The mass extinction paved the way for them, it is said. However, it is possible that not only the dinosaurs limited the development of the ancestors of today’s mammals in the Cretaceous period, but also their mammalian relatives that still existed at the time, according to a study. Accordingly, there was a complex competition between different groups of mammals during this period. Only the new circumstances after the mass extinction then created the basis for the careers of the ancestors of our modern mammals.
From bat, kangaroo, blue whale to humans – most of the mammal species living today can be traced back to groups that developed explosively after the catastrophic end of the Cretaceous 66 million years ago. Traditionally it was assumed that they had previously been suppressed by the dinosaurs: Since the multifaceted rulers of this era occupied the ecological niches, the development of the later so successful representatives of the Theria – the pouch and placenta animals – was inhibited. It seemed like these mammals could only create new forms and ways of life after the dinosaurs opened the stage for evolution.
Limitations on the trail
But the current study now paints a more complex picture of the former developmental limitations, where the focus is now on competition between the different lines of development of mammals. Because in the era of the dinosaurs there were other groups of mammals in addition to the small ancestors of the pouch and placenta animals. They included the eutriconodonta, some of which weighed over ten kilograms. “There were mammals during this period that included gliding, swimming, and burrowing species – but none of these specialized mammals were in the modern group – they all descended from earlier branches of the mammalian family tree,” said Oxford University co-author Elsa Panciroli .
As part of their study, she and her colleagues used statistical methods to investigate the extent to which different groups of mammals were restricted in their evolution before and after mass extinction. Characteristics of the different mammalian species and their lines of development were recorded and evaluated that lived next to the dinosaurs. As the scientists explain, based on certain patterns, conclusions can be drawn about the evolutionary limitations that could have arisen as a result of competition with other animals.
The relatives stood in the way
As they report, their results suggest that it was not primarily the dinosaurs who placed the greatest restrictions on the ancestors of modern mammals, but rather their closest relatives. The researchers were able to make it clear that in comparison to the other representatives of mammals in the era of the dinosaurs, the Theria produced few changes in features such as the size or structure of teeth and bones: They remained comparatively unspecialized. The direct competition with the dinosaurs as the reason for their low development does not seem plausible, the researchers emphasize.
According to them, the results suggest instead that the Theria were in the shadow of the other mammal forms: Since these already produced larger bodies and special ways of life such as climbing, swimming or sliding, the Theria therefore had little room for development. That is why they remained small and generalistic in their way of life during this time. “Our results suggest that, before extinction, the representatives of the earlier mammalian lineages kept modern mammals away from many ecological roles by displacing them,” summarizes first author Neil Brocklehurst of the University of Oxford.
It was only after many of the specialized mammals had become extinct along with the dinosaurs that the ancestors of modern mammals were able to develop and finally prevail, the study results suggest. Similar to their relatives before, they first developed new forms and ways of life and in the course of their further evolution they were finally able to fill many of the ecological niches that had once been reserved for the dinosaurs.
“Most studies of the history of mammals have focused on how quickly they evolved, but analysis of the limits to evolution has opened up interesting new perspectives,” says Brocklehurst. “Paleontology is currently in a period of upheaval due to new research possibilities. We will have to rethink many of the previous assumptions, ”the scientist concludes.
Source: Oxford University, professional article: Current Biology, doi: 10.1016 / j.cub.2021.04.044