Did dinosaurs die in the flood?

The evolution of birds : Survival after the impact

When a ten-kilometer boulder struck from space in what is now Mexico 66 million years ago, it caused a devastating pressure wave, huge flood disasters and earthquakes. Around three quarters of all plant and animal species on earth died out at the end of the Cretaceous period, including dinosaurs. As scientists working with evolutionary biologist Daniel Field from the British University of Bath now report in the magazine “Current Biology”, global forest dieback also followed. The birds were among the survivors. The researchers are now trying to explain why they made it

Survived - but just barely

Scientists are still debating whether it was the meteorite impact alone that led to global mass extinction. There could also have been a number of changes, some of which lasted longer and to which the primeval lizards were unable to adapt. But why did some animal groups such as the ammonites and belemnites known from fossil finds, both squid-like animals, die out completely, while others survived? The latter include reptiles such as monitor lizards, turtles and crocodiles, shellfish such as mussels and snails, the mammals that are so dominant today, and also the group of birds, which is rich in color and shape.

In any case, “survival” often seems to have been a scarce matter for these animal groups as well. For the birds, for example. Field's team shows that the mass extinction by no means left its development without a trace. "We put this story together from very different sources," says Field. This also includes plant fossils and prehistoric pollen deposits as well as the relationships and lifestyles of today's and earlier birds. "It's exciting when so diverse data sets point in the same direction and something becomes recognizable that happened 66 million years ago and - at least according to geological standards - at lightning speed," says Field.

Small and down to earth

The results suggest that only a small group of ground-dwelling bird species survived the disaster and its aftermath. The situation was worse for tree-dwelling bird species. Gerald Mayr, ornithologist at the Senckenberg Research Institute in Frankfurt, suspected this in his specialist book “Avian evolution”, published in 2016: Changes in terrestrial vegetation such as forest diebacks could have led to the extinction of tree-dwelling bird species. Bird species that did not rely on forests were less affected. “Fields theses don't seem completely new to me, but they are correct,” says Mayr now.

The investigated fossil pollen deposits at the boundary between soil layers from the Cretaceous and the subsequent geological age known as “Paleogene” show that ferns temporarily dominated the earth's vegetation at that time. Tree forests only recovered after centuries or even millennia.

Sulfuric acid that darkens the sun

The global death of forests and the wave of extinction in the animal kingdom could also be related to another and persistently fatal consequence of the impact: thrown up dust and soot clouded the atmosphere and blocked the heat of the sun. Temperatures on the ground dropped dramatically. However, many cubic kilometers of rock were also evaporated in the heat of the explosion upon impact. Large amounts of sulphate found their way into the upper layers of the atmosphere. As a result, a thick veil of sulfuric acid droplets probably formed. This veil probably blocked the sun for a while after the dust had settled long ago. Researchers at the Potsdam Institute for Climate Impact Research came to this conclusion in 2016 after computer simulations.

Hot impact, frosty consequences

The global temperature average on the ground could therefore have fallen by more than 26 degrees Celsius. Fauna and flora of the warm, humid climate of the Cretaceous Period were given a cold shock, so to speak. For a few years the temperatures never climbed above freezing point. Only after three decades did they reach a level similar to that before the impact. But even after that, its consequences continued to influence the flora and fauna; he also had an effect on the world's oceans. The global cooling not only disrupted the complex structure of ocean currents, it could also have caused poisonous algal blooms and fundamentally changed marine ecosystems as a whole.

Given this, Field's team wanted to understand why a small group of bird species survived the disaster. "Today birds are global and the most biodiverse group of terrestrial vertebrates," says Field. About 11,000 species are known. "But only a few lines of development survived the mass extinction 66 million years ago."

Toothless into the future

How and why did they do it? One factor could have been diet. At the end of the Cretaceous period, a number of bird species had already developed a toothless beak. Maybe after the felling they were particularly competitive in using hard-shelled seeds as food. Compared to other foods, these lasted longer and could therefore still provide calories in the dark phase if the bird found them. However, among the bird relatives with teeth there were also grain eaters, which nevertheless became extinct.

Genetic analyzes also confirm that the ancestors of today's birds that survived at that time were rather small. If the food supply was dwindling, this could also have been a decisive advantage. Because whoever has to eat less, needs less energy from food - and is more likely to be full. And there are other possible explanations: The researchers suspect an increasing size of the eggs, which would have given the offspring an initial advantage. Flexibility in nesting could also have been a factor, or improved digestive abilities of the surviving species. How exactly all of this might interact, however, needs to be investigated further, according to the researchers.

Field wants to next investigate how long it took the forests to recover from the impact and how the new tree-dwelling bird species evolved.

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