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DThe inosaurs were the architects of whole ecosystems on earth. Matching of the leaves, deputy on the ground and ground deposit by herds of large herbivores such as the Tricerops and Edmontosaurus kept the forests of the Cretaceous, helping to maintain open savannas which would have otherwise been thick with trees. In turn, it probably had a deep effect on the way the rivers crossed the landscapes, both while the dinosaurs were alive and after their extinct in an apocalypse of asteroids.
When the giant reptiles were alive, the rivers tended to flow straight, to overflow their banks and to spread through the landscape. After their disappearance, the forests went up and the rivers established winding paths. These are among the results of a new study, published this week Nature communications Land and environment by a team of scientists from the University of Michigan.
“The disappearance of these dinosaur megafaunes has fundamentally reorganized and reconfigured landscapes and ecosystems as we know them today,” explains Luke Weaver, paleontologist at the University of Michigan and the main study of the study. “An extinction event, which is organic, can have dramatic consequences for the non -biotic aspects of the landscape.”
After the disappearance of the dino, the forests became thick and the rivers established winding paths.
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The newly proposed link between dinosaurs and rivers is the result of an in-depth examination of sediment layers deposited before and after killing it from the meteorite of chicxulub dino-killing 66 million years ago. The best known marker of this border between the age of dinosaurs and the age of mammals – known as the Cretaceous -Paleogenic border – is a thin layer of sediments enriched with iridium which has been delivered by the meteorite and dispersed in the world. But the strata on each side of this border also record a brutal change in many rocky outcrops found in the old flood plains.
Below the border, the sediments tend to be thick with silt and sand, explains Weaver. This indicates the rivers which regularly overflow their banks and delivered sediments through the flood plain. Above the border, there is much less silt and sand and much more coal – the charred trash of old plants. This scheme suggests that the banks have started to stabilize, to correct sediments in wide and established winding channels.
Geologists had already proposed explanations for this change, none of which had to do with the dinosaurs. An idea is that at the beginning of the paleogenic, the earth was simply more humid in general, due to more precipitation or on the rise, which raised the water table. The fires inflamed by the impact of the meteorite could also have changed what the sediments crossed the rivers. But Weaver and his colleagues suspected that these explanations lacked an important part of history: the dinosaurs. “These things are an absolutely enormous presence in the landscape,” he says.
The disappearance of dinosaurs fundamentally reorganized landscapes as we know them today.
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To have a better opinion, the researchers examined the outcrops across the west of North America which clearly show the strata before and after the border of the Cretaceous-Paleogenic, including five limits newly identified in Montana and Wyoming. They found that a change in the sediments of sand and coal silt is coherent in western North America and persists for more than a million years after the impact. In their study, they argue that a lag of this scale is better explained by a brutal and spectacular regrowth of the forests, which were previously controlled by hungry dinosaurs.
“With this hot and humid landscape, there is no reason why you should not have dense forests that are formed, unless you prevent them from coming,” explains Weaver. In a few decades at a few centuries after the devastation encouraged by the meteorite, he said that the forests would have pushed back to form closed awnings which are not contrary to today’s temperate forests of the Northwest Pacific. These forests would then have rooted the soil in place and stabilized rivers wherever such trees could push. “This is a first step towards the test of a very big idea,” said Weaver, adding that more paleobotanic evidence of how forests have changed would help strengthen history.
Other research supports the idea that forests quickly increased after the disappearance of dinosaurs, explains Scott Wing at the Smithsonian National Museum of Natural History, which was not involved in research. For example, he found an increase in the size of the seeds found in the sediments after the border of the Dino-Mammal era. These larger seeds suggest that plants adapt to a denser forest environment, because larger seeds can survive longer without sunlight before ignition. The link with rivers “is a new line of evidence. It’s a different way of looking at the problem, ”he says.
The new theory also brings dinosaurs in much greater history of the way in which plants have shaped the flow of rivers since their emergence on land 450 million years ago, explains Mathieu Lapôtre at the University of Stanford, who did not participate in the study. “Once the plants have evolved, there were a lot of things that changed the distribution of vegetation on earth.”
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In new forests without dinosaurs, small mammals began their ascent. Finally, the primates arrived, some of which are reshaping the rivers of the planet on an even larger scale. “It is a story that is important for the whole history of the land of the first terrestrial plants today,” said Lapostre.
Image of lead: Catmando / Shutterstock
