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Paleontologists have identified the earliest example of a placental mammal in the fossil record to date, which could provide new insight into how our furry ancestors came to dominate Earth after the extinction of the dinosaurs.
They made a breakthrough by studying the odontological (tooth) equivalent of growth rings — growth lines and elements preserved in fossil teeth — which they used to reconstruct the daily life of one of our distant relatives: pantolambda batmodon, stocky a dog-pig-like creature that prowled about 62 million years ago, shortly after the extinction of the dinosaurs.
At the same time, it turned out that Pantolambda mothers were pregnant for about seven months before giving birth to one well-developed baby with a full mouth of teeth, who suckled for only 1–2 months before becoming fully independent.
“I have been studying dinosaurs for most of my career, but this mammalian growth project is the most exciting research I have ever been involved in as I am amazed that we were able to identify the chemical impressions of teeth at birth and weaning. so old,” said Professor Stephen Brusatte of the University of Edinburgh, who took part in the study.
Placental mammals make up the majority of modern mammal species, from humans to tiny shrews and giant whales. They give birth to relatively mature young, which have mostly grown inside their mother, feeding through the placenta.
Although mammals existed during the time of the dinosaurs, it wasn’t until they became extinct that mammals really started to diversify and get big. One idea is that their ability to give birth to the large, well-developed babies that the placenta previously fed was the key to their success. This style of growth and reproduction also allows human children to be born with such large brains.
Exactly when this lifestyle originated, however, remains a mystery. Because the bones of early mammals were small and fragile, fossil remains, such as femurs, that could be used to provide information about the reproductive styles of species are often missing. Better preserved teeth, the size and shape of which have long been studied by paleontologists to learn about the lifestyle of extinct mammals.
The new technique builds on this tradition. It involves cutting fossil teeth into very thin sections to study growth lines and vaporizing them to understand their chemical composition at different stages of development. “This allows us to look at almost any fossil mammal and reconstruct things like how long it was pregnant, how long it nursed, when it reached maturity, and how long it lived — things that we really couldn’t do with fossil mammals before. now,” said Dr Gregory Funston of the University of Edinburgh, who led the study.
When PantolambdaFunston was surprised to find how advanced this trait was at this stage in mammalian evolution.
“One of the closest analogues in terms of its development is things like giraffes, which are born right on the plains, and they have to move for a few seconds, otherwise they will be hunted,” he said. “We expected these types of life stories to emerge slowly and then become more and more specialized over time, but we see that Pantolambda, just 4 million years after the extinction, is already experimenting with this completely new way of life history.”
Funston hopes this research will open up new frontiers in the study of fossil mammals and their evolution. “This method opens up the most detailed window we could hope for into the daily lives of extinct mammals,” he said.