Lisa has studied natural history for over 15 years and is a fossil collector as well. She loves sharing her knowledge with others.
A Pivotal Time For Both Aquatic and Terrestrial Life
Imagine a world where fish never existed and vertebrates never colonized the land. Could another group of animals have taken their places as evolutionary tycoons, or would arthropods have been left to rule the planet unopposed? If insects did not face competition or predation from fish and amphibians, could they have evolved into massive super predators and held that role to the present day? It is impossible to know for sure, but many people believe a world without vertebrates would be bleak and insignificant.
The Devonian Period (419-358 million years ago) saw the emergence of many exceptional lifeforms, including the world's first terrestrial vertebrates. It is often known as the "Age of Fishes" due to its impressively diverse fish population, which consisted of everything from lobe-finned fish (the ancestors of terrestrial vertebrates) to placoderms (armored fish). While fish expanded and took their first steps onto land, arthropods branched off into various new groups and began colonizing the land in larger numbers.
When these new animals appeared, they discovered a landscape occupied by sparse forests that contained the planet’s earliest trees. The environment provided a safe haven for which they could escape terrifying aquatic predators like the whale-sized placoderm Dunkleosteus and the dolphin-sized eurypterid (sea scorpion) Jaekelopterus. As the time passed, plants and animals became better suited to life on land and developed some of the most monumental adaptations in Earth's history.
This article takes an in depth look at the Devonian Period and its remarkable lifeforms, including the following animals:
During the Devonian Period, the atmosphere contained a high amount of carbon dioxide that resulted in relatively warm temperatures throughout most of the globe. There were likely no ice caps, and sea levels were quite high around 400 million years ago. Approximately 85% of the world was covered by water, including large portions of North America and Europe.
Conditions were ideal for reef building, and extensive reef ecosystems could be found in the shallow waters surrounding each continent. These ecosystems contained numerous horn corals, tabulate corals, trilobites, brachiopods (shelled animals that frequently resemble clams and oysters in appearance), gastropods (mollusks such as snails and slugs), and crinoids (plant-like animals that are related to starfish and sea urchins). Brachiopods were particularly abundant, reaching their widest diversity during the Devonian Period.
Ammonites (cephalopods that possessed external shells) colonized the seas in large numbers as well. Many had coiled shells, while others possessed straight, cone-shaped shells. They were free swimmers that spent some of their time feeding along the ocean floor. Ammonites probably had similar diets to today's squids and octopuses, feeding on brachiopods, crustaceans, and fish. It is likely that they consumed trilobites on a regular basis, as these arthropods were quite abundant and would have been fairly easy to catch.
Arthropods were one of the most prominent groups of the Devonian aquatic fauna, especially trilobites and eurypterids (sea scorpions). These two types of shelled creatures appeared in many diverse forms and thrived in aquatic environments throughout the world.
Trilobites are known for having the widest species diversity, and their group contains some of the most bizarre and puzzling animals of the Devonian Period. Numerous species were covered in spines like a porcupine, while others had an extremely long, beak-like projection jutting from the front of their head. Some even possessed an elongated, forked appendage on their head that may have been used for hunting. Devonian trilobites ranged in size from under 0.5 inch (1.3 centimeters) in length to over 8 inches (20 centimeters) in length. Various species were free swimmers, but most were likely bottom dwellers that fed on soft-bodied creatures or plants.
Eurypterids were the rulers of their arthropod family. They were capable of growing to more than 8 feet (2.4 meters) in length, and they terrorized both saltwater and freshwater ecosystems. A large number of eurypterids resembled modern terrestrial scorpions, possessing a pair of pincers and a deadly spiked tail that they used to subdue and finish off their prey. However, despite the similarities, their closest living relative actually is the horseshoe crab.
In the past few decades, researchers have obtained substantial evidence supporting the idea that some eurypterids were capable of terrestrial locomotion. Fossilized trackways have been discovered in many locations throughout the world, including the United States, Germany, Scotland, China, and Australia. The tracks resemble those of their modern horseshoe crab cousins, and some are quite large, indicating the eurypterids that made them were probably around 7.5 feet (2.3 meters) in length.
In addition, CT (computed tomography) scans have revealed pillar-like trabeculae on some eurypterid specimens' gills, which suggests they had a dual respiratory system that enabled them to survive on land for short periods of time.The trabeculae helped prop the gills apart and prevented them from collapsing if these animals exited the water. Some scientists believe eurypterids occasionally travelled onto land so that they could move between different pools of water and lay their eggs in safer areas.
Jaekelopterus was one of the most formidable arthropods of the Devonian Period. This animal was estimated to reach up to 8.5 feet (2.6 meters) in length, making it the world's largest known eurypterid and the largest arthropod that ever lived. Its strong mouthparts, serrated claws, and sophisticated compound eyes indicate that it was a powerful apex predator. Jaekelopterus probably ate anything it could capture, from smaller arthropods to fish and primitive amphibians.
Similar to many eurypterids, Jaekelopterus had four pairs of walking legs and one pair of paddle-like swimming legs. It also possessed a flat, fan-shaped tail that aided in swimming. While it likely was not a very fast swimmer, its tail design suggests it was capable of hovering and making quick turns. Some scientists believe this arthropod mainly hunted in lagoons and estuaries, and it probably spent much of its life in shallow, freshwater environments.
The first fish appeared around 530 million years ago, and they branched off into a wide variety of remarkable species during the Devonian Period. Jawless fish (including ostracoderms) and placoderms reached their peak diversity during the Devonian Period, and sharks, lobe-finned fish, and ray-finned fish began colonizing the seas in large numbers. Fish competed with arthropods (like the eurypterids) for food and territory, and they probably fed on these invertebrates whenever possible.
The Devonian Period saw the emergence of numerous notable shark and shark-like groups. Each group possessed a shark-like skull and a skeleton that was made almost entirely of cartilage rather than bone. However, many did not have a traditional torpedo-shaped body. Some looked more like an eel than a shark, and others had spines in front of each of their fins or a large, anvil-shaped dorsal fin topped with bristles. Most of these animals probably were agile carnivores that fed primarily on other fish. Surprisingly, they only grew to around 6 feet (1.8 meters) in length, unlike many modern sharks.
Cladoselache is one of the best known shark-like fish of the Devonian Period, partially due to its well preserved fossils that are abundant in the Cleveland Shale of Ohio (located in the United States). Some are so well preserved that their cartilaginous skeletons have remained mostly intact, and they still contain traces of skin, muscle tissue, and internal organs. Fossil evidence indicates that Cladoselache's head shared some similarities with modern frilled sharks. However, this animal possessed a torpedo-shaped body and forked tail like today's mackerel sharks, including the great white. Its streamlined body suggests it was a fairly fast swimmer that was capable of making quick, sharp turns.
The largest Cladoselache specimen ever discovered was only about 6.6 feet (2 meters) in length, and many other specimens were significantly smaller. Like most prehistoric sharks and shark-like fish, it possessed a fin spine, which was located in front of its first dorsal fin. The spine curved backward and likely served as a defense weapon against larger predators. Cladoselache also had thin, smooth-edged teeth that it would replace periodically. Its teeth were suitable for grasping prey, but not for tearing them apart. Studies of this animal's stomach contents indicate that it feasted on hagfish-like proto-vertebrates, ray-finned fish, and shrimp-like arthropods.
Although sharks proved to be quite successful during the Devonian Period, they were not the apex predators of their aquatic world. Armored fish were the dominant aquatic vertebrates at the time. They came in a wide array of diverse forms, and many had cartilaginous skeletons like those of sharks. These animals were split into two distinct groups, known as the ostracoderms and placoderms. Both groups were known for their articulated armored plates that covered their heads and thoraxes, but they possessed different diets and feeding habits from one another.
Ostracoderms were the first fish to appear on Earth, and most scientists believe they were also the first vertebrates to have ever existed. They were jawless, making them incapable of biting or snatching prey in a manner similar to other carnivores. Instead, they had to use their muscular pharynxes to suck small, slow moving prey into their mouths. Some were active swimmers that probably filter fed near the water's surface like modern humpback whales, while others were bottom feeders that scavenged like today's catfish. Their diets were likely limited to foods such as plankton, decayed plant and animal matter, worms, and algae.
These bizarre, jawless fish were among the earliest animals to possess bony heads, and researchers have discovered that the head armor of many ostracoderms was actually comprised of tiny teeth. The armor contained thousands of teeth that each had their own enamel and a pulpy center like teeth inside a mouth. This bony plating helped protect them from a predator's bite. Ostracoderms were also different from other fish in that they used their gills exclusively for respiration and never for feeding. They possessed separate gill pouches along the sides of their heads, which were permanently open and lacked protective gill covers.
Many ostracoderms were known for their perplexing body designs, which ranged from large, rounded thoraxes with long, skinny fins to wide, crescent-shaped heads with flat, tapered bodies.
Larnovaspis was especially strange looking, as it possessed an elongated, horn-like nose and had no fins other than a lobed tail fin. Although this ostracoderm lacked fins, it probably was a good swimmer due to a pair of rigid, wing-like projections located on each side of its body. Larnovaspis also had a streamlined body, which was perfect for swimming.
Researchers believe this fish was capable of maneuvering with ease, but its head and thorax armor likely prevented it from being a swift swimmer. This was presumably the case with all ostracoderms and placoderms. Larnovaspis grew to about 7.9 inches (20 centimeters) in length and possessed a row of stiff spikes along its back, which may have provided additional protection from predators. A sharp spine protruded from its armor and probably played an important role in defense as well. Some scientists believe Larnovaspis primarily fed on plankton just beneath the ocean's surface.
Placoderms were among the first jawed fish to appear on Earth, and they were the rulers of the aquatic vertebrate group during the Devonian Period. Some were capable of reaching around 30 feet (9.1 meters) in length, which was larger than any other known animal from that time. Their powerful jaws contained razor-sharp plates that functioned as teeth and could crush the armor of many fish. Contrary to other jawed vertebrates, most placoderms did not have any true teeth and did not descend from toothed animals. While some species may have had reasonably good vision and fed on anything they could catch, others likely were blind bottom feeders with limited diets.
Placoderms are the earliest vertebrates for which researchers have some insight regarding their coloration in life. In 1997, scientists discovered that a placoderm fossil from Antarctica contained preserved pigment cells. The specimen's dorsal side (back) contained red pigment cells, while its ventral side (belly) contained iridescent silver pigment cells. Based on this data, one can assume at least some placoderm species had color vision like that of many modern fish. They may have used their varied pigments to attract mates or camouflage themselves from larger predators.
There is still much debate over whether placoderms should be classified as the direct ancestors of all other jawed fish, as a group between sharks and bony fish, or as a sister group of sharks. Not all placoderms may have been closely related to one another, and it is quite possible that sharks actually originated from one placoderm group. Placoderms and sharks possess numerous affinities, including similar jaws and nasal regions, postcranial cartilage, claspers (used to transfer sperm to a female's body during copulation), and cartilaginous fins.
However, these armored fish were distinct from bony and cartilaginous fish on the basis of their jaw muscles (which were located on the inside of the jaw) and their skull bones (which were incomparable to those of other fish). Placoderms were also different in that they had a single gill opening.
Dunkleosteus was one of the largest known placoderms that ever existed on Earth, and it was estimated to reach about 30 feet (9.1 meters) in length. Its fossils have been discovered in numerous countries throughout the world, including the United States, Canada, Belgium, Poland, and Morocco. This terrifying fish reigned as the Devonian Period's apex predator, feeding on anything from ammonites to members of its own kind. It was a powerful swimmer that lived in both deep water and shallower coastal ecosystems.
While Dunkleosteus was best known for its thick, durable armor, its inner skeleton was mostly comprised of cartilage rather than bone. This placoderm's gill arches, brain case, vertebrae, and fin supports were all cartilaginous like those of today's sharks. Dunkleosteus and most other placoderms may have also been among the first vertebrates to reproduce via internalized egg fertilization, which is found in some modern sharks.
Fossil evidence suggests that juveniles possessed true teeth. However, these teeth wore down with age, creating sharp, opposing edges which served as highly effective cutting surfaces. An interesting trait of Dunkleosteus was its ability to sharpen its jaw plates simply by opening and closing its mouth. Its powerful jaws were capable of delivering a high bite force that could puncture the armor of virtually any animal, including another Dunkleosteus.
Proof of cannibalism among these fish has been discovered by studying their armor, coprolites (fossilized feces), and regurgitated meals. Researchers noted that the fossilized neck armor of one medium-sized Dunkleosteus had been bitten into two pieces, and the bite marks of a larger Dunkleosteus were clearly visible. Their coprolites and regurgitated food have been known to contain the semi-digested bones of other massive predators as well. Most scientists are quite confident these bones belonged to members of their own genus, as we don't know of any other predator that matched them in size.
Two other distinct fish groups that swam the Devonian waters were ray-finned and lobe-finned fish. Unlike sharks and many armored fish, their skeletons were bony rather than cartilaginous. Ray-finned fish were much less diverse during the Devonian Period than they are today. In fact, they currently make up almost the entire bony fish population, with over 20,000 species in existence. Their pectoral and pelvic fins are supported by thin, bony spines (rays) and are powered primarily by muscles within the body trunk.
Lobe-finned fish were the dominant bony fish group of the Devonian Period, and they were much more diverse than they are today. Lungfish and coelacanths currently are the only two types of fish from this group that still exist. Lobe-finned fish are known for their long, muscular pectoral and pelvic fins, which articulate with the shoulder or pelvis via a single bone. This advanced feature makes locomotion on land possible for such fish, and it enabled ancient lobe-finned fish to evolve into the planet's first terrestrial vertebrates around 365 million years ago.
Most Devonian lobe-finned fish possessed two dorsal fins with separate bases, rather than a single dorsal fin like ray-finned fish. Many also had a symmetrical tail, and they all possessed teeth that were covered with true enamel. Besides this, they sported a set of gills and one or two lung-like organs. These early lungs allowed them to survive in low-oxygen, shallow waters and terrestrial environments. Similar to modern lobe-finned fish, they were carnivores that thrived in both freshwater and saltwater ecosystems and fed on a large variety of different animals, including smaller fish, cephalopods, snails, worms, and crustaceans.
Tiktaalik was a particularly remarkable member of the lobe-finned group. Its fossils have been recovered from the Fram Formation of Ellesmere Island, Canada, and it was estimated to reach up to 9 feet (2.7 meters) in length. This creature shared many similarities with tetrapods (four-limbed animals) and filled an evolutionary gap between them and fish. In fact, it may have even been the common ancestor of all terrestrial vertebrates. Tiktaalik was best known for its muscular, arm-like fins, primitive lungs, sturdy ribcage, movable neck, and flat, crocodile-like head. Its fish-like features included lobed fins, body scales, gills, and a tubular, streamlined body.
Tiktaalik's front fins consisted of a shoulder, elbow, wrist, and simple rays that slightly resembled finger bones. Its fin bones were strong and showed large muscle facets, which suggests that they were weight bearing and capable of flexing like a wrist joint. This animal also had massive shoulders with expanded scapular and coracoid elements. Tiktaalik could utilize its advanced fins to anchor itself to the bottoms of fast moving streams, walk along the floors of shallow water bodies, and support itself on land for brief periods.
A trait that was especially important to Tiktaalik's success was its unique respiratory system, which enabled it to survive in both aquatic and terrestrial environments. This fish likely spent around 99% of its life in the water, and its lungs would have helped it thrive in shallow, low oxygen waters that many other aquatic animals could not survive in. As a result, Tiktaalik would have enjoyed the benefit of few predators and little competition. The ability to travel onto land would have added to its success, as it could allow Tiktaalik the opportunity to safely explore an ecosystem which had not yet been dominated by any other large animal.
Unlike most fish, this extraordinary creature lacked bony plates in its gill area that restrict sideways head movement, making it the earliest known fish to have a flexible neck. Its neck would have been particularly useful when hunting. Tiktaalik also possessed two rows of sharp teeth and strong, well developed jaws that were capable of grasping and shredding prey.
As the Devonian Period progressed, many lobe-finned fish developed an increasing number of amphibian-like traits and evolved into the world's first aquatic tetrapods. They were the earliest animals to sport limbs, fingers, and well developed lungs, but scientists are uncertain as to how much time they spent on land. These primitive tetrapods may have occasionally traveled onto land to escape aquatic predators or move between different water bodies for breeding purposes.
Tetrapod footprint fossils have been discovered in multiple countries, including Ireland and Poland. Some date back to 395 million years ago, and researchers found that one of the trackways was created by an animal as large as 9.8 feet (3 meters) in length. These fossils have taught researchers that tetrapods did not only live in freshwater environments like ponds and streams. Many also thrived in saltwater environments, such as marine intertidal zones.
Scientists initially believed vertebrates took their first steps onto land from a freshwater setting, and they had associated this event with the development of a terrestrial ecosystem. However, tetrapods actually may have first left the water to consume marine life that was stranded by the receding tide, rather than for the purpose of feeding on terrestrial arthropods or vegetation.
Ichthyostega was one of the earliest four-limbed vertebrates to appear on Earth, and it also was one of the first to have weight-bearing adaptations for terrestrial mobility. Its limbs and lungs helped it navigate through swamps and other shallow water bodies. It shared numerous physical traits with modern amphibians and looked fairly similar to a salamander. However, this animal is not considered to be a true amphibian in the narrow sense, as modern amphibians (which belong to the group Lissamphibia) did not appear until the Triassic Period (252-201 million years ago). Rather, it was a stem tetrapod with a mixture of fish and amphibian features, but it possessed more amphibian qualities than other tetrapodomorphs did.
Ichthyostega grew to about 4.9 feet (1.5 meters) in length and possessed a broad skull with large teeth. Its diet likely consisted of various fish and aquatic tetrapod species. Its limbs were fairly large, and each hindlimb had seven digits. The exact number of digits on its forelimbs is currently unknown due to a lack of Ichthyostega hand fossils in the record. This animal's feet functioned like fleshy paddles rather than fins, but it probably was a strong, fast swimmer. It also possessed strong shoulders and hips that were better adapted to life on land than those of many other Devonian tetrapodomorphs. In addition, Ichthyostega had more supportive ribs and a stronger backbone.
This creature's spine was largely fish-like, and its massive ribcage was composed of broad, overlapping ribs. It is highly likely that Ichthyostega was capable of using its limps to pull itself out of the water. It probably traveled on land for brief periods in search of food or a safe place that was devoid of predators. However, Ichthyostega's hindlimbs were smaller than its forelimbs, meaning that it presumably could not bear full weight at the rear half of its body. This animal's forelimbs possessed the necessary range of motion to push its body up and forward, but they lacked the rotary motion range that is required for normal quadrupedal locomotion. As a result, Ichthyostega may have only been capable of traveling across flat land, and it could have moved in a manner similar to that of a mudskipper or seal.
The Dawn of Earth's Forests
While many aquatic lifeforms were expanding and diversifying, Earth's terrestrial landscape was beginning to blossom into a vast network of new, complex plant species and large, woody trees. Plants with complex root systems, seeds, megaphylls (leaves containing a web of branched veins), and woody tissues all first appeared during the Devonian Period.
Advanced root systems improved water and nutrient absorption and enabled plants to anchor themselves into soil. Seeds possessed facilities for nourishment and defense, which helped to better prepare plants for the outside world and increase their chances of survival. Seeds also were capable of sprouting almost anywhere, unlike spores, which required a wet environment for germination. Plants with megaphylls were capable of capturing more solar energy than those that had microphylls (leaves containing a single, unbranched vein). Woody tissues allowed plants to survive in harsher conditions, and they also offered more support so that the plants could grow taller.
An increase in plant diversity and complexity led to the world's first forests. These forests included ferns, horsetails, lycopods, and large, wooded trees like Archaeopteris. Archaeopteris was one of the earliest known trees. It appeared about 385 million years ago and grew over 80 feet (24 meters) tall. The major expansion of plant life had significant impacts on Earth's atmosphere. A substantial rise in photosynthesis levels greatly increased the concentration of oxygen in the air, aiding in the terrestrialization of tetrapods. However, these plants removed large amounts of carbon dioxide from the atmosphere and may have been responsible for global cooling, which is considered to be a contributor of the Late Devonian Extinction.
Terrestrial invertebrates of various forms began colonizing the planet during the Devonian Period, and extensive forests of leafy, seed-bearing plants provided the perfect environment for them to thrive and spread. These animals included wingless insects, arachnids (like spiders and scorpions), centipedes, and millipedes. Some (including the centipedes and millipedes) were quite similar in appearance to their modern descendants. While many of these small creatures were herbivores that fed on vegetation, some were carnivores that hunted other terrestrial invertebrates. They probably also were a secondary food source for numerous early tetrapods.
The Devonian Extinction
The end of the Devonian Period is considered to be the second of Earth's "big five" mass extinction events. The Devonian Extinction did not occur in just one event. Instead, it occurred in at least two long episodes of species depletion and several shorter periods. The Kellwasser Event of the middle Devonian was mainly responsible for the mass depletion of great coral reefs, trilobites, and jawless fish. The Hangeberg Event of the Devonian/Carboniferous Boundary wiped out all placoderms and ostracoderms, as well as most ammonites.
Scientists are uncertain as to what triggered these Devonian extinction events. However, it is likely that episodes of reduced dissolved oxygen levels in the oceans resulted in the extinction of many aquatic species, especially marine animals. Falling sea levels during the Devonian Period could have contributed to their demise as well. It is possible sea levels fell due to tectonic events that may have caused the sea to change its configuration, or due to a global event like a climatic shift. Some researchers believe the first forests caused global cooling and were primarily responsible for the Late Devonian Extinction.
By the end of the Devonian Period, over 70% of marine invertebrate species had disappeared. However, terrestrial plants and animals were largely unaffected. Forests continued to grow and diversify, while land invertebrates and early tetrapods became better adapted to a terrestrial lifestyle. Sharks replaced armored fish as the dominate aquatic predators, and they began to reach impressive sizes that would have rivaled Dunkleosteus. Early tetrapods became the dominate terrestrial predators and eventually evolved into today's amphibians, reptiles, birds, and mammals, creating a legacy like no other.
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