The first radiations of plants had already appeared before the Devonian. The proof of this is the deposit of Rhynie Chert in Scotland, where there are traces of the first vascular plants. It is a fact that the oldest ones known today began in this period.
The vegetation from the early era was largely made up of small plants, the tallest being about 1 meter high. Towards the late phase, ferns, horsetails and various vegetations that already reproduced with seeds appeared. This is how the first trees were born and forests developed.
Two large groups of animals colonized the earth. First came the tetrapods, which were followed by the marine arthropods. Wingless insects and arachnids were also added to this.
Crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common in this period.. In addition, the variety of fish increased.
Then there were three large continental masses. North America and Europe settled together near the equator, with much of their surface covered by deep seas.
In the north was a part of modern Siberia. In addition, a continent made up of South America, Africa, Antarctica, India and Australia occupied the southern hemisphere.
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The seas in the Devonian
In the seas, brachiopods such as spiriferiaceae and corals that formed reefs were abundant. In the lower stage, ammonoids emerged, leaving large limestone deposits from their shells.
Added to this are bivalves, crinoid echinoderms, blastoids, grapolites and trilobites. Although the latter disappeared at the end of the Devonian.
Something important happened too. An important diversification of fish occurred. There were benthic, jawless and armored types that were abundant in the late phase. They included different groups and towards the Middle Devonian the placoderms flourished, which were the first with jaws.
Many grew to large sizes and became predators. Added to this is the emergence of sarcopterans, which were fish with lobe fins. At the end of this stage the first tetrapods also emerged.
The Devonian landscape
It was an era when many life forms colonized the earth. Before this there was no organic accumulation in the soils, which had a reddish color. This indicated an underdeveloped landscape, possibly colonized only by mats of bacteria and algae.
Initially, early terrestrial vegetation began to spread. The plants initially had no roots or leaves and some lacked vascular tissue. So they spread by spores. Furthermore, they were only a few centimeters high.
These included zosterophiles and trimerophiles. Among these plants lived a fauna of mites, trigonotartibis, wingless insects and myriapods.
In the late phase, lycophytes, sphenophytes, ferns and progymnosperms evolved. All of these with roots and leaves mostly, which also gained more height. An example of this is the Archeopterisa really big tree that had wood.
At the end of the Devonian, the first plants with seeds also appeared, which was part of the Devonian explosion, in which a wide range of vegetables and plant groups emerged. In turn this supported the diversification of arthropods.
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Tectonics and paleoclimate
The most significant changes in the geography of the planet occurred in the Devonian. It was then that the super continent of Euramerica was formed and on the other side that of Gondwana was consolidated.
They were two land masses fairly close to each other in a single hemisphere, while the ocean covered the rest of the world. They were also surrounded by subduction zones that set in motion a major collision, which resulted in the continent of Pangea in the Permian.
Added to this are important regional activities such as the collision of North America and Europe, which caused massive granite intrusions and the Appalachian uplift. The vigorous erosion of these mountains also gave rise to a large amount of sediment, which was deposited in vast lowlands and deep seas nearby.
Extensive reef construction gave birth to the most complex reefs in the world. This continued as stromatoporoids and corals of increasing size appeared. The equatorial seas were formed between the continents and large shallow seas in North America, Central Asia and Australia. These were transformed into basins in which there were large precipitations of rock salt, gypsum and other minerals.
In the final stage of the Devonian, a mass extinction event occurred. Glaciation and falling sea levels triggered a crisis that affected marine species.
It is also thought that there were meteorite impacts or changes in carbon dioxide in the atmosphere. Another possibility is that the expansion and evolution of forests and the first plants with complex root systems may have profoundly altered the climate. It was finally the time when the first vertebrates arrived on earth.
Life in the Devonian
This period experienced a great diversity of marine fauna from the Paleozoic Era. The ocean was dominated by sharks, bony fish and ammonoids. Trilobites declined, while other brachiopods became the most abundant marine organism. The oceanic conditions and biological richness resulted in the greatest carbonate production of the Paleozoic Era.
Fish presented evolutionary advances, more diversification and dominance, whether in the sea or freshwater. In addition to jawless fish and placoderms, they appeared in lobe-finned, ray-finned fishes in the fossil record.
Finally changing terrestrial and freshwater environments encouraged the evolution of some fish to the first tetrapods who evolved into terrestrial vertebrates at the end of the Devonian.
Land animals such as scorpions, spiders, and insects also thrived in the environments that vascular plants provided.
This vegetation transformed the Earth’s environments, giving rise to extensive swamps. The forests that emerged with the new trees created the biosphere and altered the carbon cycle. Thus complex soils formed, links between land and water expanded, and habitats became stable. Organic matter increased both on land and in the oceans through runoff.
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terrestrial vertebrates
Amphibians were the first four-legged animals or tetrapods to conquer the land. They had well-defined limbs and feet, a supportive ribcage, and a neck that allowed the skull to rotate. They were more prepared to move and rest their bodies on the ground than their ancestors.
Despite their modified bone structure, amphibians maintained a strong connection to water. The adults fed on fish in the shallow areas. In addition, it was also the place where they spawned, laying a number of smaller eggs that become swimming larvae. These early specimens did not have much in common with the frogs or salamanders we know today.
Adaptations
As with the first land plants and arthropods, several adaptations were key in the transition of vertebrates to life on land. Amphibians developed skeletal and muscular features to support body weight, walk, and keep their heads off the ground.
However, some believe that in reality those who conquered the surface were the ripidistian fish that had fleshy, lobed fins with which they reached the shore.
There are many similarities between these ripdistia and the first terrestrial vertebrates. That is, amphibians. The fossil evidence, skulls, teeth and vertebrae in some are virtually identical.
Some are fleshy ventral fins that are attached directly to the lobe fin skeletons, near the tail end of the fish. There are also mobile bones and muscles here. The bones inside the fins match the legs of amphibians.
They also share the ability to breathe air, since ripidistian fish had nostrils and lungs.
These physical adaptations, although used on land, were developed in water. Fish such as Lobfins would have used ventral fins to improve their swimming ability and direction in the water.
Ripidistian fish probably evolved the ability to breathe air to cope with environments where oxygen levels in the water were exceptionally low. Such is the case of lagoons.
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Extinction event
This period also witnessed an extinction event that affected 50 to 55% of marine invertebrates, and 70 to 80% of invertebrates in general. It is believed to have combined meteorite impacts, volcanism, changes in ocean chemistry, oxygen depletion and glaciation.
Initially, the conditions were in place for the species to recover satisfactorily. But this changed as the final phase of the Devonian approached. Pulses of extinction appeared that affected marine populations.
The rough corals and stromatoporoids that were responsible for building the reefs are almost gone. This also put the brachiopods and trilobites in check. Conodonts also suffered heavy casualties and jawless fish were completely eliminated. The placoderms had the same fate.
Those who emerged victorious were the land plants and animals that were mostly intact. Although they could also be affected by ocean chemistry and rapid cooling. One hypothesis suggests that the impact of a meteorite and volcanism were important triggers of the extermination.
The selective effect
The Late Devonian extinction affected marine ecosystems more than life on land. It ended almost all jawless fish and also every last placoderm became extinct. Instead, bony fish and sharks survived. This sometimes raises questions regarding the randomness of extinctions or survival of specimens.
Knowing whether mass extinctions have random consequences or whether the risk varies from species to species or group to group would greatly improve our understanding of both extinction and evolution.
The idea of selectivity considers that certain factors, such as body size, geographic range, or feeding behavior, will determine whether species, groups, or entire families are more or less likely to survive an extinction event.
Some data seem to reinforce…