The origin of placental mammal life histories

The jungle is quiet. Slinking through the understory towards a break in the trees is a small, dog-like animal. Its ears perk up as an unfamiliar grunt breaks the silence. Emerging into the sunlight, the dog-like animal spots a much larger creature, frozen in place, across the clearing. Resembling an up-sized red panda, the larger mammal stares warily back. Its bulky body, seemingly invulnerable to the diminutive dog-like animal, is mismatched with its small, bear-like head, full of plant-grinding teeth. After a moment, the cause of its uncertainty becomes apparent: a baby, still wet from birth, emerges from the forest and reaches up to nurse from its mother. Upright but shaky on its legs, the baby looks over at the dog-like animal and bleats at it, flashing its own mouthful of teeth. The smaller mammal pauses to allow the mother and calf to move on. Quiet returns to the jungle.

Sixty-two million years ago in a Palaeocene jungle, a Pantolambda mother suckles her young, already mobile but born just moments ago with a full pelt of fur, open eyes, and a mouthful of teeth. Our new study reveals this lifestyle for the first time in a fossil animal, based on thin sections like that at the bottom of the image, from an upper milk tooth of a 2.5-month old juvenile. Artwork by Henry Sharpe.

Fast forward 63 million years. Three large primates gather around a machine that vaporizes material with a laser to determine its composition. Under the laser is a tooth of the young baby from the jungle, what we call Pantolambda, only two-and-a-half months old when it perished and was preserved forever as a fossil. Like a needle on vinyl, the laser plays out a record of the baby’s life, captured in a series of chemical changes reflecting its birth, early life, and death. Because although the story in the jungle is fiction, it is based on cutting edge science, which allows us to paint a vivid picture of life in the deep past. Just published in Nature, my team and I have developed a new method, using the chemistry of the teeth, to reconstruct the daily lives of ancient mammals. And this approach is revealing the earliest origins of our own lifestyle, in unprecedented detail.

Our study relies on daily growth lines recorded in our teeth, reflecting our 24-hour circadian rhythm. Onto this framework, we map out chemical changes in the tooth that reflect major transitions in early life: high levels of zinc deposited at birth, and enriched barium during the suckling period. Together, these lines of evidence allow us to reconstruct how long ancient mammal mothers carried their young, how long those babies suckled, and when their teeth erupted, all on the order of days.

For Pantolambda, one of the largest mammals of its time, this technique shows that its lifestyle was remarkably similar to large hoofed mammals of today. Our data show that it carried its young for 7 months, and the baby—likely just one born in each litter—probably had fur and open eyes. It was born well-developed, with a full complement of teeth, and needed only 75 days of nursing before it was completely independent. This is the first record of a precocial lifestyle in a mammal, a way of living, much like in horses, deer, and sheep, where the young are born ready to go. This lifestyle relies on the specialized tissues that give us and our allied mammals our names: placental mammals.

Comparison of the life history of Pantolambda (black diamond) to living mammals (coloured dots). Dots closer together represent mammals with more similar life histories, and the arrow indicates the trend in the data, where more precocial species are towards the top left, and more altricial species towards the bottom right. Pantolambda plots close to placental mammals (green dots), far away from marsupials (blue dots) and a monotreme (the Echidna; purple dot).

Placentals are the most common mammals around today, dominating every continent except Australia, the last holdout of the Cretaceous-conquering marsupials. Marsupials and placentals reproduce in different ways: marsupials give birth to tiny, jellybean-like babies that crawl to the pouch, latching onto a nipple to undergo most of the growth that placentals do while still inside their mother. Scientists have long wondered whether the placental strategy, enabled by a more complex placenta, underlies the greater success of placental mammals since the extinction of the dinosaurs (the delayed recovery after this extinction is why the jungle in our story was so quiet). While the story is certainly more complex, our study shows that this innovation may have given placental mammals an edge by giving them a fast-track to reach bigger body sizes.

Perhaps most importantly, though, our study provides a powerful new tool that opens a radical new window into the lifestyles of ancient mammals. This approach has the potential to answer major questions in mammal evolution, and it has some key benefits, not least of which is that it makes use of the abundant fossil record of mammal teeth, which had until this point limited our ways to understand reproduction and growth in these creatures.

The work was a major team effort, involving researchers from the University of Edinburgh, University of St Andrews, Carnegie Museum of Natural History, and New Mexico Museum of Natural History and Science. Major funding for the study came from a Royal Society Newton International Fellowship, with contributions from ERC Starting Grants, NSF grants, a Philip Leverhulme Prize, a SNSF Mobility Fellowship, and the University of Edinburgh School of GeoSciences.

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