The early mammal Pantolambdas took over the world after the dinosaurs went extinct 66 million years ago.
The early mammal Pantolambdas took over the world after the dinosaurs went extinct 66 million years ago.
62 million years ago a mother gave birth to a baby. The baby got over the shock of birth in minutes and began exploring the world around her. The baby began suckling on its mother, a natural instinct shared by all mammals of its kind.
Every day it grew, and after a month or two it began to feed on shoots and leaves. It would have become independent shortly after, but tragedy struck. It died after only two and a half months.
But this baby’s story doesn’t end there. For 62 million years later, his distant cousins (humans) would discover his skeleton fossilized in the harsh desert of New Mexico in the southwestern United States.
It is classified as an early mammalian species, a bear-like animal named Pantolambda Bathmodon. The clade to which the species belongs (Pantodonta) became extinct in the Eocene, about 10 million years after the birth of the fossil baby, leaving no living descendants.
An international team of scientists and I used his bones and teeth to reveal his life in unprecedented detail. And our findings could help explain how mammals like Pantolambda took over the world after dinosaurs went extinct 66 million years ago.
Your skeleton tells the story of your life. Trapped in the dense minerals of your dazzling smile, tiny lines mark the growth of your teeth every day, which grow inwards for most of your life.
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Your chemistry will reveal your diet. Elemental building blocks from your diet are recycled to build your tissues while preserving their original chemical fingerprints. Your bones grow like trees and leave annual rings.
So your skeleton acts as a kind of diary, recording some of the most important events you experience, such as birth, starvation, or injuries. Scientists can reveal this calendar by slicing the bones and teeth into wafer-thin slices—so thin that light can shine through.
We did this for the baby Pantolambda fossil along with several adults. We followed the daily growth of the two and a half month old baby’s teeth. This told us that his teeth were growing rapidly, some formed in two months. Others, like the large molars, took up to six months (our molars take three years to form).
But his teeth revealed something even more important. As in so many of our own calendars, one big day was highlighted: his birthday. A distinct birth line marked the day this baby was born, and the same line also appeared in the teeth of the adult Pantolambda, evidence that adult teeth began to form before birth.
Our analysis showed that Pantolambda babies were born with a full set of baby teeth and their adult teeth replaced them within a year of birth. Most mammals today have milk teeth, but they are not usually replaced for years. We also delved deeper into Pantolambda’s early life by mapping the changing chemistry of the tooth. For that we needed lasers.
Using a microscopic laser, we vaporized the tooth bit by bit. Like a needle on vinyl, the laser played back a recording of the chemical changes our bodies undergo as we are born, breastfed, and weaned. By matching this record to our timeline using daily growth marks, we were able to determine how long Pantolambda mothers were pregnant and how many days the baby was nursing.
Our study, the first of its kind in a fossil mammal, showed that Pantolambda had a long gestation (seven months) and a short lactation (one to two months). The data also show that Pantolambda’s lifespan would have been quite short, four to five years, with some reaching around ten years.
This lifestyle is similar to the placental mammals, the group to which we belong. It is the earliest evidence of this type of lifestyle in the fossil record.
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Placenta species are special, you guessed it, our placentas. While many mammals and even other animal species (e.g. fish) have a placenta, we and our relatives have a more developed placenta that provides better nutrition, waste disposal and protection for the fetus.
This last aspect is crucial. Other mammals may not have long pregnancies because the mother’s immune system can reject the fetus if it grows too large. Our special placenta prevents this, and our study suggests that Pantolambda must have had it too.
Longer gestations allow placental mammals to give birth to larger babies, which we hypothesized was a shortcut to larger sizes in adulthood. They grew bigger and faster than the other mammal species.
These are the marsupials (including kangaroos and koalas) and the egg-laying monotremes (like the platypus). But history, as always in science, is ambiguous. A recent study showed that an extinct group of mammals, the small, rodent-like multituberculosis, also appears to have had placenta-like reproduction, or at least brief lactation periods, like Pantolambda.
We now know Pantolambda’s life story better than just about any other fossil mammal. In fact, we probably know Pantolambda’s life history better than some rare mammals in existence today (one of several recently discovered bat species, for example). And it’s early placental species like Pantolambda that we have to thank for the amazing diversity of mammals around us.
Although the mammals living at the same time as Pantolambda evolved so rapidly that we have difficulty tracing their ancestral tree, we do know that they created the first mammalian-dominated ecosystems. These mild-mannered survivors who inherited Earth made the most of the opportunity and established one of the greatest dynasties the planet has ever seen.
(The conversation)
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