The Devonian Period is a geologic period of the Paleozoic Era that began approximately 419.2 million years ago and ended 358.9 million years ago.
The period is named after Devon, a county in southwestern England where rocks from this period were first studied.
Commonly referred to as the “Age of Fishes,” the Devonian Period is known for the greatest diversification and expansion of fish species in Earth’s history.
During the Devonian, various fish species evolved unique adaptations, which allowed them to occupy new ecological niches.
The first jawed fish appeared, and they became the dominant group in Earth’s marine ecosystems.
In addition to the various fish groups that rose, the Devonian Period also saw the rise of diverse marine invertebrates, including brachiopods, corals, and ammonoids.
Trilobites were present, but their population was starting to decline.
Away from the water, the Earth’s terrestrial habitats took on a new form during the Devonian Period as plants began to colonize the landscape.
Liverworts and mosses were the earliest plants to dominate freshwater environments and gradually spread across the land.
This set the stage for the development of terrestrial ecosystems that grew even more sophisticated during subsequent periods.
The Devonian Period is also known for the appearance of the first true forests.
These early forests included various primitive plants, including lycophytes and ferns.
They played a crucial role in stabilizing soils and creating habitats for other organisms, such as the terrestrial arthropods and the first terrestrial vertebrates that became more common as the period progressed.
Additionally, the establishment of forests made up of more advanced plants with leaves and true roots likely had a significant impact on global climate during the latter half of the period.
The period ended with an extinction event that occurred around 360 million years ago.
Marine life was the most affected by this event which resulted in the decline of many reef-building organisms, trilobites, and several groups of fish.
In this article, we’ll highlight all the events and key developments that took place over the course of the period and how they shaped the Earth’s ecosystem over time.
Timeline of the Devonian Period
The Devonian Period is the fourth period of the Paleozoic Era.
This 61 million years period began roughly 419.2 million years ago and lasted till 358.9 million years ago.
The Devonian was preceded by the Silurian Period and was followed by the Carboniferous Period.
The Devonian Period is commonly divided into three sub-periods or epochs, which represent specific geological and biological events. They include:
- Early Devonian (419.2 to 393.3 million years ago)
- Middle Devonian (393.3 to 382.7 million years ago)
- Late Devonian (382.7 to 358.9 million years ago)
Early Devonian (419.2 to 393.3 Million Years Ago)
At the start of the Devonian Period, the Earth’s Southern continents started coming together to form the supercontinent Gondwana.
This supercontinent included the landmass of present-day Africa, South America, Antarctica, Australia, and parts of Asia. It was located close to the South Pole.
The climate was warm, and the levels were high during this period which favored the evolution and diversification of various marine animal groups.
The first jawed fish appeared, marking a significant milestone in vertebrate evolution.
The bony skeletons and jawed mouths of these fishes provided them with a competitive advantage in their marine environments.
The first ammonoids also evolved from bacteroid nautiloids during this period.
This class of cephalopod mollusks remained a dominant part of the marine fauna for the rest of the Paleozoic Era.
Plants began to gradually colonize the terrestrial habitat during the Early Devonian.
The Devonian Plant Explosion, which was a period of rapid diversification of plant life, began about 428 million years ago, shortly before the Devonian began.
Early terrestrial ecosystems were dominated by mosses and liverworts, which gradually spread across the continents.
Early forest habitats started to take shape and were composed of lycophytes and ferns.
Middle Devonian (393.3 to 382.7 Million Years Ago)
During the Middle Devonian, the continents began to drift towards each other.
The landmass Laurussia (which included North America) gradually started coming together with Baltica (Western Europe) to form the Eurasian landmass.
Gondwana began a northward drift during this period as well.
The diversification of the marine ecosystem continued during this Middle Devonian.
The jawed fishes evolved into more specialized forms, including predatory species.
The growing abundance of these predator species led to the decline of jawless agnathan fishes.
The expansion of forests on land was a notable feature of the Middle Devonian.
Early trees emerged, transforming the landscape, and plant diversity increased.
This period saw the rise of new plant groups, such as the progymnosperms, which were precursors to seed plants.
The early tetrapods, ancestors of modern terrestrial vertebrates, began to evolve in the shallow, oxygen-deficient ecosystems formed by inland lakes during the Early Devonian.
Certain groups of early fish began to develop features such as lungs and modified pelvic fins.
These adaptations allowed them to gradually crawl onto land for short periods.
Late Devonian (382.7 to 358.9 Million Years Ago)
As the Devonian drew to a close, the continents continued to drift towards each other to form a single supercontinent that would later be known as Pangea.
The climate started to cool gradually, and glaciation began in the Southern Hemisphere.
The changing climate had significant effects on life in the Late Devonian, and the period ended with a major extinction event.
Marine life was the most affected by this event, with the disappearance of several reef-building organisms.
Trilobites, a group of marine arthropods that have been around since the Cambrian, also began to decline during this period.
Fish diversification did continue during the Late Devonian, with the appearance of more complex species.
Forests continued to expand and diversify during this time with the emergence of several new plant groups.
Climate and Geography of the Devonian Period
During the Devonian Period, the Earth’s landmasses were organized into two main supercontinents.
Gondwana was in the Southern Hemisphere, while Laurussia was closer to the Equator (on the northwest of Gondwana).
There were also smaller continents and microcontinents, such as Baltica to the east of Laurussia and north of Gondwana.
As the period progressed, all these land masses began to drift toward each other.
The climate of the Devonian Period was predominantly warm for most of the Devonian Period.
Sea levels were rising, and the movements in the continents led to the formation of oceanic basins, shallow seas, and inland lakes where aquatic organisms could evolve and diversify.
Diverse fish species and reef ecosystems developed in these aquatic environments.
The warm climate also contributed to the colonization of land by plants.
By the Middle Devonian, there were some changes in the Earth’s climatic patterns.
Although the climate remained warm, sea levels began to fluctuate.
The expansion of forest habitats on land due to the “greening of the Earth’s surface” may have also contributed to the changing climate.
The climate started to cool significantly towards the end of the Devonian.
Glaciation occurred in the Southern Hemisphere, indicating a shift towards a more temperate climate.
This climatic cooling and other factors contributed to the Late Devonian extinction event that led to the decline of numerous marine animal groups.
Key Events and Developments of the Devonian Period
The Devonian Period was characterized by major evolutionary advancements in marine and terrestrial ecosystems.
Although it is often known as the “Age of Fishes,” several other significant events happened during this period which includes the following:
Devonian Plant Explosion
One of the significant events of the Devonian Period was the colonization of land by plants.
The Devonian explosion began shortly before the end of the Silurian Period, about 428 million years ago and lasted till the end of the Devonian.
During this relatively short period, the diversity of plant groups increased significantly.
Early plants, such as mosses and liverworts, transitioned from freshwater environments to full terrestrial habitats and soon spread across the continents.
This colonization was a critical step in the development of terrestrial ecosystems and laid the foundation for the evolution of more complex plant life during subsequent periods.
Evolution of Fish
The diversification and expansion of fish populations that occurred during the Devonian is why the period was nicknamed the “Age of Fishes”.
Although Jawless fishes were present before the Devonian Period, new jawed fish with more complex skeletal structures (including armored varieties) evolved during this period.
Development of Early Forests
The Devonian Period saw the establishment of the first forests on Earth. These early forests were primarily composed of primitive plants, including lycophytes (such as club mosses) and ferns.
The establishment of forest habitats was quite significant because it allowed new organisms to evolve to fill the emerging niches.
The development of extensive forests also significantly impacted the global climate because it changed the planet’s atmospheric composition.
Major Groups of Organisms in the Devonian Period
The Devonian Period was characterized by the emergence of several new ecosystems caused by favorable climates and the Earth’s changing geography.
These changes led to the evolution of several plant and animal groups both on land and in aquatic ecosystems.
Some of the major groups of organisms that evolved during the Devonian include:
Placoderms
Placoderms were a group of armored fish that dominated the Devonian seas.
They had bony plates covering their heads and upper bodies, which is why they’re commonly regarded as the earliest known group of vertebrates with jaws.
Placoderms exhibited a range of body sizes and shapes.
While most of them were small, bottom-dwelling species, a few of them grew to large, predatory forms.
Acanthodians
Acanthodians are also known as “spiny sharks.”
This was a diverse group of fish with a shark-like appearance, and they’re considered the ancestors of modern sharks.
Acanthodians played a significant role in the Devonian marine ecosystems, occupying various ecological niches as both predators and prey.
Sarcopterygians
Sarcopterygians were a group of lobe-finned fish that would eventually give rise to the early tetrapods towards the end of the Devonian.
Members of this group were characterized by fleshy lobed fins supported by bony structures.
These lobed fins would later evolve into the limb-like structures of the early tetrapods.
Some sarcopterygians exhibited adaptations for shallow water and ventured into occasional terrestrial environments.
Ammonoids
Ammonoids are a group of marine mollusks that first evolved during the Devonian Period.
They are related to cephalopods like octopuses and squids and were quite abundant in the Devonian seas.
Many ammonoids survived the Late Devonian extinction and were alive until the end of the Cretaceous Period.
Reef-building Organisms
Many of Earth’s reefs that are still present today were built by carbonate-secreting organisms that lived during the Devonian.
The reef-building organisms of the Devonian were mostly autotrophic cyanobacteria or coral-stromatoporoid.
The warm conditions of the Devonian seas favored the abundance of this group.
Bryophytes
Bryophytes, such as mosses, liverworts, and hornworts, were among the earliest land plants to evolve during the Devonian.
These nonvascular plants did not have true roots, stems, or leaves.
They reproduced through spores and were mostly found in moist environments such as swamps.
Lycophytes
Lycophytes, such as club mosses and spike mosses, were the dominant land plants during the Early Devonian.
They were the first plants with advanced vascular tissues which allowed them to transport water and nutrients within their structures.
Lycophytes formed the small forests of the Early and Middle Devonian.
Seed Plants (Progymnosperms)
Primitive seed plants emerged towards the end of the Devonian Period.
Progymnosperms were transitional plants because they had the characteristics of both ferns and gymnosperms (seed-bearing plants that emerged later).
They had woody stems but still reproduced through spores instead of seeds.
Notable Species from the Devonian Period
Dunkleosteus
Dunkleosteus was a massive, predatory fish that lived in the Devonian seas.
It evolved during the Late Devonian and was the largest marine predator of that period, growing to lengths of up to 10 meters (33 feet).
Dunkleosteus was a placoderm with an armored head and formidable jaws lined with sharp, bony plates.
Tiktaalik
Tiktaalik is considered a transitional species because it had both fish-like and tetrapod-like characteristics.
Lobe-finned fish species like this represent a critical link in the evolutionary transition from a fully-aquatic life to early land-dwelling vertebrates.
Tiktaalik was probably able to support itself in shallow water or venture onto land.
Archaeopteris
Archaeopteris is an extinct genus of progymnosperm trees that lived in the early forests of the Devonian Period.
It is considered one of the earliest known woody trees.
Although it had a woody trunk, Archaeopteris still had fern-like leaves and reproduced through spores instead of seeds.
Trilobites
Trilobites were arthropods that existed throughout the Paleozoic Era.
They emerged during the Cambrian Period and were still quite abundant when the Devonian Period began, playing an essential role in marine ecosystems.
Although they were still alive at the end of the Devonian, their population began to decline during the Late Devonian extinction event.
Fossils and their Significance in Understanding the Devonian Period
Marine and terrestrial deposits from the Devonian Period are remarkable because they preserve tangible evidence of the animals that lived during this period and the evolutionary changes they went through.
The Devonian was previously referred to as the “Old Red Age” because of the red and brown terrestrial deposits from Devon (United Kingdom), where the earliest fossils from this period were found.
The Period is often referred to as the “Age of Fishes” due to the abundance and evolutionary diversity of fish fossils found in rocks of that period.
Although many fish groups existed before the Devonian, the emergence of heavily armored groups during this period led to better representation in the fossil record.
The presence of transitional fossils from this period has also made it easier for scientists to trace the emergence and diversification of various plant and animal groups.
For instance, fossils of progymnosperm plants represent an important transition from spore-bearing to seed-bearing plant groups.
Similarly, fossils of transitional species, like Tiktaalik, provide critical evidence for the transition from aquatic to terrestrial habitats and tetrapods’ evolution, which started during the Devonian.
The Devonian Plant Explosion and its Impact on Evolution
Also known as the Silurian-Devonian terrestrial revolution, the Devonian explosion was a period of rapid diversification of terrestrial plants which occurred during the Devonian Period (between 428 to 359 million years ago).
It technically started during the Silurian, but most of it took place during the Devonian.
The Devonian explosion is characterized by the proliferation of different plant groups, including mosses, liverworts, lycophytes, ferns, and early seed plants.
The diversification of plant life is often compared to the Cambrian explosion in terms of its scale and effect on plant and animal life.
Before this period, the early plant groups were primitive species that lived in the aquatic environment.
The emergence of new plant groups that soon spread across the continents had a significant impact on the biotic composition of the Earth’s soil and its atmosphere.
The new plant groups also organized into extensive forests that created diverse habitats and niches.
These new complex ecological systems allowed new plant and animal life to evolve and diversify further.
The Devonian plant explosion gave rise to new ecological interactions among organisms.
Plants became the foundation of terrestrial food webs, providing habitats, shelter, and food sources for other organisms.
The evolution of plants also led to the emergence of herbivory, as animals began to feed on plant tissues.
This, in turn, drove further adaptations and co-evolutionary processes between plants and herbivores.
The increasing abundance of land plants during the Devonian also led to a significant rise in atmospheric oxygen levels.
This change in atmospheric composition had profound implications for the evolution of organisms, as it provided an oxygen-rich environment necessary for the evolution of more complex life forms seen during subsequent periods of the Devonian.
Extinction Events and Their Effects on the Devonian Period
The Devonian Period ended with a series of extinction events (rather than just a single event).
This extinction event which began during the Late Devonian Epoch, was one of the five largest mass extinction events in Earth’s history when considered collectively.
Most records group the Late Devonian extinction into two main events, namely:
Kellwasser Event (Frasnian-Famennian Extinction)
The Kellwasser event started about 372 million years ago during the Late Devonian.
It mainly affected the marine ecosystems wiping out about 19% of all aquatic families and 50% of all genera.
It resulted in the decline and extinction of numerous marine species, including trilobites, brachiopods, and reef-building organisms.
Hangenberg Event (End-Devonian Extinction)
The Hangenberg event is a smaller extinction event that occurred 359 million years ago, bringing the Devonian Period to a close.
It was caused by multiple factors, including climate change, sea level fluctuations, and the release of toxic substances into the atmosphere due to volcanic activities.
This event resulted in the extinction of various fish groups, including some placoderms and acanthodians.
Sources
- https://www.britannica.com/science/Devonian-Period/Devonian-life
- https://en.wikipedia.org/wiki/Devonian
- https://ucmp.berkeley.edu/devonian/devonian.php
- https://www.nationalgeographic.com/science/article/devonian
- https://www.livescience.com/43596-devonian-period.html
- https://natmus.humboldt.edu/exhibits/life-through-time/visual-timeline/devonian-period
- https://www.livescience.com/43596-devonian-period.html
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