Earth History: The Sixth Mass Extinction – Science Spectrum


© Spectrum of Science, According to Penzlin, H.: The Phenomenon of Life. Springer Spectrum 2016, Figure 4.4 and Sepkoski, J.J. Jr.: Biodiversity: past, present and future. Journal of Paleontology 71, 1997 (extracts)

Fever curve of the races | In Earth’s history, major climatic changes have led to major extinctions at least five times. In the 1980s, American paleontologist Jack Sepkosky compiled extensive data on the diversity of marine animal forms in order to reconstruct changes in fauna on land. Among other things, this led to the curve describing the evolution of the number of races. It shows, for example, those wounds in which a large part of the underwater animals died and which so shaped the history of the Earth. However, over time, genera follow the trend of growth. However, it is unclear how accurate the curve inversion is for the actual evolution. The numbers given differ from those in the text, where species are indicated; Species are the highest rank in a biological system.

Big Five الخمسة

1 A frozen fresh start

450 to 440 million years ago, at the end of the Ordovician, much of the biodiversity was lost for the first time. In the preceding Cambrian period, almost all important animal species evolved explosively in the oceans: arthropods, sponges, echinoderms, mollusks, and vertebrates. In the Ordovician, an ever-increasing range of species has evolved in the water. It was unusually hot at first due to the extremely high levels of carbon dioxide. However, as the giant continent of Gondwana drifted towards the South Pole, vast tracts of land began to freeze more and more. As a result, the sea level decreased significantly, in addition, the land was very cold. This eventually led to the disappearance of about 85 percent of all species living at the time.

2 disaster with question marks

Between 372 and 359 million years ago the so-called Kellwasser event occurred, named after a fossil site. Fish wandered the seas, and the first animals began to explore the already green land. Why it fell is uncertain. It is assumed that factors such as a lack of oxygen in the oceans, volcanic eruptions, and low carbon dioxide concentrations, played together, causing the climate system to turn upside down. There is also speculation about an asteroid collision or even a near-Earth supernova. 50 to 75 percent of all species were lost, especially a number of fish and corals. Thirteen million years later, another mass extinction not among the Big Five most likely occurred on the cusp of the Carboniferous. Subsequently, up to 90 percent of all marine animal species became extinct, while remarkably few plants suffered from disasters.

3 devastating volcanic eruption

252 million years ago, when moving from the Permian to the Triassic, the Earth changed more dramatically than any other animal that was cut down. More than 90 percent of all fossil-forming species have been exterminated. These included the last remaining trilobites, which are arthropods that live in the sea. But this time it also hit wild animals: about three-quarters of them died, even insects. In addition, a large part of the vegetation cover was lost, while most of the terrestrial plants recovered. Experts believe that huge volcanic eruptions in Siberia caused the collapse of almost all ecosystems. Hundreds of thousands of years ago, lava flooded vast areas, burning entire swaths of land and emanating huge amounts of greenhouse gases.

4 Sinking of conical teeth الأسنان

When moving from the Triassic to the Jurassic period, three-quarters of all species disappeared from the Earth’s surface 201 million years ago. Only a small percentage of coral reefs remain. Conodonta (Greek for “conical tooth”), which inhabited the oceans for at least 340 million years, was completely destroyed. The fossilized remains, showing a solid conical dental apparatus, are today among the most important major fossils of the ancient world. The few soft-bodied fossils indicate a body shaped like an eel. With the exception of crocodiles, all large crurotarsi, a group of archosaurs, have disappeared from the mainland. The cause of the mass extinction is not clear. Among other things, an asteroid impact or strong fluctuations in sea level are suspected. However, exceptional volcanic activity is likely to be the main culprit here as well.

5 fatal injuries

The overwhelming victims of the mass extinction in the Cretaceous to Paleogene transition 66 million years ago are perhaps the most remarkable of all: the dinosaurs. But the reason for this is also astonishing: an asteroid 10 to 15 kilometers in diameter struck the Yucatan Peninsula in Mexico – perhaps the most powerful blow the Earth has received in the last 600 million years. The theory known as the impact hypothesis is now considered safe. The impact (there were probably several) along with greatly increased volcanic activity triggered a variety of environmental changes, which spanned at intervals from a few days to several hundreds of thousands of years. As a result, animals and plants have changed profoundly, and perhaps nearly 60 percent of all species have died. After all, today’s birds are directly descended from dinosaurs.


MacLeod, IN: Species die. A turning point in evolution. wbg Theiss, 2016

Benefits of mussels

As a prime example of a lack of adaptability, Stanford University’s Jonathan Payne cites the brachiopoda, which were replaced by the mussels (Bivalvia) at the end of the Permian. Although mussels and arm pods are different anatomically and at different evolutionary levels, the normal eye can hardly distinguish between them: they both have two lobes, live underwater and feed on organic matter that they filter from the water. Representatives of bivalves are widespread today and are well known to us: for example, mussels, oysters, oysters and their relatives. In contrast, only a few species of Brachiopoda are still found in small numbers in the dark, cold waters.

However, in the Paleozoic Era, it was clear that the armpits were numerically superior to the mussels. This was reversed after the mass extinction at the end of the Permian period. This is due, according to Payne, because mussels have a more dynamic metabolism and a more diverse lifestyle. Animals can move, some even run away from enemies, burying themselves in the sand and feeding on sediment. Armfeet can’t do any of this. For millions of years, these deficits were not a problem, only during the crisis turned into a solution to it.

deterioration of ecosystemsLaden…

Ecosystem degradation | البيئية The graphic shows how the relationship between mobile marine animals, such as arthropods and fish, and those that move little or no movement, such as Crinoidea and Brachiopoda, has evolved over the course of Earth’s history. Vertical lines indicate animal fragments at the end of the Permian period 252 million years ago and the end of the Cretaceous period 66 million years ago. The ratio of mobile marine species to non-motile marine species, which remains stable from Silurian to the end of the Permian, then gets out of equilibrium: while it initially fluctuates relatively evenly around the mean value, it suddenly falters with each extinction event. This demonstrates that mass extinction events alter ecosystems, at least partially or even completely destroying them.

In the early 2000s, David Jablonsky of the University of Chicago formulated the thesis that every mass extinction event follows non-constructive selectivity. In simple terms, it means that the survival of a species is not necessarily related to its biology, but also does not happen by chance. The most important factor in the survival of the species is its geographical distribution. There is a simple reason for this: the event leading up to the Faunenschnitt never really affects all regions of the world equally. If representatives of the species occur all over the world, then it is likely that at least a few specimens will pass – especially because they can usually live under different environmental conditions. Species that are restricted to only one area, eg an island, are likely to become extinct. However, there are also other cycles, such as the extinction at the end of the Triassic period, in which not all species living on Earth were largely affected.

In contrast to the cause of a mass extinction, which is usually a sudden event of short duration – such as an asteroid impact – the subsequent recovery phase spans over millions of years. From 2018 in “Nature” magazine. Research Analysis From the sedimentary rocks of the Chicxulub Crater in the Yucatan Peninsula, it appears that the first invertebrates colonized the site again a few years after the collision. Even after 30,000 years, a relatively large number of life forms appeared on the sea floor. However, this biodiversity was far from what it was before. After this event, the carbon cycle and water pH values ​​take over a million years to return to normal levels. It would take millions of years for a species richness comparable to its predecessors to emerge.

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