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Live fast and die young, this was the life history of the first large mammals

When the asteroid wiped out the dinosaurs 66 million years ago, some mammals survived and grew in size very quickly to fill ecological niches vacated by Tyrannosaurus rex, Triceratops and other giant dinosaurs.

Pantodons (Pantodonta) are ancient beasts, about 62 million years old, that were the first known group of large herbivores that evolved into a wide range of forms, but became extinct in the Eocene. These and other mammals responded to the extinction of the dinosaurs by diversifying and increasing their body size.

A team of scientists, led by Gregory FunstonCanadian paleontologist at the University of Edinburgh (United Kingdom), photographer and explorer, has discovered through dental analysis of fossil remains from the site in Badlands National Park, South Dakota (USA), that the first large mammals after dinosaurs grew twice as fast as modern dinosaurs of equivalent size and had a comparatively shorter lifespan.

In their study, published in the journal Nature, the authors used various methods, such as dental trace element mapping, to shed light on the life history of the pantodon Pantolambda bathmodon.

“We know they grew fast and died young from growth marks on their bones and teeth. These fossils show a slowing rate of development just before the first annual growth mark, which occurs when an animal reaches sexual maturity and deviates the energy from growth to reproduction”, Funston told SINC.

This shows that P. bathmodon reached sexual maturity before its first birthday, which is quite fast for a mammal of its size. “We found that it died at an equally rapid rate, because we sampled multiple individuals from a site where they are kept together,” he continues.

“Being able to produce large offspring, which mature for several months in the womb before birth, helped mammals transform from humble mouse-sized ancestors, which lived alongside dinosaurs, to the wide variety of species such as humans to elephants and whales, which exist today,” Stephen Brusatte, who is part of the international team from the University of Edinburgh, the University of St Andrews, the Carnegie Museum of Natural History and the Museum of Natural History and Sciences of New Mexico.

Reconstruction of ‘Pantolambda’, juvenile and adult. /S Shelle

Short lives with well-developed pups at birth

Most of the individuals at the site studied were between three and four years old when they died, but the oldest was eleven. In this way they found that even the oldest specimen found died at an age close to half the life expectancy of an animal of that size today: about twenty years.

“This tells us that the Pantodonta were short-lived. It’s not clear why they adopted this strategy of living fast and dying young, as is the case with the evolution of many of their traits. But once they adopted this lifestyle, they would have provided a way to reproduce very quickly for an animal of its size By giving birth to well-developed young that have a high chance of surviving, and having them reach sexual maturity quickly, the number of individuals reproducing would have been maximized Funston says.

This fact almost always ensures the success of a species, because it allows the population to grow very quickly. Only later could it have become a disadvantage, competing with others that need fewer resources to survive.

An unclear relationship to later mammals

This life story has no modern analogue. P. bathmodon reproduced as a placental mammal. The young were born in an advanced stage of development and were weaned in one or two months; then they died before reaching the age of ten.

“We suspect they’re a good example for other placental mammals from the same era. We know they’re closely related to many of them, but pantodons are special because they were the first type of mammal to get really big, and they were the biggest we’ve ever seen.” existed at that time,” says the paleontologist.

For this reason, they are a central group to the question of size in this time period, revealing the mechanism they used—larger offspring—to become huge on rapid evolutionary timescales. “We don’t think all mammals at the time adopted the same strategy, but it’s likely that they were at least similar to what we see in Pantolambda,” he continues.

It is unclear what place the pantodons occupy in the mammalian family tree. They are thought to be closely related to placental mammals, which includes us. However, it is difficult to unravel the relationships of these living things, just after the extinction of the dinosaurs, because they do not yet have the sets of characteristics that are used to group living species today.

“Furthermore, many of these species evolved separately with similar characteristics to help them tackle the same problems, so it’s difficult to distinguish which similarities are due to shared ancestry and which have evolved independently to tackle the same ecological problems. No However, we know that species like Pantolambda were in the mix of the first placentals that arose to take over the ecosystems left behind by the dinosaurs”, stresses the expert.

Drone image of the Badlands Fossil Bed. /G Funston

Drone image of the Badlands Fossil Bed. /G Funston

Specify the date of birth of these extinct animals

With the advanced methods used in this work, scientists can pinpoint the date of birth of extinct mammals. It is a cutting-edge technique that reveals when a fossil animal was born, how many days it nursed and how old it was when it died.

“Trace element mapping is done by a very precise laser that vaporizes small bits of material at a time and sends these gases through a machine that reads the elemental composition. By vaporizing the entire tooth little by little, we can reconstruct its elemental composition by translating element concentrations in pixel colors for the corresponding part of the tooth,” explains Funston.

In this way, they obtain great precision. This ‘mapping’ had already been used in some living mammals and in some recent fossils, but never in such ancient remains. “Our study shows that it can still produce important results, which opens up a new opportunity to use the technique in other fossil animals,” he emphasizes. “It’s like reading your diaries, but engraved on your teeth,” says the paleontologist.

An appearance that has nothing to do with his genealogy

Pantolambda is not a mix of dog and pig in the genealogical sense, it was not closely related to either type of animal. However, it seemed like a strange mixture of dog, pig and bear, because it had a series of physical characteristics that we now associate with those groups. These include robust limbs (bears), a large, deep torso (pigs), and a face with a short snout and long tail (dogs). However, it was a completely different type of animal, about whose lifestyle we now know much more.

“We know that they were herbivores and that they weighed about 42kg as adults. In addition, they were fast-lived and born ready for action. the decade,” says Funston.

The lifestyle was that of mammals on our side of the family tree, called placental mammals. Unlike marsupials (such as kangaroos, whose young grow in a pouch), or monotremes (such as the platypus, which hatch from an egg), placental pups do most of their early growth inside the mother. facilitated by a specialized placenta that nourishes and cleanses the baby.

A challenge and a stimulus for researchers

For Stephen Brusatte, this study has been very inspiring: “Most of my career I have studied dinosaurs, but this project on the growth of mammals is the most exciting study I have ever been involved in, as I am amazed that we have been able to identify the chemical traces of birth and weaning on such ancient teeth.

Previous studies, using the same methods, were limited to living species or recent ancestors, such as Australopithecus, some two million years old. Funston and his team’s success is to use them on a much older fossil, setting up a new future to learn more about the hidden life of even older mammals.

“We are reconstructing the daily life of these mammals, not by comparisons and estimates, but with actual data corresponding to the days when these animals were alive. It is extraordinary that we now know more about the life history of Pantolambda than we do about some rare mammals.” living today. Perhaps most exciting is that we will be able to learn about other mammals with the same amount of detail, using the same techniques,” Funston concludes.

Reference:

Stephen L. Brusatte et al. ‘The origin of placental mammal life histories’. Nature‘.

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