The rise to greatness of the mammals

New fossil discoveries show that evolution began to set the stage for mammals to dominate the world long before the asteroid that wiped out the dinosaurs cleared the way for them.

By Steven Brossat וDeja-shi Lu, The article is published with the approval of Scientific American Israel and the Ort Israel network 01.09.2016

• Scientists have long wondered how and when mammals became the world's dominant vertebrates. But relevant fossils eluded them.
• A stream of discoveries, collected over the past 15 years, have helped document the rise of this group and clarify the role the dinosaur extinction played in the game.
• The findings revealed that mammals started much earlier than experts had imagined possible, and they developed some specializations as early as the time of the dinosaurs.
• The final extinction of the dinosaurs allowed placental mammals in particular to thrive.

One winter evening in 1824 he got up William Buckland, an English naturalist and theologian, to address the audience of the Geological Society of London who filled the room. Buckland was known for his impassioned lectures at Oxford University. He would flaunt his academic vestments and pass animal parts and fossils to his adoring students. Rumors have been circulating for years that Buckland possessed some fossils of giant bones found by workers when they were cutting paving stones in the English countryside. After almost ten years of research he was finally ready to make a statement. He told the audience that these bones belonged to a lizard-like animal, much larger than modern lizards, which he called Megalosaurus. The crowd was enthusiastic. Buckland has just revealed the first of the dinosaurs.

That evening was a defining moment in the history of science, a moment that ignited the enduring fascination that dinosaurs have on the human race. But what has been forgotten is that Kland issued another statement that day, which referred to a discovery much smaller in physical dimensions but no less revolutionary. Buckland said that in a review of the other fossils, which were found alongside the Megalosaurus in the stone quarries, there was another "particularly noteworthy" discovery: two tiny mammalian jaws, similar in size to the jaw of a mouse. Until then, experts believed that mammals were a relatively new creation and appeared long after the ancient geological era, which was dominated by giant salamanders and lizards. The two tiny jaws, the pointed teeth distinctly characteristic of mammals, were the first sign that the history of this group was much longer than previously thought.

These jawbones raised many questions. When, in the distant past, were mammals created? What did they do during the long period of time when the dinosaurs ruled? How did the classic mammals form in the pattern we know, which is characterized, among other things, by fur, mammary glands in the chest, a large brain, complex teeth and pointed senses? And why exactly did this unique group of mammals, who have a placenta like us (who give birth to well-developed living offspring), rise to greatness with a variety of more than 5,000 species scattered across the planet today, starting with featherweight bats and ending with the monstrous blue whale?

Nearly two hundred years after Buckland's lecture, these questions remained unanswered, because only a few fossils of early mammals had been found. A burst of spectacular fossil discoveries in the last 15 years is finally allowing scientists to chart their evolutionary journey, from those tiny creatures that lived in the shadow of Megalosaurus, to the vast variety of forms that have come to dominate the modern world.

Humble beginnings

Like all lineages, mammalian origins were humble. According to the scientific classification, the mammal branch on the tree of life includes egg-layers (Monotremes), pocket-sized (Marsupians, marsupials that raise their tiny babies in their pockets (and have placentas)Placental), as well as other descendants of the common ancestor who disappeared from the world. The first creatures that looked and behaved like modern day mammals were a variety of mammal-like, Mammaliaforms, a fancy name for the closest mammalian relatives. They evolved fromCynodonts, primitive biological types that still retained many organs of the ancient lizards.

The earliest traces of the mammal-like lineage are from 210 million years ago, from Upper Triassic which was an exciting time in evolution. A few tens of millions of years earlier, almost all life was wiped out in a mass extinction, probably caused by volcanic activity, marking the end of The perm turn and the beginning of the Triassic period. After most of the giant amphibians and reptiles that dominated the Permian period became extinct, a vacuum was created in the animal world during which most of the important animal groups emerged today. Turtles, lizards, frogs, alligators, dinosaurs (which eventually became birds) and mammal-like, pre-mammals all gained an advantage during this period of radical change.

Some of the best Triassic mammal-like fossils have come from rocks surrounding an icy spur of the Arctic Ocean that juts into the east coast of Greenland and is called Fleming Fjord. In 1990, an abundance of tiny teeth and jaws were found there that were invaluable in sketching the portrait of the closest mammalian ancestor. These fossils were not easy to come by. Frisch Jenkins, a legendary paleontologist from Harvard University who died in 2012, with his daring team pulled them from the frozen rocks. Jenkins was also as captivating and dramatic in his lectures as Buckland. The elegant professor, dressed in well-tailored suits, chalked up precise drawings of skeletons and organ systems in his anatomy classes. Jenkins, who had previously served as a marine in the US Navy, was a daring leader of fossil-hunting expeditions and always carried a rifle to protect the Arctic crews from the constant danger of fieldwork in northern latitudes: polar bears.

Jenkins' teams found fossils of three types of mammal-like: Caenotherides, Morganucodonts וThe Ramaidans. All were small animals between the size of a mouse and a shrew that had already developed several important mammalian landmarks. The most noteworthy is a fur cover that provides insulation from the cold and helps dissipate body heat when the temperature rises. The temporomandibular joint was simpler than that of today's mammals. It worked using broad jaw muscles to strengthen and allow for more delicate chewing movements compared to the random biting and swallowing technique of the cynodonts. The spikes on the teeth, especially on the molars at the back of the jaw, greatly optimize chewing.

The fossils from Greenland and other continents reveal that a change that took place in the development of the teeth and is considered a milestone, accompanied these innovations in the jaw. While cynodonts had teeth that constantly grew, wore down and grew back throughout their lives, the mammal-like ones displayed the familiar pattern of baby teeth alternating with mature teeth. Although we humans may curse this unusual set of teeth, when we lose teeth as adults and cannot grow new teeth, this unique way of growing teeth and replacing them is related to one of the hallmarks of mammalian biology. Young people without teeth or with milk teeth, can suckle from their mother's milk created in the mammary glands and hence the group got its name: the group of mammals. It is thought that these mammals nursed their young in the way that modern mammals do, a watershed evolutionary change that allowed mammals to grow faster, gave higher survival to the young and allowed them to increase their metabolic rate, a change that helped the mammals to be active in a colder environment, And especially in the dark of night.

The mammals also showed the beginnings of other important traits, including some traits that increased the level of intelligence and sensory perception. Advances in CT imaging in the past decade have allowed paleontologists to see details in the internal anatomy of the fossil bones, including the brain ventricles and nerve pathways. With this technology, they discovered that these early mammals had a huge brain compared to that of their ancestors, although it did not reach the dimensions of the brains of modern mammals. They also had larger olfactory bulbs and auditory regions, which gave them sharper senses of smell and hearing and expanded brain areas that processed tactile input from the skin and hair. They even upgraded the inner ear by surrounding it with solid bone to insulate those sensitive hearing aids from the loud chewing sounds.

Although these tiny mammal-like creatures have begun to acquire some impressive characteristics of modern mammals, they are far from the most prominent animals of the Triassic. This honor belongs to dinosaurs and crocodiles that reached monstrous dimensions and qualified for the top of the food chain. But what these mammalian ancestors lacked in size they made up for in variety. Recent studies led by Pamela Gil From the University of Bristol in England they discovered a surprising diversity of nutritional adaptation among these creatures. Gil and her colleagues used both a particle beam in an accelerator (synchrotron) to scan the teeth of mammal-like animals and engineering software to learn about their function, and showed that morganocodonts had teeth strong enough to crush large insects with a hard exoskeleton, such as beetles, while chenotherids had thin jaws and fine teeth that could, Most likely, deal only with a little more soft worms or moths. Another work of one of us (Lu) demonstrated that the hermiaidans could cut and grind small plants with their uniquely moving jaws.

Big Bang as the shooter

The prevailing view of the evolution of mammals holds that the ancestors of these mammals have not changed over tens of millions of years, which are a significant part ofThe Mesozoic Era (The period of time between 252 million years ago and 66 million years ago, which includes the Triassic, Jurassic and Cretaceous periods). At that time the dinosaurs were supreme rulers, while the ancestors of the mammals were inferior and their existence as small insect eaters running in the grass under the trees was unimportant. But the flood of new discoveries of fossils from different places on the globe put an end to this misconception. The adaptability of the mammal-like lineage was the common feature that accompanied them in the evolution of mammals, including the long period in which they lived alongside the dinosaurs, and it seems that this tendency to adapt to changes through diversity was the key to their success.

In the case of the mammals, their keen senses and coordination of small muscle movements (two skills enhanced by the larger brain), along with their increased metabolic rate allowed them to thrive in the cold, dark nights. And these were exactly the qualities that could help them survive when another disaster struck the earth. When the Triassic gave way to the Jurassic, approximately 200 million years ago, the supercontinent Pangea was torn apart and separated into subcontinents. Volcanoes erupted from the widening cracks between the rising continents while poisoning the atmosphere and collapsing the ecosystems. The mammoths were apparently able to survive this overwhelming event, hiding in ecological niches inaccessible to many other vertebrates at the time.

Many dinosaurs managed to survive the mass extinction of the end of the Triassic period and this group still dominated the Jurassic. But in the first 30 million years of this time period, the mammalian lineage underwent another vigorous episode of much faster evolution. Most of the evidence for this explosion of new genera and species came from the thousands of amazing fossils collected in the last decade from the rock formation Tiejishan in northeastern China. These wonderfully preserved specimens include fossils of insects, feathered dinosaurs, and dozens of mammal skeletons, many of which have a halo of fine hair around them. Unfortunately for them, these animals lived about 160 million years ago in an area of ​​lakes and forests that were sometimes bombarded with materials emitted from volcanoes that killed and buried them before they could rot.

Studies that led him and other researchers, including a team of Jin Meng from the American Museum of Natural History in New York, showed that these creatures had an extraordinary variety of body types, which allowed them to invade a wide range of ecological niches. Castorukauda, a late survivor of the mammal-like, was a dog-sized creature that lived in the prairie and had webbed hands and feet and a flat beaver-like tail. It is known as the earliest swimmer among mammals. Ducophosor It dug into the soil with shovel-like hands with claws and broad, jointed fingers, similar to the African golden mole of today. Agilodocodon It was a nimble tree climber that fed on the trunk by gnawing the bark with its shovel-like teeth. The strangest is probably theVolticotherium, who looks like a flying squirrel as he darts between the branches, rides air currents with a membrane stretched between his arms and legs. Such unique mammals were found not only in China. Lou andJohn Wible From the Carnegie Museum of Natural History in Pittsburgh they discovered in Colorado theThe protophosor, who used to dig and eat ants. Overall, the Jurassic mammal fossils found in recent years show almost every major lifestyle we see in small mammals even today.

During the Middle Jurassic period, from 174 million years ago to 164 million years ago, the number of mammal species skyrocketed. Robert Close Currently working at the University of Birmingham in England, he conducted a statistical analysis that compared skeletal characteristics and built a family tree through this analysis. A calculation he made to determine the rate of anatomical change over time showed that these genera evolved during the Jurassic almost twice as fast as the mammal-like evolution of the Triassic. This accelerated rate of change laid the foundation for the mammalian family tree: the tree shows that the lineages leading to the bivalves (Monotremes) of today (egg-laying mammals) andtherians (the wider group that includes the marsupials and the placental animals) split from each other, like two brothers who came out of the same routine.

Although many of the diverse Jurassic mammals described here belong to extinct lineages, such as were found on the family tree between bibs and therians, they are important and essential to understanding the origin of today's mammals because they help reveal the morphology of their ancestors. Those long-extinct branches of the family tree flourished alongside the precursors of today's mammals in the Jurassic, then the Cretaceous, before disappearing. They also conducted evolutionary experiments in many styles of nutrition and movement. These creatures interbred with each other and also with creatures that are the ancestors of today's mammals in an evolutionary spate. Researchers are therefore eager to understand why these early "experts" failed to survive into modern times.

flower power

at dawnCretaceous, about 145 million years ago, the mammalian pattern was founded. Big brains and a fast growth rate continued to be the key feature of the group. But a new evolutionary change, which at first glance seems marginal, appeared and changed the rules of the game: the appearance of a molar tooth with three ridges, when a ridge in an upper molar fits into a depression in a molar below it in the lower jaw, and the two work together in crushing the food just like a pestle and mortar. This structure of the teeth opened up a whole new realm of nutritional possibilities for mammals.

Equipped with teeth with three ridges, which allow for a versatile function, the therians began to diversify. Two evolutionary lineages diverged from each other and began separate evolutionary paths that eventually led to the main groups of mammals: The Utherians, which later developed into placental owners, andMetatherians who developed into the pickpockets. The oldest and most primitive remains belonging to these lineages came from China, where they scurried about on the forest floor, under the feet of feathered dinosaurs, as long as 125 million years ago.

Although these pioneering therians were around as early as the Cretaceous, their golden age had not yet arrived. They were few and small, it was rare to find someone bigger than a gerbil. In contrast, slightly more primitive mammals known by names Trichonodonts וCimetrodonts Mammals took center stage in the first 30 million years of the Cretaceous as they continued their early Jurassic successes. Some of these types were the largest mammals of the entire Mesozoic era - like The rephenomamus, a creature the size of glutton, who was one meter long and weighed 14 kilograms, who lived in the early Cretaceous period in China, whose stomach contents included the fossilized bones of small dinosaurs.

Then something unexpected happened, an event that changed the historical course of mammals: the development of a completely new type of plant, the angiosperm plants. These are the shrubs and trees bearing the flowers and fruits that are among most types of plants today, which provide many components of our diet and decorate our gardens. During the mid-Cretaceous period, seed-covered plants spread throughout the world and provided mammals with new sources of food: the fruits, the flowers themselves, and the insects that fed on them. The molar teeth (having three ridges) of therians, with the dual ability to crush and tear, were ideal tools for processing this new food and as a result therians multiplied. Meanwhile, mammals with more primitive dentitions such as the dinosaur-eating Rephenomamus dwindled in number and did not survive the Cretaceous period.

However, even in the face of these new opportunities on the menu, the success of the Therians was not yet guaranteed. A new competition began to ferment. While the therians were feasting on insects in the middle and end of the Cretaceous, some others, from more primitive groups of mammals, had developed complex tooth sets that were well suited to shearing and grinding the plants from the new seed coats. The northern continents were flooded bMultituberculates, rat-like vermin with "rabbit teeth". Despite their size, these creatures were not close to real rodents, but took on a similar body shape because they fed on the same foods. New studies of Gregory Wilson from the University of Washington andDavid M. Grossnickel from the University of Chicago showed through sophisticated statistical analysis of databases that contained fossil measurements that these creatures thrived in the latter part of the Cretaceous. They evolved into many types, grew in size and developed more complex molars, in a common evolutionary dance with the seed coats whose distribution expanded.

The southern continents also harbored competitors for the Therians. Paleontologists know very little about those late Cretaceous mammals, but intriguing new findings point to the flourishing of a strange group: Gondwanatharians (who, despite their name, were not really Therians). For decades, the only findings of these mysterious creatures were a few teeth: high-tipped, enamel-coated molars that grow throughout life like the teeth of horses and cows—ideal for grinding down hard plant material. In 2014, a team headed by David Kraus from Stony Brook University the first skull of a Gondwanatherian, a new named species Vintana who lived in Madagascar at the very end of the Cretaceous. It resembles a beaver and may have fed on the first grasses that developed in the Cretaceous.

From disaster to opportunity

At the very end of the Cretaceous, 66 million years ago, the mammals did everything. They have come a long way since their first appearance in the Triassic. Many of them were insectivorous therians, plant-chewing multituberculates or gondontherians integrated into the food web left behind by large dinosaurs such as Tyrannosaurus. They were still confined to the bottom of the food chain and were unable to spread to new areas of habitation.

But their fate, like that of many other creatures, suddenly changed: an asteroid hit the Earth and left behind huge fires, tsunami waves, earthquakes and volcanic eruptions that reshaped the Earth in days and weeks. This catastrophe and the long-term climatic and environmental changes that followed were beyond the powers of the dinosaurs. And as if by magic, these majestic creatures, who ruled the earth for more than 150 million years, were swept into the dustbin of prehistory.

The mammals also suffered a severe blow of extinction. Evidence of their decline came from a large fieldwork project he originally conducted William Clemens from the University of California at Berkeley, and is now run by Greg Wilson. During the 50-year project, fossils were meticulously collected in the state of Montana in the USA from the post-extinction time period. The findings show that many large mammals, and those that specialized in a unique diet, went extinct with the dinosaurs. The metatherians, which began to flourish at the end of the Cretaceous, were almost wiped out, and if it weren't for a few brave species that survived the disaster, their modern descendants, the kangaroos and koalas in Australia, would have had no chance to evolve.

Among the other mammals that managed to survive were some of the early placentals, those types that, like us, give birth to fairly developed offspring. Studies of the molecular clock, which calculates when distant ancestors diverged from each other based on differences in DNA between species living today, indicate that the common ancestor of placentals already evolved alongside the dinosaurs in the Cretaceous. But only after the extinction at the end of the Cretaceous did those advanced mammals develop and split into the main modern subgroups, including the rodents and great apes. The reason for the sudden extreme change is clear. When dinosaurs like Tyrannosaurus, Triceratops and their relatives were eliminated from the fray, these placental owners were left with a free playing field to conquer. Again they developed rapidly and filled every available niche.

Although researchers have long suspected that the names of dinosaurs served as a means of the rise of mammals, we now understand much better the exact role they played: specifically, it was the spark that ignited the placental revolution. Like all revolutions, this revolution also happened very quickly, over several thousand years, like the blink of an eye in geological terms. one of us (breast) did fieldwork in New Mexico to better understand the many aspects of that fateful moment, from which mammals continued to evolve and to learn how the diet and behavior of these survivors helped them in the post-disaster world. The colorful benefits of Nasimiento formation in western New Mexico, contain among them the best record in the world of the prosperity that mammals experienced after the death of the dinosaurs. Thomas Williamson, Brost's colleague from the New Mexico Museum of Nature and Science, has been wandering over these rocks for more than 25 years, during which time he has collected many thousands of fossils. Thanks to his photographic memory, he can remember every detail accurately in almost every one of them. The fossils include jaws and teeth belonging to a huge number of mammal species ranging from hoe-sized insectivores to saber-toothed carnivores and cow-sized herbivores. They existed already 500,000 years after the asteroid hit, a testimony to the speed with which the mammals took over the planet, once they were given the chance.

Thanks to their success, we humans live here and are able to tell their story. Among the placental mammals discovered by Williamson in New Mexico is the skeleton of a puppy-sized creature called Turrehonia, has long and thin limbs, long fingers and toes. It lived about 63 million years ago, but when you look at its graceful skeleton, you can almost see it leaping up the trees with its skinny toes clinging to the branches. Torrejona is one of the earliest known primates. Our near far away. An additional 60 million years of evolution were required to transform these small pre-primates into philosophizing, bipedal monkeys. This is just another chapter in the evolutionary journey of mammals, which has lasted for 200 million years and is still not over.