era. The Devonian Period is further divided into Early Devonian, Middle, and Late Devonian.
Major Events of the Devonian Period
The earth underwent a lot of changes during the Devonian. The period is characterized by major changes in earth’s geology, climate, and lifeform. Two major events during this time were the formation of a supercontinent and a mass extinction event that caused the disappearance of about 70% of marine life.
Collision of the Continents
The giant supercontinent, Gondwana, was steadily forming in the north in the early years of the Paleozoic Era. Laurussia (Euramerica) is another supercontinent formed around the equator. The supercontinent was a combination of the landmass of North America, Russia, Greenland, and northern Europe.
During most of the Devonian, these landmasses gradually united into a single northern hemisphere landmass. This union of the paleo continents of Laurentia (comprising much of North America, Greenland, northwestern Ireland, Scotland, and the Chukotsk Peninsula of northeastern Russia) and Baltica (now most of northern Europe and Scandinavia) occurred near the beginning of the Devonian.
Devonian Period Mass Extinction
This mass extinction during the Devonian Period is the second most brutal of the five greatest mass extinctions on earth. It is also called the Late Devonian extinction. Unlike many mass extinctions that occurred within a short time, the Devonian mass extinction was more prolonged and lasted for the last 20 million years of the period.
The effects were so severe that three-quarters of all the existing animals at that period went extinct. The Kellwasser and Hangeberg events were the two longest episodes of the mass extinction. The former killed off coral reefs, jawless fish, and trilobites, while the latter caused early ammonites and placoderms to die off.
The cause of the extinction has been attributed to the global cooling of the climate and oxygen loss in the Devonian oceans. Despite the intensity of these events, terrestrial organisms were not as affected as the marine species.
The Devonian Climate
Scientists have found little to no evidence of polar ice caps from the Devonian period. This suggests that the climate was warmer at the time. The Devonian climate was mild, with warm waters. There was no significant difference in the temperature at the equator and the polar regions. Other pieces of evidence that point to higher temperatures during this period include:
- The wide distribution of evaporite basins in the Northern Hemisphere
- The abundance of coal deposits in some parts of the world
- Widespread desert conditions and carbonate reefs
- Devonian salt deposits, which indicate high evaporation rates
By studying coral reefs, scientists also discovered that the Devonian had long years. The earth took 420 days to complete its revolution around the sun instead of 365 days. The conditions favored the production of some of the largest reef complexes in the world in the equatorial seas. Stromatoporoids, as well as corals, increased greatly during the Devonian. These organisms helped cover the seas into large basins with widespread mineral deposition.
Terrestrial Life of the Devonian
Plants started to show up in the Age of Fishes. The first plants sprang up in wetlands and adapted until they could survive away from the water. The earliest plants on the planet had no roots or leaves. They were also void of vascular tissues and were only a few centimeters tall. The animals that thrived among these plants were mainly arthropods.
The rise of vascular plants altered the environment and formed marshlands and forests. This eventually caused the formation of complex soil and more stable habitats. Stemmed plants with strong structures (some as tall as 100 feet) made up the first forests. The trees had numerous branches and leaves.
The first ferns, horsetails, and seed plants appeared at the end of the Devonian Period. The fast appearance of so many plant groups at the end of the Devonian Period has been termed the “Devonian Explosion.” The diversification also brought about an increase in the variety of arthropods that existed. It is important to note that the arthropods were already on land before the Devonian. However, they became more widespread as the plants gradually shaped the landscape and provided favorable habitats for their proliferation.
Marine Life of the Devonian
There was a significant increase in organic matter in the water bodies and on land during the Devonian. The marine world of the Devonian Period had a large number of brachiopods. They (alongside red algae) were responsible for creating reefs in the waters. Ammonoids existed during the lower Devonian. Their shells formed large mineral deposits of limestone in the seas. Most trilobite groups, bivalves, and echinoderms were still present. However, most of them died out at the end of the period.
Devonian waters were also full of different kinds of fish. This is why it is referred to as the “Age of Fishes.” Fish existing in the Early Devonian were benthic, jawless, and armored. Placoderms and the first groups of jawed fish began to appear in the Middle Devonian. Many grew to impressive big sizes (as large as 33 feet) and terrorized the seas as predators.
However, the early fishes did not last long. The ancestors of the present fish belonged to two major groups in the Devonian, and they were not armored. They are cartilaginous and bony fish. The cartilaginous fish had cartilages instead of skeletons in addition to scales, fixed fins, and teeth. They eventually developed into modern-day sharks and rays. The majority of the fish present nowadays belong to the bony fish group. They possess fins that can be easily maneuvered, and swim bladders.
Lobefins were important Devonian bony fish. They became the ancestors of all land vertebrates with four limbs. Their fossils have been found in the red Devon Rocks. The lobefin species still exist in modern times as six species of lungfish and two species of coelacanths.
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- Britannica, Available here: https://www.britannica.com/science/Devonian-Period/Devonian-life
- University of California Museum of Paleontology, Available here: https://ucmp.berkeley.edu/devonian/devonian.php
- National Geographic, Available here: https://www.nationalgeographic.com/science/article/devonian