Mammals switched from night to daytime activity after the dinosaurs extinction

A possible scenario in the Kayenta formation (Image courtesy Mark Witton)
A possible scenario in the Kayenta formation (Image courtesy Mark Witton)

An article published in the journal “Nature Ecology & Evolution” describes a research on the evolution of mammals from nocturnal animals to diurnal ones. A team of researchers co-ordinated by Tel Aviv University (TAU) and University College London (UCL) analyzed information about 2,415 extant mammalian species using computer algorithms to reconstruct their ancestors’ activity patterns, concluding that they started switching to daytime activity after the extinction of dinosaurs.

Even today most mammalian species exhibit sensory adaptations to nocturnal activity regardless of their actual activity period. Because of this remarkable diffusion, this characteristic is considered a heritage dating from a crucial period for the evolution of early mammals.

The most plausible theory is that in the Mesozoic era nocturnal mammals survived while those that were active in daytime and then faced dinosaurs ended badly. This created an evolutionary bottleneck with the consequence that today’s mammals descend from relatively few species from that era.

The image shows an artistic interpretation of the possible scenario in the Kayenta formation, an area in Arizona, Colorado, Nevada and Utah where several fossils were found. Among them, there are those of Kayentatherium wellesi, a cynodont shown at the center of the image that was about a meter long and could be an ancestor of modern mammals.

The problem is that so far the evidence collected by examining the fossils were ambiguous preventing from appropriately reconstructing the evolution of mammals. A team led by Roi Maor of TAU and UCL tried to accomplish this task by carrying out a complex reconstruction of early mammal activity patterns starting from the characteristics of 2,415 species of extant mammals that represent 135 of the 148 current families.

The researchers had to base their research on hypotheses regarding the history of mammalian evolution and consequently decided to test their analysis of their pattern of activity’s evolution on two of the main hypotheses. Analyzing both possible genealogies, however, it turned out that mammals started shifting from night to day activity after the dinosaurs’ extinction.

That passage was gradual and the two possible family trees show different timelines but the correlation with the disappearance of the dinosaurs looks very close. The ancestors of gorillas and gibbons were probably the first to switch to daytime activities and other families followed the same way over time.

This research is based on many simulations and deductions so the resulting window for switching from night to day activity is vague, between 52 and 33 million years ago. It’s not easy to get accurate results precisely because mammals kept on maintaining adaptations to nightlife with the consequence that even today you need to observe the animals directly to understand exactly how they live.

Various adaptations are visible in mammals’ soft tissue that get generally lost in fossils. It’s one of the typical problems in paleontology and in this case the researchers intend to keep on collecting information to obtain a more accurate family tree that can improve the results of their analyzes.

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