The Devonian is a geologic period and system of the Paleozoic Era, spanning 60 million years from the end of the Silurian Period, about 419.2 million years ago (Mya), to the beginning of the Carboniferous Period, about 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied. The Devonian period experienced the first significant adaptive radiation of life on dry land. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the "Age of Fish". The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida. The paleogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small continent of Euramerica in between. -Wikipedia, Devonian
Need more about fish
Jaekelopterus was the size of a crocodile. One of the largest sea scorpions in existence.
A species of cavefish in Thailand has been documented walking and climbing waterfalls in a manner similar to four-footed creatures such as salamanders, in a find researchers call “huge” in evolutionary terms. In a new study published in the journal Nature Scientific Reports, New Jersey Institute of Technology (NJIT) scientists describe the behavior in the blind, walking cavefish Cryptotora thamicola. Study co-author Brooke E. Flammang, an assistant professor of biological sciences at NJIT, said in a press release that the fish has anatomical features previously known only in tetrapods — four-limbed vertebrates that include amphibians and reptiles. “What these fish do, in complete darkness, is stick to the rock and climb waterfalls, completely underwater,” Flammang said. While some other fish species have means of moving on land, the NJIT researchers write that no other living fish has the gait employed by the cavefish, which uses a tetrapod-like, “robust pelvic girdle” to climb. “The pelvis and vertebral column of this fish allow it to support its body weight against gravity and provide large sites for muscle attachment for walking,” Flammang said. The NJIT team says the find could tell scientists more about how the anatomy for land-walking evolved, as tetrapods made the long transition from finned to limbed beginning in the Devonian period about 420 million years ago. “From an evolutionary perspective, this is a huge finding,” Flammang said. “This is one of the first fish that we have as a living species that acts in a way that we think they must have acted when they evolved from a fluid environment to a terrestrial environment.”
From around 420 to 350 million years ago, when land plants were still the relatively new kids on the evolutionary block and “the tallest trees stood just a few feet high,” giant spires of life poked from the Earth. “The ancient organism boasted trunks up to 24 feet (8 meters) high and as wide as three feet (one meter),” said National Geographic in 2007. With the help of a fossil dug up in Saudi Arabia scientists finally figured out what the giant creature was: a fungus. (We think.) The towering fungus spires would have stood out against a landscape scarce of such giants, said New Scientist in 2007. “A 6-metre fungus would be odd enough in the modern world, but at least we are used to trees quite a bit bigger,” says Boyce. “Plants at that time were a few feet tall, invertebrate animals were small, and there were no terrestrial vertebrates. This fossil would have been all the more striking in such a diminutive landscape.” Fossils of the organisms, known as Prototaxites, had peppered the paleontological findings of the past century and a half, ever since they were first discovered by a Canadian in 1859. But despite the fossil records, no one could figure out what the heck these giant spires were. The University of Chicago: "For the next 130 years, debate raged. Some scientists called Prototaxites a lichen, others a fungus, and still others clung to the notion that it was some kind of tree. “The problem is that when you look up close at the anatomy, it’s evocative of a lot of different things, but it’s diagnostic of nothing,” says Boyce, an associate professor in geophysical sciences and the Committee on Evolutionary Biology. “And it’s so damn big that when whenever someone says it’ssomething, everyone else’s hackles get up: ‘How could you have a lichen 20 feet tall?’”" That all changed in 2007 when a study came out that concluded the spires were a fungus, like a gigantic early mushroom. But not everyone was sold on the idea that Prototaxites was an early fungus. No one’s questioning the spires’ existence—people just have trouble trying to imagine that such a huge structure could be a fungus. Researchers trying to refute the fungus idea thought that Prototaxites spires were gigantic mats of liverworts that had somehow rolled up. But in a follow-up study, the scientists who had proposed the fungus idea doubled down on their claim. So science is messy, and despite more than a century of digging, we still don’t really know, for sure, what these huge spires that dominated the ancient Earth really were. But even though the spire-like mushrooms of yore—or whatever they were—are long gone, don’t feel too bad for funguskind. The largest organism on Earth, says ABC, is still a huge fungal mat, a single organism spread over 2,200 acres of forest in eastern Oregon. -Colin Schultz, Long Before Trees Overtook the Land, Earth Was Covered by Giant Mushrooms
They didn't have giant caps though, they were just stalks.
Then when trees evolved, there was no fungus or other organism capable of consuming dead trees (wood); in other words they would not rot. This was for millions of years, and the earth literally piled up with dead trees. Occasionally there were fires, and the deep piles of wood would burn and burn. This era is where most of the world's coal comes from, and also why that time was called the carboniferous period.
Also, because wood is mostly made of carbon and there was nothing that could decompose it, the inability for the carbon to be re-released into the air caused the amount of Oxygen near ground level to spike. This caused insects to be able to become HUGE, like the 3 dragonflies with 3 foot wingspans. -Reddit community, TIL that before trees were common, the earth was covered in giant mushrooms
Devonian Forest. Plants had covered the land by the middle of the Paleozoic era, 320 million years ago.
- Charles Knight, Field Museum of Natural History Chicago
Giant insects emerge onto land
Their hard exoskeletons let them conserve moisture.
They developed tubes on the underside of their bodies that let them absorb oxygen.
Their legs remained largely unchanged.
Initially may have come out to lay eggs
If so they may have resembled horseshoe crabs, who do this today. These creatures can spend about a week on land.
Moving onto land
It was a difficult transition. The exoskeletons of invertebrates allowed them to both conserve moisture and walk unimpeded from gravity, without major changes.
But as plants swept across the continents, the invertebrates likely stayed for food
A final development—fins supported by fin bones--led to the ray-finned fish, a form of animal so effective in its movement it proliferated through the oceans, eliminating altogether the armored form of fish that had started the era. The first heroes to venture onto land were plants. The animals dared not. This was not because of any particular difficulty with air. There were most likely already some animals who could handle air with ease. When we reflect upon the nature of the continents back then, we can speculate that to have eyes was to eliminate any courage. From the tip of the sea, one faced a lifeless, thousand-mile expanse of baked rocks and rubble and dust, barren as a moonscape. There was no living soil. There was nothing green. Hell with its truculent citizens would have been more inviting. But the ultimate obstacle to movement onto land was even more ferocious. On land Life faced an invisible enemy, a reality so pervasive and so strange and so overpowering that life remained exclusively in the oceans for ninety per. cent of Earth's history. This invisible power came to be called, hundreds of millions of years later, gravitation. Gravity is an ordering principle of the universe whose effects are sus-pended for organisms in the oceans. Thus life learned enough about the electromagnetic powers to create a haven where the fierce demands of gravity could be forgotten, at least for a time; and had it not been for the ancient heroes of the Paleozoic, perhaps for all of Gaian time. Waves splashed high and smashed across the granite rocks, then slid away in a foaming betrayal of their living cargo. Plants that had mastered the three-dimensional swirl in the ocean currents now found themselves flattened and baked by the sun. How many tons of life perished into crisp green flakes that scattered across the lifeless continents? In the sleepy semi-consciousness of those plants, how many experiences of distress as they lay plastered to the rocks? Yet the genius of creativity swelled within those plants that were un-avoidably engaged with this world-encompassing enemy. There at the edges of sea, continents, and air, a new being appeared, Capaneus, a hero who invaded an alien world. Capaneus invented the wood cell and became the first terrestrial creature able to withstand the flattening power of gravity. Capaneus built these solid structures with vascular vessels to transport food and material through its body. Possibly Capaneus began as a semi-aquatic plant surviving at the water's edge, and then developed enough strength to endure gravity as water levels dropped back or the seas dried up. Capaneus's descendents improved their vascular transport and deep-ened their root system as they formed forests along the edges of the rivers and oceans and throughout swampy areas. These lycopod plants required the wet and darnpy areas for their reproduction. Just as with their ancestors in the oceans, lycopod sperm needed a moist world to find their way to iycopod eggs. The novelty that completed the Paleozoic plant creativity was the naked seed organism, the gymnosperm. Now trees themselves without need of a moist surface could bring the male and female gametes together to create a seed. Having fully mastered the challenges of both gravity and aridity, such gymnosperms marched across the continents to become full forests that re-dounded back and replaced the previously dominant lycopods. Just as the ray-finned fish represented the most masterful design for fish life in the ocean and became the principal marine vertebrate, so too the naked seed plants represented the Paleozoic's supreme accomplishment in bringing the mesocosm into the previously uninhabitable dry continental interiors, and so became the principal form of terrestrial plant life. The last of the Paleozoic's achievements was the mastery by animals of dry land. The first animals to follow Capaneus 425 million years ago were arthropods, probably millipedes, soon to be followed by their predators. These arthropods met the challenge of land by developing an exoskeleton that could keep water within. They became, in a sense, mobile living ponds. Such insects underwent a profound proliferation as they entered and adapted to the immense number of new worlds found within the lycopod and then gymnosperm forests and swamps. Dragonflies developed wing spans of eighteen inches. Millipedes grew to eight feet in length. One scorpion was large enough to kill and consume small vertebrate animals when they appeared later in the Paleozoic forests. By the last part of the Paleozoic, the insects had achieved subtle adaptations that would never be significantly improved upon, becoming enduring features of the terrestrial biosphere. Vertebrates joined the adventure onto land 370 million years ago. Some 10 million years previously, at least one branch of the fish family had developed lungs. With so much life having already invaded the land, any fish that could survive even briefly along the shores of the rivers, lakes ponds, or oceans would have found its way into a paradise of food. Not only were there trillions of insects, the vast majority of these would not be capable of recognizing the large hungry object as a predator. Having evolved in a vertebrate-free forest, the insects would have no instinctual responses stored in their DNA for protecting themselves from such predation. These air-breathing fish would need only the most rudimentary locomotion. Of all the types who attempted this move, the lobed fish proved to have a slight advantage anatomically and thus prevailed as the form of vertebrate to survive on land. The descendents of the lobed fish, the amphibians, retained the ancient fish strategy of laying eggs in the water. Amphibians began their lives as fish, as tadpoles with gills, growing through their juvenile stages in the water; but then quit the seas, grew lungs, and made their way into the world of swamps and forests, reaching sizes of twenty feet in length. -Brian Swimme and Thomas Berry, The Universe Story
The kronosaur emits a deep exhalation and then belches. A triple wave crashes over its back. The forty-foot-long body wriggles in a clumsy wide turn, like a multi-ton worm. The kronosaur bends its head back toward the sea. It cannot breathe out of water. It has lungs, but the crushing weight of gravity pulls its body bulk down onto the pulmonary chamber, squeezing the lung apparatus. The kronosaur's flippers are not attached firmly to the backbone, so they cannot prop the body up against the sand. A paroxysm of wriggles and head movements finally pull the kronosaur into the water. -Robert Bakker, Raptor Red, p168