Why shouldn't we shrink dinosaurs

When dinosaurs became birds : The big shrinking

Birds have been around for a good 160 million years. Some of them are only a few centimeters tall, thanks to a colorful plumage, the animals, which weigh a few grams, rise elegantly into the air. With these specimens in particular, it takes a lot of imagination to see in them the descendants of massive dinosaurs that hunted prey on two sturdy legs on the ground millions of years ago. At the same time, hardly any paleontologist doubts that this is how things developed. Nevertheless, evolutionary biologists are amazed at the strong miniaturization of the dinosaurs, which are more known for their gigantism. Researchers have long wondered how long this process of shrinking took. As recently as 20 years ago, it was estimated to have been around ten million years ago, but this has been refuted by more recent fossil finds.

A group led by Michael Lee from the South Australian Museum in Adelaide has now presented a new calculation. As they report in the journal “Science”, it took around 50 million years for two-legged “theropod” dinosaurs to become birds. Scientists examined 1,549 fossils of 120 different species in this lineage.

Thigh bones survive millions of years particularly well

Above all, they based their analysis on thigh bones, which are large and therefore often survive millions of years well. Another advantage is that the weight of an animal can be better estimated from the size of the bone than from other fossils. To track the development of the early birds in time, the researchers linked the characteristics of the fossilized bones to their respective geological ages. With the help of sophisticated statistics, they were able to understand the history of evolution.

According to this, such a dinosaur walking on two legs weighed an average of 163 kilograms 198 million years ago. 174 million years ago it was only 46 kilos, 167.5 million years ago the once powerful dinosaurs had reached the weight of a gray goose living today at around three kilos. Finally, 163 million years ago, the first primeval birds weighed just 800 grams and were therefore just about the size of a mallard. When the researchers repeated their analysis using two other methods, they came up with similar results: in at most 50 million years, the massive dinosaurs that trudged across the earth 210 million years ago must have evolved into much smaller birds.

The dinosaurs had a baby face

During this miniaturization, the physique of the theropods also changed, report Lee's scientists. Often the development from the youth to the adult form broke off. The shrinking dinosaurs kept the baby face of newly hatched young animals, and they no longer grew a long snout. The relatively large brain and the noticeably large eyes were also preserved into old age. This enabled the small animals to see three-dimensionally and to do gymnastics over the branches so much more skillfully. The larger brain, in turn, controlled the complicated movements with which the little dinosaurs swung from branch to branch.

Obviously, with this miniaturization and the changed body shapes, the animals had adapted to a life in the branches of the woods, according to paleontologists like Mike Benton from the University of Bristol, who wrote an accompanying commentary in "Science". “In the crowns, the animals find protection from larger predators that are too heavy to climb trees,” argues Benton. In addition, numerous insects live up there, many of which are out in the dark because cold-blooded animals are usually too cold to hunt at night. But here, too, evolution had given the budding birds an advantage: they had long been wearing feathers that kept them warm and thus enabled them to hunt at night.

Archeopteryx managed the hurdle to independent flight

170 to 120 million years ago, many of these feathered dinosaurs experimented with different ways to move in the air between two trees. In addition to the jumps from branch to branch, which the squirrels still use today, they also tried a kind of parachute and, above all, gliding. In turn, there is a new design for the feathers that previously warmed the primeval birds in the branches: the first flight feathers increased the sliding surface of the arms. Over time, larger and larger wings developed on which the animals could glide to distant trees. “A group, which also included the ancient bird Archeopteryx, finally overcame the decisive hurdle to a flight powered by the wings,” explains Benton. "The birds were born."

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