Could modern mammals survive among dinosaurs?

Unexpected diversity of early mammals at the time of the dinosaurs

Many lines of the various Jura mammals described here have long since died out. Most of them have their place in the family tree between the monotremes and the theria. Nevertheless, the origin of today's mammals can only be understood to some extent on the basis of the representatives of those branches that have disappeared. Because these types and forms allow conclusions to be drawn about the physique and appearance of their ancestors - and thus also about the ancestors of the higher mammals.

Those extinct groups thrived in the Jurassic and the Cretaceous that followed - while the ancestors of today's mammals went their own way. These ancient lines experimented many times with the same modes of nutrition and locomotion as the predecessors of modern mammals at that time. Within these lines, too, many similarly specialized forms and ways of life developed independently of one another. How could it then be that they finally perished completely and ultimately only the ancestors of today's mammals asserted themselves?

145 million years ago, at the beginning of the Cretaceous, the essential characteristics of modern mammals were established. They continue to rely on a big brain and rapid growth. There was also a new feature that at first glance might seem insignificant: so-called tribosphenic molars. An outgrowth of the respective upper tooth fits into an indentation of the opposite lower tooth. When grinding food, they work together like a mortar and pestle.

Such teeth were particularly versatile and versatile. This opened up completely new nutritional options for their owners. The Theria soon began to diverge in various directions and split into the forerunners of the placental animals and the marsupials. We know fossils of very early, still quite original-looking representatives of these two lines from China. Well over 125 million years ago, they lived on the forest floor, so to speak, under the feet of feathered dinosaurs.

Although these Theria pioneers definitely lived in the early Cretaceous, their heyday was still a long time coming. They were few in number and seldom larger than gerbils. During the first 30 million years of the Cretaceous, the more primitive Triconodonts and Symmetrodonts determined the mammalian scene, which followed on from their successes in the Jura. They include the largest mammals of the Mesozoic. So was Repenomamus more than a meter long and weighed 14 kilograms. The predator, somewhat reminiscent of a wolverine (bear marten), lived in the Lower Cretaceous in China and even ate young dinosaurs, as the fossilized stomach contents reveal.

An event of a completely different kind gave the mammalian revolution in the middle of the chalk a completely new turn: the flowering plants had emerged - the angiosperms, often incorrectly called flowering plants. At that time they spread worldwide. Today they represent a large part of our usual flora, the bushes and trees, "flowers" and grasses. We are not the first to feed on their flowers and fruits, sprouts and leaves. They offered the mammals of that time unprecedented sources of food, including, not least, an abundance of lured insects. The tribosphenic molars of the Theria, which could both cut and grind, were ideally suited to adapting to the new food palette, so that this group of mammals now got an upswing. Unmodern animals like Repenomamus with its old-fashioned teeth couldn't seem to counter this enough. After the chalk ran out, they were gone.

But the Theria faced competition from two other sides in the Mesozoic Ages: from two more primitive groups of mammals who also developed complex dentures with which they could cut and grind the new plants.

The multituber culates, the "rodents of the Middle Ages", had already originated in the Jura. They are not closely related to modern rodents, but they had a similar ecological meaning. With their protruding front teeth, mouse to rat size and body shape, they are somewhat reminiscent of modern rodents and a similar diet to them. This branch of the Mammalia flourished in the late Cretaceous, almost inundated the northern continents and, with its diverse life forms, is considered to be the most successful group of mammals of that time. Gregory P. Wilson from the University of Washington in Seattle and David Grossnickle from the University of Chicago have shown this with extensive statistics. At that time, the multituber culates formed many different and increasingly larger species. Quasi spurred on by the spread of the opaque species and their constantly refined adaptations, they developed increasingly sophisticated molars. This group apparently only died out around 34 million years ago, long after the end of the dinosaurs.

On the southern continents, the Gondwanatheria apparently competed with the real Theria in the late Cretaceous. Paleontologists still know very little about them. For decades it was mainly known that they - like horses and cows - had molars with a high crown that pushed themselves up from below for life. Accordingly, they ate hard vegetable foods, which heavily rubbed their teeth. Researchers working with David Krause from Stony Brook University (New York State) found the first skull in Madagascar in 2014. They baptized the rather large animal that lived at the very end of the chalk and whose head resembled that of beavers, Vintana. It could have eaten the first grass that emerged shortly before.

The almost immediate replacement of the dinosaurs after they had ruled for 150 million years

Right before the dramatic end of the Cretaceous around 66 million years ago, mammals were doing pretty well on the whole. Since their beginnings in the Triassic more than 200 million years ago, they had come a long way and found various niches in the process. They still preferred to stay in the undergrowth and fit themselves into food webs, at the top of which were gigantic predatory dinosaurs, above all Tyrannosaurus rex. But they had produced, among other things, the Theria - the forerunners of today's mammals - with many insect-eating species, and also the Multituberculaten and the Gondwanatheria, both of which knew how to utilize the most modern plants of the time and were important links in the food web.

When a large asteroid impact triggered huge natural disasters - which overturned the living conditions on earth within days and weeks and had a massive impact on the global climate - the doom of the dinosaurs was sealed after 150 million years of reign. Only a small group of them survived, that of the birds. Mammals were also hit hard. This is proven by an American research program whose employees in Montana have been meticulously collecting fossils from the relevant period for 50 years. First it ran under William Clemens from the University of California at Berkeley; it is now headed by Gregory Wilson from Seattle.

In fact, many of the larger mammals perished at the same time as the dinosaurs. Species with specialized diets also disappeared. The Metatheria (the marsupials and their relatives), which had just begun to flourish in the Upper Cretaceous, almost became extinct.

But a few of them survived. Otherwise there would be neither kangaroos nor koalas today. The remaining mammals that survived the inferno included some of the first placenta animals: Eutheria, which gave birth to their young at an advanced stage of development and until then fed on a well-developed placenta.

According to DNA family trees, a common ancestor of the placental animals originated in the Cretaceous. But their most important modern subgroups, such as rodents or primates, did not differentiate themselves until later. The background to their sudden upsurge seems obvious: as them Tyrannosaurus and Co no longer stood in the way, they had a free path, so to speak, and penetrated into all possible niches within a short time according to the proven evolutionary pattern of the Mammalia.

The pace of this evolution - almost a revolution - is nowhere more evident than in fossils of the Nacimiento Formation in New Mexico. This is a barren hill country, because of the different deposits, gently striped, with layers from the first millions of years directly after the dinosaurs went down. One of us (Brusatte) took part in field research in these "badlands" in order to understand in detail what was going on at the time. Among other things, he wanted to know more precisely which mammals had survived the asteroid impact and with which nutritional and behavioral patterns they survived the difficult conditions afterwards.

His colleague Thomas E. Williamson from the New Mexico Museum of Natural History & Science has been researching this rock formation for more than 25 years. He found thousands of fossils there, mostly jaws and teeth, and thanks to his photographic memory, he has an exact idea of ​​almost every single piece! It looks as if the placentals were just waiting for a spark to ignite. Because they unfolded almost instantly according to geological standards: practically within millennia. As early as 500,000 years at the end of the Cretaceous, they had produced innumerable new, completely different species, including shrew-sized insectivores, carnivores with saber teeth, and herbivores the size of cows. In other words, when the opportunity arose, the placenta animals immediately took over the planet.

Williamson also recovered a skeleton of an animal the size of a puppy in New Mexico: Torrejonia. If you look at its graceful skeleton, slender limbs and long, thin fingers and toes, you can well imagine how this lanky creature jumped around in the trees, grabbing twigs and branches. This was one of the first primates - 63 million years ago!