All you'd ever need to know about the Devonian Period
- The Devonian period is a geological interval in the Paleozoic Era that spans between the Silurian and the Carboniferous.
- It marks the first transition of vascular plants and certain animals from water onto land.
- And this process led to the evolution of some of the strangest creatures and plants to ever live and changed life on the planet forever.
The Devonian Period, named after a picturesque county in the United Kingdom, was one of the most critical periods in Earth's history for life on Earth. The changes in plant and animal life during this time set Earth on a path that would define the next tens of millions of years until today.
But why was it so important? Let's find out.
What was the Devonian Period?
The Devonian is a period of time or interval that encompasses the end of the Silurian, about 419 million years ago (Mya), to the start of the Carboniferous, around 359 Mya. It is, therefore, a geologic epoch and system of the Paleozoic ("ancient life") era, which spans from the Cambrian explosion to the Permian extinction.
The period is called the Devonian because the first studies of these rocks were done in the English county of Devon.
The Devonian Period is one of the most critical times in the evolution of most modern animal and plant groups because it is when many significant changes in life on land occurred. Vascular plants with free spores started to take over dry soil and grow into huge forests that covered the continents.
At the end of the Devonian, when the first plants with seeds appeared, many groups of plants already had real roots and leaves. Myriapods, arachnids, and hexapods (six-legged arthropods, like insects) have been on the land since the Ordovician period but became well-established early in this period.
Because there were so many different kinds of fish during this time, the Devonian is sometimes called the "Age of Fishes." Placoderms (which had heavy bony armor on the head and neck) began to rule practically every known aquatic ecosystem. The strong pectoral and pelvic fins of the ancestors of all four-limbed vertebrates (tetrapods) progressively transformed into legs. However, this adaptation to walking on land did not fully take place until the Late Carboniferous.
During this time, there were more primitive sharks in the seas than during the Silurian and Late Ordovician.
A group of mollusks known as ammonites exploded during the Devonian, too. While they appear to have evolved in the preceding Silurian period, the Devonian is when they began to dominate the ancient seas.
The enormous coral reefs, mollusk-like brachiopods, and trilobites were also very common creatures during the Devonian. All Placodermi (armored fish) and trilobites, except a few species of the order Proetida, perished during the Late Devonian extinction, which began around 375 million years ago.
Gondwana, a supercontinent, dominated the Devonian paleogeography to the south; Siberia, a small continent, was to the north; and Laurasia, a medium-sized continent, was to the east. Three important tectonic events are the closing of the Rheic Ocean, the split of South China from Gondwana, and the growth of the Paleo-Tethys Ocean.
As Laurussia and Gondwana moved closer, several critical mountain-building events happened during the Devonian.
These included the Acadian Orogeny in North America and n Europe, the beginning of the Variscan Orogeny. These early collisions occurred before the late Paleozoic extinction event and the emergence of the supercontinent known as Pangaea.
Devonian Period major events
We've already pretty much covered this in the section above, but for reference, the most significant events of this period were as follows: -
- The first marine organisms developed legs and began to walk on land as tetrapods, laying the foundations for most land-dwelling animals today.
- The earliest arthropods, insects, and spiders began occupying terrestrial habitats.
- The earliest seed-bearing plants colonized land, establishing massive forests.
- The first lobe-finned and bony fish, as well as the first sharks, emerged in the waters.
- The first ammonite mollusks arose, and extensive coral reefs and trilobites, the brachiopods that resemble mollusks, dominated the seas.
- The Devonian had several significant extinction events, culminating in an ice age that lasted well into the following Permian period.
- The Gondwanan supercontinent to the south, Siberia to the north, and the early creation of the small Euramerican supercontinent in the center dominated the paleogeography.
What kind of animals lived during the Devonian period?
The Devonian experienced generally high sea levels, and, as a result, fossils from the period are dominated by marine animals. For example, the bryozoa, ammonites, diverse and numerous brachiopods, the mysterious hederellids, microconchids, and corals are some of the most common fossils from Devonian rocks.
Lily-like crinoids ("sea lilies", animals despite their similarity to flowers) and trilobites were also rather prevalent. Bivalves became very common in deep water and conditions on the outer shelf.
When it comes to vertebrates, the diversity of the jawed fish (gnathostomes) grew in fresh and saltwater, while that of the jawless armored fish (ostracoderms) fell. In the early Devonian Period, armored placoderms were common; they went extinct in the late Devonian, possibly due to competition with other fish species for food.
Early fishes with bones and cartilage (Chondrichthyes and Osteichthyes) also developed into a varied group and were quite important in the Devonian seas. The Devonian Period saw the emergence of Cladoselache, the first widely distributed shark genus.
Because of the explosion in different types of early fish throughout the Devonian, it is sometimes dubbed the "Age of Fish."
The Devonian saw a significant increase in the diversity of nektonic marine life, dubbed the "Devonian Nekton Revolution" by many scientists. This diversification was likely caused by the abundance of planktonic microorganisms in the free water column and by high ecological competition in benthic habitats, which were extremely saturated with life.
However, it should be noted that this is hotly debated.
Devonian reefs, unlike today, were either microbial reefs made up mostly of autotrophic cyanobacteria or coral-stromatoporoid reefs made up of coral-like stromatoporoids and tabulate and rugose corals. In the warmer early and late Devonian, microbial reefs predominated, whereas coral-stromatoporoid reefs did in the cooler middle Devonian.
On land by the time of the Devonian Period, colonization of the land by life was well underway. Early in the Silurian period, primitive rooted plants joined the moss forests, bacterial mats, and algal mats to form the first stable soils and support arthropods like mites, scorpions, trigonotarbids, and myriapods.
But, arthropods first appeared on land much earlier than in the Early Devonian, and fossils such as Protichnites suggest that amphibious arthropods may have appeared as early as the Cambrian.
The mysterious Prototaxites (giant towering fungi), towering over the low, carpet-like flora during the early Devonian period, were the largest land organism at the time. Its exact composition is unknown. It may have been the fruiting body of an even bigger fungus, a rolled liverwort mat, or another organism of unknown affinities.
The period also saw the rise of early insects on land, from about 416 Mya.
Although it has been questioned, and an interpretation that they were traces of fish-feeding (Piscichnus) has been advanced, there is evidence for the earliest tetrapods (four-limbed vertebrate animals) in the form of trace fossils in shallow lagoon habitats inside marine carbonate platform/shelf during the Middle Devonian.
What are some examples of Devonian animals?
We've already discussed a few examples above, but some famous creatures from this period give you an insight into how alien the Devonian world was. Let's take a look at some of them.
1. Dunkleosteus was a bizarre creature
One of the earliest apex predators ever seen on Earth lived in the Devonian oceans. Called Dunkleosteus, this was a jawed fish that lived approximately 382–358 million years ago.
A placoderm (armored fish), this creature was giant, and its "teeth" might have been its most intriguing feature. Instead of teeth, Dunkleosteus had razor-sharp, self-sharpening bone plates.
This created a beak-like device that may have been able to pierce the shells of numerous fish and other armored organisms from the Devonian Period.
To date, Dunkleosteus species have been reported to number up to ten. With lengths of up to 29 feet, Dunkleosteus Terrell was this group's largest and best-known species.
Dunkleosteus may have been a slow swimmer despite being designed for hunting prey. According to scientists, the two-part bony armor of this fish is responsible for this. Experts also think the Dunkleosteus was among the first vertebrates to internalize egg fertilization, as with several modern sharks.
2. Ichthyostega is your distant ancestor
Aquatic animals started venturing onto land at the end of the Devonian. One of the earliest animals to truly live on land, Ichthyostega, existed 370 million years ago.
This genus's members were among the earliest creatures to have limbs that could support their weight on land.
The Ichthyostega is interesting because it incorporates traits from several groups of extinct animal species. This creature possessed tetrapod-like limbs and digits.
It is not, however, a true "tetrapod." It also lived in wetlands and shallow water and had an amphibian physique, although it wasn't a part of that group. Because of these characteristics, Ichthyostega is frequently regarded as an intermediary species between fish and tetrapods.
The 4.9-foot-long (1.5 meters) Ichthyostega was a sizable animal that may have had lungs for land-based breathing. It possessed huge teeth and dorsally positioned eyes on a translucent-skinned body. This species would have had difficulty on land even if it had feet rather than fins and could pull itself out of the water.
Although it is often reconstructed to appear like a salamander, it probably moved more like a seal or mudskipper.
3. Adelophthalmus was an interesting sea scorpion
Arthropods called eurypterids first appeared during the Ordovician Period and are also called sea scorpions. Although later eurypterid types lived in fresh or brackish water, the earliest eurypterids were marine creatures.
The group was the marine ecosystem's primary highlight during the Silurian Period. Many persisted until the Devonian Period when they evolved into some of the most amazing creatures ever.
One of the longest-lived was Adelophthalmus.
They existed from the Early Devonian to the Permian, a period of more than 120 million years. The eurypterid group's last surviving members included Adelophthalmus.
The Devonian was home to only one genus of swimming eurypterids; the others were bottom crawlers. The name of this relatively small sea scorpion alludes to what appears to be an absence of eyes, and it was presumably blind.
The length of the largest species in the Adelophthalmus genus was roughly 12 inches.
4. Megamastax was once the biggest vertebrate in the sea
In 2014, scientists discovered a fossilized fish that was once the largest vertebrate. Megamastax amblyodus, which translates to "large mouth, blunt teeth," was a predatory sea creature that roamed the oceans 423 million years ago.
It utilized its flat teeth to shatter the shells of its slow-moving, hard-shelled prey.
According to specific climate models, the period was characterized by hypoxia or low amounts of atmospheric oxygen. According to the study, marine fishes generally are also known to be less tolerant of hypoxic conditions than many aquatic invertebrates.
The maximum body size and accessible niche options of the first gnathostomes, or jawed vertebrates, may have been restricted in some way by low oxygen levels, according to research.
Fish jaw and teeth fragments were discovered in the Kuanti formation in China by vertebrate paleontologist Brian Choo and his Chinese Academy of Sciences colleagues.
M. amblyodus could have reached a length of up to 39 inches (1 meter), nearly three times larger than the next-largest fossilized animal of the epoch. This estimate is based on the size of the jaw.
5. Arctolepis was a very weird-looking fish
The extinct genus Arctolepis belonged to a class of fishes known as the arthrodires, or jointed-neck fishes, that lived in the early Devonian Period (416 million to 360 million years ago).
The body of Arctolepis tapered to a pointed tail; a tail fin only grew on the bottom edge of the seat. It had a bony head and trunk shield but was unarmed behind the trunk region.
Arctolepis was less than 12 inches (30 cm), yet it had incredibly developed pectoral spines that must have helped it swim and perhaps served as a deterrent to, presumably, avoid being devoured.
What did Devonian Period plants look like?
Despite the presence of vascular tissue in many of these plants, many Early Devonian plants lacked the roots and leaves of modern plants. Some of the earliest land plants, including Drepanophycus, were probably propagated by spores and vegetative development (think modern-day vines).
The earliest terrestrial plants, including Cooksonia (though it first emerged in the Silurian), were often quite short-statured and didn't rise much higher than a few centimeters. They were composed of leafless, dichotomous axes and terminal sporangia.
The oldest known plants with woody tissue can be found in the 400 million-year-old fossils of Armoricaphyton chateaupannense. By the Middle Devonian, primitive plants had developed into shrub-like forests, including lycophytes, horsetails, ferns, and progymnosperms.
Many of these plants were extremely tall, and most had real roots and leaves. In the Middle Devonian, trees first became visible. Among these were cladoxylopsids, the progymnosperm Archaeopteris, a clade of lycopods, and another arborescent, woody vascular plant.
These tracheophytes possessed the ability to biosynthesize lignin, which provided them physical rigidity and increased the efficiency of their vascular system while giving them resilience to infections and herbivores, which allowed them to grow to enormous sizes on dry land.
As far as we know, these are the oldest trees and formed the first recorded forests.
The first seed-forming plants also emerged by the end of the period.
All of this contributed to the "greening" of this period's continents, which served as an enormous carbon sink, perhaps lowering carbon dioxide concentrations in the atmosphere. This could have been so significant that it led to a plant-based global extinction event from global temperature changes and the influx of nutrients into shallow seas from land plants (more on that later).
What was the climate like in the Devonian Period?
Since the Devonian was a relatively warm epoch, glaciers were probably absent for most of it. The difference in temperature between the equator and the poles was not as significant as it is now.
The weather was also extremely arid along the equator.
Tropical sea surface temperature reconstruction from conodont apatite (a mineral derived from bone) suggests that the average temperature was 30 °C (86 °F) in the Early Devonian.
Throughout the Devonian Period, CO2 levels significantly decreased due to the proliferation of photosynthesizing organisms like plants. Carbon from the atmosphere was also drawn out of the atmosphere by the newly formed woods and buried in sediments.
This could have led to the apparent cooling in the middle of the Devonian of about 9 °F (5 °C).
The Late Devonian warmed to levels equivalent to the Early Devonian. The climate would have had an impact on the predominant creatures in reefs; in hotter eras, microorganisms would have been the primary reef-forming organisms, whereas, in cooler periods, corals and stromatoporoid sponges would have assumed the lead.
This then culminated in the "The Late Palaeozoic Ice Age," which began around the end of the Devonian when Earth quickly cooled to become an icehouse.
During the Devonian, the topography of the planet underwent significant alterations. During this time, the land on Earth was divided into two supercontinents, Gondwana and Euramerica. While a large ocean blanketed the rest of the planet, these enormous landmasses were located quite close to one another in one hemisphere.
There were subduction zones all around these supercontinents; with the formation of the subduction zone between Gondwana and Euramerica, a significant collision that would combine the two into one world-continent — Pangea — in the Permian was set in action.
Numerous significant regional geological actions also took place in addition to the global patterns of change. The collision of the North American and European continents caused enormous granite intrusions and raised the eastern North American Appalachian Mountains. Massive amounts of sediment were produced by the vigorous erosion of these recently raised mountains and deposited nearby in huge lowlands and shallow oceans.
As more stromatoporoids and corals emerged, extensive reef building continued, resulting in some of the largest reef complexes on the planet. These were constructed between the continents in the tropical seas. Large shallow oceans formed basins where significant amounts of rock salt, gypsum, and other minerals crystallized across North America, Central Asia, and Australia.
A huge extinction event happened toward the end of the Devonian. Since warm water marine species were most likely impacted, glaciation and lowering the world's sea level may have caused this crisis.
According to Berkeley University, changes in atmospheric carbon dioxide and mass extinction have been attributed to meteorite impacts.
What are some interesting facts about the Devonian period?
We've already covered a lot of ground about the period, but if you want some rapid-fire facts about the period, then please read on.
1. The first insects arose during the Devonian
Before amphibians started to inhabit the continents but not too long (geologically speaking) after the early land plants developed (in the Ordovician period), insects first appeared in the fossil record during the Devonian. According to some paleontologists, the oldest known insects are collembolans (springtails), whose fossils have been discovered in Devonian strata.
Something worth noting is that the first insects had no wings. However, they did develop flight shortly after appearing on land.
Interestingly, some studies support evidence that insects may have evolved during the Silurian or early Ordovician. But this needs a lot more work to verify.
Even in the Paleozoic, insects had diverse behaviors and environments, indicating that their evolution was likely closely linked to terrestrial plants and animals.
Regarding the number of species and individuals, insects are currently the most abundant terrestrial creatures on Earth.
2. The end of the Devonian was one of the "Big Five" mass extinction events
The phrase "mass extinction" may imply an immediate worldwide calamity, although these occurrences can take millions of years.
For instance, the End-Devonian, which lasted for more than 20 million years, was composed of a succession of pulses in climate change, including the "Hangenberg Crisis," which some scientists believe to be a distinct mass extinction event in its own right.
Intense volcanic activity in Siberia likely decreased ocean oxygen levels, and other environmental changes may have contributed to this event.
So, how severe was it? According to some estimates, about 70 percent of species and 35 percent of taxa became extinct. Sadly, all the placoderms were wiped out. Numerous coral species and other trilobites perished as well.
Most smaller creatures appear to have been the best able to adapt and survive in the aftermath, especially vertebrates less than a meter long (about 3.3 feet). Tetrapods, four-legged creatures that were moving from the sea to land and would later give rise to reptiles, amphibians, and mammals, were among the survivors too.
3. The Devonian was a great time for fish, well most of them
The Devonian, as we've previously explained, is also called the "Age of Fish" for a good reason. This is because a remarkable diversity of fish emerged during this time.
The most formidable of these were the armored placoderms, a subclass with strong jaws lined with bladelike plates that served as teeth that first appeared during the Silurian.
Early placoderms consumed invertebrates like mollusks, but later species evolved into aggressive fish-eating monsters up to 33 feet (10 meters) long. Other jawless fish with bony plates evolved a variety of strange shapes. There are fossil examples of species with rounded shield-like heads and some with horseshoe-shaped skulls.
These ancient fish weren't made to last, despite their robust defense. Fishes still alive today descended from two primary nonarmored groups during the Devonian Period.
Sharks and rays, for example, were subsequently descended from the cartilaginous fish, so named because of the cartilage that made up their skeletons. They featured sharp, replaceable teeth, fixed fins, and tiny, rough scales. The second group of bony fish had scales covering their bodies, movable fins, and swim bladders filled with gas to regulate their buoyancy. Modern fish are primarily bony fish.
Lobefins were among the bony fish that also appeared during this period. Known for their thick, fleshy fin bases, lobefins are considered the ancestors of all four-limbed land vertebrates, including dinosaurs and mammals.
They are attributed to the significant evolutionary step that gave rise to amphibians. These fantastic animals' fossils were found in Devon's red rocks. Some lobefins are alive and well today, including the well-known coelacanth fish, a "living fossil."
4. Plants also had a good time during the Devonian
Plants underwent a massive explosion in diversification during the Devonian, most notably on land.
To sustain their weight in the air, which is significantly less dense than water and consequently less supporting, plants needed stronger architecture as they migrated from their aquatic surroundings onto continents. Species with thick stems, and eventually wood, gained an advantage in this and the need to preserve water.
Many land plants also developed a vascular system to move food and water through these stems during this time.
Terrestrial plants at the start of the Devonian were typically small (about an inch tall) and lacked roots, seeds, leaves, or woody tissue. Because the organisms' tissues were incapable of withstanding the stresses brought on by considerable vertical growth, plant height was constrained. The emergence of roots, seeds, leaves, and woody tissues contributed to the diversification of species, the growth of big trees, and ultimately the creation of the first forests.
The ability of plants to "anchor" into the soil and increase water and nutrient absorption also occurred during the Devonian. As plants spread throughout the continents, soil as we know it developed with a high concentration of organic matter from decomposing vegetation (and other organisms).
Longer distance dispersal was made possible by the development of seeds, which also shielded reproductive tissue from dry seasons. A greater photosynthetic surface area was provided by leaves, allowing for more effective food production. Finally, a method for allowing extensive vertical growth was supplied by developing woody tissues (and effective vascular networks).
5. And they (plants) almost wiped out animal life on Earth
A recent study in the Geological Society of America suggests that a sequence of extinction events that took place between 419 and 358 million years ago may have been caused by plants. Although animal life hadn't fully migrated to land during the Devonian, plants had, and they took off.
Marine habitats were severely impacted by a succession of extinction events throughout the Devonian, including one of the five major mass extinctions, as we previously described. More than two-thirds of all species on the earth had been wiped off by the time everything was said and done.
According to scientists, the evolution and spread of the earliest land-based plants may have directly contributed to the extinction of marine animals. The theory is that when plants continued to colonize land, they formed root systems that loosened the soil and released previously landlocked nutrients. An enormous amount of nutrients from the land entered the oceans as soon as roots developed.
But how? Today, when large amounts of fertilizer enter water environments, we tend to see large algae blooms that quickly kill thousands or millions of fish. The algae consume all the oxygen and release toxins into the water column as dead algae die and are decomposed.
Called eutrophication, the global-scale mass release of nutrients from the land leading to a significant decline in available oxygen would have made it very difficult for water organisms to thrive, possibly eradicating entire species or ecosystems.
And that is your lot for today.
The Devonian period was one of Earth's most exciting and important geological periods. Vascular plants and animals made their first attempts to colonize the land and, in a manner of speaking, literally laid the foundations for the future of all life on the planet.
Ultimately, you and everything around us on land today can thank these early pioneers millions of years ago.
Why do we do it, how can we stop it, and who else is at it?