Before the first animals could reach land, marine life suffered a catastrophic Devonian mass extinction.
What is the Devonian period?
This was the age of fish. 360 million years ago there were no terrestrial creatures, and the largest animals still lived in the oceans. One of the largest was dunkleosti. This animal more than others fits the description of a sea monster. The fish grew up to 10 meters in length and was covered with thick armor. It was a predator that had thick bones sticking out of its jaw instead of teeth. Dunkleosteus literally ate sharks for breakfast.
You might think that such a fish can withstand anything, but its time has come. Mass death of marine life began, and by the time it ended, from 79 to 87% of all species, including dunkleosti, had died out.
What are the reasons for the mass extinction of animals?
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This was one of the most massive extinctions in the history of our planet. Obviously something terrible was going to happen, but what? There is no indication that anything dramatic was happening. It seems that everything was harmless, but what event led to the domino effect and ultimately turned out to be catastrophic?
Technically, the Devonian is called the Fish Age. This was long before even dinosaurs walked the Earth (from 419 to 359 million years ago). But at that time there were few land animals, and the oceans were literally teeming with life. Fossilized reefs in Western Australia give an idea of what life was like in the Devonian seas. They were overflowing with fish, and it is very strange that now there is nothing like it. All of these fish were destroyed during the extinction that occurred in the late Devonian.
Falling asteroid?
When the dinosaurs went extinct 65 million years ago, it was obvious who was the culprit. The fall of the asteroid, as scientists suggest, led to the rise of a cloud of dust throughout the planet, which changed its climate. They even managed to find an impact crater located in Mexico.
In Western Australia, there is a similar crater from an asteroid fall, which is 359 million years old. However, in comparison with the Mexican, it is much smaller, so it can hardly be the answer to this story.
How terrestrial plants could affect ocean dwellers
One of the ideas was put forward by scientist Thomas Algeo of the University of Cincinnati. Perhaps the Devonian marine life was killed by plants.
In the Devonian period, the development of "vascular" terrestrial plants is observed. This group includes all trees, ferns and flowering plants. Moreover, primitive "non-vascular" plants such as mosses and lichens lost ground before the early Devonian.
Vascular plants have had a strong impact on the Earth. Their roots penetrated deep inside, breaking stones and creating soil. After breaking stones, they released certain substances and minerals. They gave advantages to plants, but they also ended up first in rivers and then in oceans. It was at this point that marine life changed. These nutrients became food for microscopic algae in the oceans, and they began to proliferate. Soon their thickets filled the oceans. Dead algae were decomposed by bacteria, which used oxygen for this process. This means that its amount in the water column has greatly decreased. The result was anoxic zones - areas in the ocean where there was not enough oxygen for the animals to breathe.
Today, this is the case in parts of the ocean where nutrients from farms enter the water. The resulting dead zones can span many square kilometers.
In the Devonian period, dead zones spread over many thousands of years, and gradually the animals found themselves in closed areas. If Algeo was right, the animals in the oceans began to fight for oxygen sites, but eventually died out, thanks to the success of the terrestrial plants.
But even if Algeo's hypothesis is correct, it does not explain why some species survived. For example, many sharks survived, although they needed oxygen as much as many other fish.
This can be partly explained by the fact that not all of the ocean has become a dead zone. For example, the upper layer of the ocean could not remain completely oxygen-free, since oxygen penetrated there from the air.
Anoxia levels could also vary from location to location, with some continents releasing more nutrients than others.
However, it seems that lack of oxygen cannot explain the extinction of many species. There must have been other problems that sea animals faced besides suffocation.
Ocean toxicity
It is possible that the oceans have become toxic. Australian scientists studied fossils from the Devonian period and found large amounts of sulfur in them. Its appearance was the result of the activities of other organisms that fed on algae. These species do not use oxygen, so they cannot be blamed for anoxia. But instead, hydrogen sulphide is a waste product. This chemical is highly toxic, so it could kill many animals and even affect terrestrial species.
Lowering ocean levels
But that's not all. The same terrestrial plants could have caused the Ice Age. The fact is that the more they grew, the more carbon dioxide they took from the air. CO2 is a greenhouse gas that traps solar heat. Its disappearance from the air contributed to the cooling of the planet.
After the temperature was low on Earth, glaciers began to form. This lowered the water level in the ocean and harmed the life that existed there. With all the changes taking place at the same time, life on Earth could have become unbearable for many species.
Domino effect
These events did not happen immediately. In fact, there were two waves, 359 and 372 million years ago. Some species suffered immediately. Others, depending on their place in the food chain, also felt the domino effect, which caused their extinction. It was a slow chain reaction. During such chaotic changes, any harmful events are more destructive than they would be during normal periods. Under stressful conditions, even small external shocks of this kind could potentially trigger an increased rate of extinction.