Paleontologists have so far identified the earliest example of a placental mammal in the fossil record, which could provide new insights into how our furry ancestors came to dominate Earth after the extinction of the dinosaurs.
They made the breakthrough by studying the odontological (tooth) equivalent of tree rings — growth lines and elements preserved in fossil teeth — which they used to reconstruct the daily life of one of our early cousins: Pantolambda swimwear, a stocky dog-pig-like creature, which trotted about 62 million years ago – shortly after the extinction of the dinosaur.
By doing this it turned out that pantolambda mothers were about seven months pregnant, before giving birth to a single, well-developed, mouthful of teeth, suckling for just 1-2 months before becoming fully independent.
“I’ve studied dinosaurs for most of my career, but this mammalian growth project is the most exciting study I’ve ever taken part in because I’m amazed that we were able to identify chemical fingerprints from birth and teats in teeth that are so old,” says Professor Stephen Brusatte from the University of Edinburgh, who was involved in the study.
Placental mammals account for the majority of mammal species living today, from humans to small shrews and giant whales. They give birth to relatively mature young, which have done much of their growth in their mother, fed by a placenta.
Although mammals existed in the time of the dinosaurs, mammals really started to diversify and grow large when they became extinct. One idea is that their ability to give birth to large, well-developed babies previously fed by a placenta was key to their success. This style of growth and reproduction also allows human babies to be born with such large brains.
But when exactly this lifestyle originated was a mystery. Because the bones of early mammals were small and fragile, fossilized remains of, for example, hip bones are often missing, which could be used to understand the reproductive styles of species. Better preserved are teeth, the size and shape of which paleontologists have long studied to learn about the lifestyle of extinct mammals.
The new technology builds on this tradition. It involves cutting fossil teeth into extremely thin sections to examine growth lines, and vaporizing them to understand their chemistry at various stages of development. “It allows us to look at virtually any fossil mammal and reconstruct things like its gestation period, how long it suckled, when it matured, and how long it lived — things we couldn’t do before in fossil mammals now,” said Dr Gregory Funston of the University of Edinburgh, who led the study.
In case of pantolambdaFunston was surprised to discover how advanced this trait seemed to be at this point in mammalian evolution.
“One of the closest analogs in terms of its development is things like giraffes, which are born directly on the plain, and they have to move within seconds or else they will be hunted,” he said. “We expected that these kinds of life histories would have developed slowly and that they would become more and more specialized over time, but what we’re seeing is that pantolambda, only 4 million years after its extinction, history is already experimenting in this completely new way of life.”
Funston hopes the study can open up a new frontier for research into fossil mammals and how they evolved. “This method opens the most detailed window we could hope for into the daily lives of extinct mammals,” he said.