From rodents to primates, many of today’s mammal species form groups or colonies. Paleontologists are now presenting the oldest known evidence of such a social way of life among representatives of mammals. According to this, rodent-like mammals already lived in communities of several adult animals in the age of the dinosaurs. They probably established colonies in underground structures about 75 million years ago, similar to today’s ground squirrels, is evident from the fossils and the location of the finds.
The first representatives of mammals arose in the Jurassic Age or even earlier. But for many millions of years they led a rather shadowy existence: only after the non-avian dinosaurs fell victim to the mass extinction 66 million years ago did the great career of this group of animals begin. The mammals conquered the niches of life that had become free and diversified into many groups and species with a wide range of body characteristics and behaviors. Up until now it was assumed that the early mammals were loners and that social ways of life only developed in some species after the end of the Cretaceous period.
A more modern development?
This assumption is based on the fact that the representatives of primitive mammals still existing today do not live socially and only a few species of marsupial form group structures. In the case of the highly developed placenta animals, however, around 50 percent of all species have a social way of life that goes beyond togetherness to reproduce. It was therefore suggested that the development of more complex social behavior in mammals was linked to the spread of this subgroup. However, the current study by the team of American and Chinese paleontologists now shows that group life in mammals is much more deeply rooted in development than expected.
The fossils they report on are the remains of previously unknown representatives from the primeval group of mammals, the Multituberculata. These rodent-like small animals developed in the Middle Jurassic and even survived the mass extinction at the end of the Cretaceous – the last representatives only disappeared about 35 million years ago. The new fossils are dated to be around 75 million years old. They come from the locality “Egg Mountain” in the west of the US state Montana. Numerous remains of dinosaur sites have already been discovered there. It therefore seems clear that the Filikomys primaevus named representatives of the Multituberculata shared the habitat with the rulers of the Cretaceous period.
As the researchers report, they found the remains of a total of 22 specimens of the approximately mouse-sized animals. Examination of their anatomy shows that they had extremely strong forelimbs. Comparisons with today’s animal species suggest that it was probably an adaptation to digging – Filikomys primaevus therefore probably lived in underground structures. The really special thing about the finds, however, was the context: the fossils were grouped.
Grouped fossils provide clues
Apparently two to five individuals were recorded together by the processes that led to the fossilization. 13 individuals were discovered within an area of just 30 square meters in the same rock layer. In addition, the study found that the groups were a mix of adult animals of different ages. This suggests that it wasn’t two parent animals who raised their young, the scientists explain. “We normally deal with individual teeth and bones in the remains of early mammals. But in this case it was several, almost complete skulls and skeletons that have been preserved in the exact place where the animals lived, ”says senior author Wilson Mantilla of the University of Washington in Seattle.
According to the researchers, the plausible explanation for the location is that Filikomys primaevus lived in social groups of presumably related individuals. Communities with colony-like structures may also have formed. “These representatives of the very old mammal group of the Multituberculata apparently lived in group structures in the late Cretaceous period that were similar to those we know today from the ground squirrels, for example,” says first author Luke Weaver from the University of Washington.
The results thus suggest that social behavior was not just developments in placenta animals or marsupials. Sociality is likely a trait that evolved at different times and independently of one another in the evolutionary history of mammals, the scientists write. “How deep the history of these behaviors goes back in mammals is a fascinating finding,” says Weaver in conclusion.
Source: University of Washington, Article: Nature Ecology & Evolution, doi: 10.1038 / s41559-020-01325-8