2nd Major Mass Extinction Event: Time, Causes and Facts

The Silurian and subsequently Devonian periods saw a resurgence of life after it had survived a cataclysmic extinction at the end of the Ordovician known as Late Ordovician Mass Extinction (LOME). Ammonites and vertebrates (bony fish & sharks) all made their initial appearances as the marine life prospered. Along with the development of the first genuine woods, amphibians, and insects, this was also the time when life began to take root on land. The Devonian period, known as the "age of fish," witnessed the emergence and decline of numerous primordial marine species. Although creatures had started to emerge on land by this period, the bulk of life was still found in the marine realm.

2nd Major Mass Extinction Event: Time, Causes and Facts
Photo by Andrea Imre: pexels.com

The coral reefs, on the other hand, were the genuine Devonian victors. At this time of the Paleozoic Era, the majority of life on Earth was centered on coral- and reef-building organisms. In actuality, the Devonian waters had some of the biggest and most varied coral reefs ever to have existed. Thousands of species that were all well adapted to tropical, shallow coastal waters could all find a home on these reefs. Life would continue as normal as long as nothing adversely affected this particular environment. Unfortunately, that particular habitat did start to see some negative changes.

The majority of extinctions are thought to have taken place in shallow, warm seas throughout the late Devonian, according to fossil records. Approximately 20% of all marine families perished as a whole. Ammonites, brachiopods, jawless fish, and trilobites were among the most affected groups. Only one order of the Trilobita class survived through the Devonian. The corals were so extensively damaged that it would take another 100 million years for them to recover.

Devonian Period, age of fish, Late Devonian mass extinction
"Early shark Cladoselache, several lobe-finned fishes, including Eusthenopteron that was an early marine tetrapod, and the placoderm Bothriolepis in a painting from 1905". source: wikipedia

The Late Devonian extinction is one of the Big Five mass extinctions that occurred in the past 600 one million years ago (Ma) of Earth's history, the other four being the end-Ordovician, end-Permian, end-Triassic and end-Cretaceous massextinctions.

Benthic marine animals that lived in shallow tropical waters had exceptionally severe extinctions during the Devonian period. In actuality, many of the species that flourished during and after the extinctions were frequently cousins of the destroyed forms found in deep waters or at high latitudes. In contrast to tropical marine settings, the consequences on land flora and animals, if any, were noticeably less severe. In reality, there was a large drop in the variety of terrestrial plants in the early and middle Frasnian, but there is no evidence of a similar decline after the Frasnian. Near the Devonian–Carboniferous border, a smaller drop that is remarkable for the extinction of Archaeopteris occurs, however it happens after the Hangenberg event.

What is a Mass Extinction? Are We in a Mass Extinction?

Is the First Mass Extinction Primarily Caused by Volcanic Eruptions?

What species made extinct by the 2nd Mass Extinction?

75%, some suggest as high as 82%

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When was the 2nd mass extinction?

Well, the End-Devonian (372.2 Mya to 358.9) was characterised by a series of pulses in climatic change that resulted in repeated and abrupt reductions in biodiversity for more than 20 million years. The occurrence of two separate anoxic shale layers suggests that there may have been two closely spaced episodes at this location. The Kellwasser Event happened at the Frasnian-Famennian border known as the Frasnian-Famennian Crisis (372.2 ± 1.6 Ma) and the Hangenberg Crisis (358.9 ± 0.4 Ma) occurred at or near the Devonian-Carboniferous boundary and marks the end of the extinction. The Hangenberg Crisis is regarded by some experts as a distinct mass extinction event than the Devonian Mass Extinction.

How bad was the 2nd Mass Extinction?

About 75 – 82 per cent of species and 35 per cent of genera went extinct.

What died by the 2nd Mass Extinction?

Tropical habitats, especially the reefs of the shallow waters, were where the end-Frasnian extinction was most noticeable. The gigantic Devonian reefs, the grandest metazoan reef ecosystems that have ever existed on this planet, were the most conspicuous victims of the mass extinction. The area of reefs decreased dramatically from the Frasnian Stage to the Famennian Stage, by a factor of 5000. Corals and the reef-building sponges known as stromatoporoids also experienced declines, and stromatoporoids eventually vanished during the third extinction event at the end of the Devonian. Brachiopods related to reefs went extinct as well. At each of the three extinctions, groups of trilobites vanished, and only a small number persisted until the subsequent Carboniferous Period. Miserably, the placoderms—fish with incredible armour—were exterminated. Numerous coral species and other trilobites perished as well. A 33-foot-long (10-meter) armoured fish known as Dunkleosteus, was eradicated from the ocean. This enormous fish, a dangerous predator, possessed a helmet made of bone plates that wrapped its entire head and formed a cusp on its jaw.

Devonian mass extinction, 2nd major mass extinction, trilobites
Photo by Alejandro Quintanar, pexels.com

What thrived the 2nd Mass Extinction?

Vertebrates that are smaller than one metre long fared the best (about 3.3 feet). Tetrapods, four-legged creatures that would later give rise to amphibians, reptiles, and mammals, were among the survivors as they made the move from the water to land. In terms of the land, the Middle Devonian saw the emergence of the first trees and forests on Earth, while the Late Devonian saw a diversification and complexity increase in the flora, as well as the expansion of coastal and lowland Archaeopteris forests that produce spores. However, the patterns of plant diversity do not suggest any serious plant crises, thus the marine environment would be more severely affected by the destructive effects of Late Devonian global shifts.

Devonian Period, Devonian forests, Late Devonian mass extinction
"The Devonian Period marks the beginning of extensive land colonisation by plants. With large land-dwelling herbivores not yet present, large forests grew and shaped the landscape". Source: wikipedia

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Possible Causes of the 2nd Mass Extinction

Even though the name "Mass Extinction" may imply an immediate worldwide catastrophe, these occurrences can take millions of years. Here, Devonian mass extinction - the 2nd major mass extinction on the earth is being discussed. In this part of the write up we will see "what caused the 2nd mass extinction?" 

The End-Devonian was characterised by a series of pulses in climatic change that resulted in repeated and abrupt reductions in biodiversity for more than 20 million years. It is impossible to identify a single cause or differentiate between the cause and impact of the extinction events since they took place over such a long time. The fluctuations, which may have been triggered by extensive volcanic activity in Siberia, decreased oxygen levels in the seas and instigated other environmental alterations.

Sedimentary records show that the Devonian was a time of environmental alterations that had an impact on the organisms and caused their extinction. However, it is still up for contention as to what changed. There is proof of extensive anoxia at the ocean's bottom based on sedimentary records. As a result, the benthic creatures were destroyed, and the rate of carbon burial skyrocketed. However, there has been controversy about whether the occurrences were brought on by global warming, climatic extremes brought on by solar radiation, or mechanisms unique to the planet.

As plants developed roots, they unintentionally changed the environment in which they resided by converting rock and rubble into the soil. Following the runoff of this nutrient-rich soil into the world's oceans, a vast number of algae bloomed. These blooms effectively produced enormous "dead zones” or regions where algae deplete the water's oxygen, stifling marine life and upsetting marine food webs. Species that couldn't adjust to the reduced oxygen levels and scarcity of food perished. However, there is disagreement over this notion, and other scientists think that volcanic eruptions were the reason for the drop in ocean oxygen levels.

Approximately 75% of all species on Earth were wiped out during this extinction event, which began 383 million years ago and lasted for nearly 20 million years. Ocean oxygen levels plummeted during the Devonian in several pulses, severely harming conodonts and their ancient goniatites cousins, which are shelled ancestors of squid and octopuses. The Kellwasser event, the deadliest of these pulses, occurred around 372 million years ago. According to studies, when oxygen levels dropped, many organisms that built reefs became extinct, including a significant group of marine sponges known as stromatoporoids.

Although the source of the late Devonian extinction pulses has been difficult to determine, volcanism is one potential catalyst: After the Kellwasser event, 240,000 cubic miles of lava were erupted by a vast igneous province in what is now Siberia known as the Viluy Traps. The eruption would have released sulphur dioxide, which can result in acid rain, as well as greenhouse gases. Asteroids could have also played a role. One of the largest remaining impact craters on Earth, Sweden's 32-mile-wide Siljan crater was created some 377 million years ago.

Although it would seem strange, terrestrial plants might have been complicit in this dramatic biotic episode. Plants discovered several successful adaptations throughout the Devonian, including the stem-strengthening substance lignin and a fully developed vascular system. These characteristics made it possible for plants to grow larger and have deeper roots than ever before, which sped up the weathering of rocks.

Temperature change may have been a factor in the extinctions, which largely affected tropical populations. Geological evidence points to a lowering of the global climate during the end-Frasnian event and close to the end of the Devonian Period.

A decrease in the atmospheric concentration of carbon dioxide may have contributed to cooling. Because carbon dioxide is a greenhouse gas that contributes to global warming, cooling will occur if levels drop. The earliest woods were created during the Late Devonian when big trees first appeared. More carbon dioxide was used up in photosynthesis as plant life grew. Although some plant material (such as leaves) will be buried in swamps, lakes, and rivers, as dead plant matter decomposes, carbon dioxide is released back into the atmosphere. This subterranean plant matter, which frequently turns into coal, permanently removes carbon from the atmosphere.

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