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The extinction of dinosaurs allowed mammals to “go out into the light”

The flowering of mammals began after the extinction of the dinosaurs 66 million years ago. Fauna of the Miocene epoch (23-5.3 million years ago) in the artist’s view.
Jay Matternes, 1964 / Wikimedia Commons

British and Israeli biologists have provided evidence supporting the hypothesis of a “nocturnal bottle neck”, according to which mammals adapted to the daily lifestyle only after the extinction of dinosaurs 66 million years ago. Before that, they were active at night and thus avoided interaction with dangerous reptiles. The study is published in Nature Ecology & Evolution.

The oldest mammals, apparently, appeared in the middle or the end of the Triassic period. Judging by the structure of the retina, they led a nocturnal or predominantly nocturnal lifestyle, and vision was adapted to it. For example, in mammals in the retina there is no area that increases visual acuity in carnivorous day-old reptiles, fish and birds. Many daytime mammals, such as ungulates or predators, in the retina of the eye are dominated by sticks – photoreceptors responsible for night vision. It is also possible that a goodsense of smell , the ability to hear over a wider frequency range than birds and reptiles, and increased tactile sensitivity, which in some mammals provide vibrissae, have evolved to compensate for the lack of visual information.

Researchers believe that mammals adapted to the day-to-day lifestyle only after extinction, which occurred about 66 million years ago. According to the hypothesis of the “night bottleneck” during the Mesozoic (251-66 million years ago), the planet was dominated by dinosaurs, leading a daily life. Adaptation to nocturnal activity allowed mammals to avoid interacting with larger and stronger reptiles. During the Paleogene extinction , not only dinosaurs disappeared, but sea and flying reptiles, and mammals and birds evolved, filling the vacated ecological niches.

The hypothesis of a “night bottle neck” is confirmed by morphological,anatomical and molecular research. But, according to the authors of the article, these are only indirect evidence, and they do not answer the question, when mammals adapted to the day-to-day lifestyle – before extinction of the dinosaurs or after. To answer it, scientists analyzed the activity of 2,415 species of modern mammals, representing all the existing orders and 135 of the 148 families. To find out the time of transition from night to day mode of life, the researchers reconstructed the probable activity of ancient mammals, from the moment they appeared. They divided the views into several groups according to the type of activity, including day, night and cathedral, when the animal is active at certain intervals of time, day and night. Then the researchers put a “map” of activity on two phylogenetic trees. Both of them began with the division of the ancestors of perverters (modern platypus and echidna), and placental and marsupials. According to one hypothesis (let’s call it “Triassic”), it happened220 million years ago, on the other (“Jurassic”) – 166 million years ago.

According to the results of the reconstruction it turned out that most mammals moved from an exceptionally nocturnal to a cathedral lifestyle, either about 75 million years ago, several million years before the extinction of dinosaurs, or immediately after – about 65.8 million years ago. According to the authors of the study, such results do not contradict the hypothesis of a “night bottleneck”, since according to some assumptions, the disappearance of dinosaurs here and there began 15 million years before the Cretaceous-Paleogene extinction.

To the day-to-day mode of life, mammals passed about 64 million years ago, according to the “Triassic hypothesis”, and about 52 million years ago according to the “Jurassic hypothesis”. Presumably, the first to appreciate the benefits of daytime activity, were the elephant jumper ( Macroscelidae ). They were followed by rodents (about 62 million years ago) and camelids (about 60 million years ago). The ancestors of the ape-shaped ones were among the first to pass to the day image – about 52.4 million years ago according to the “Jurassic hypothesis”.

Earlier, scientists found that mammals almost died out with dinosaurs. Their success in the Cenozoic era was associated with a successful recovery and rapid evolution after the Cretaceous-Paleogene extinction.

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