A little furry creature scurries through the hellscape left behind by the deadliest day for living things in Earth’s history as it dodges the darkness, ash, and lethal heat. It digs through the debris, grabs an insect to eat, and crawls back to its hiding place. The dinosaurs’ dead and decaying remains, which have terrorized mammals for ages, are all around it.
These were the first few weeks and months following the stunning end of the Cretaceous period, which was caused by the collision of a six-mile-wide (10-kilometer) asteroid with the coast of modern-day Mexico. As the Paleocene epoch began, the forests were on fire, tsunamis shook the shores, and massive amounts of rock, ash, and dust were ascending kilometers into the air.
But life did exist in this universe. The first known primate, Purgatorius, who resembled a hybrid between a shrew and a little squirrel, was one among the survivors. In the midst of this worldwide tragedy, its population would have undoubtedly decreased, but the species endured.
Soon after the asteroid strike that wiped out three-quarters of the Earth’s existing species, early mammals led similar lives. Only the Great Dying, which occurred 252 million years ago, was more devastating (albeit less abrupt), eradicating 90% of marine life and 70% of terrestrial species.
Famous dinosaurs like Tyrannosaurus and Triceratops, as well as lesser-known but equally weird species like Anzu, or the “chicken from hell,” were all wiped out by the asteroid that brought an end to the Cretaceous period. There were dinosaurs with duck bills, long necks, and armor covering every inch of their bodies. But very quickly, every single one of them perished.
Mammals like Purgatorius were diminutive and tenacious under the shadow of these Late Cretaceous monarchs and queens, and many of them filled the same ecological niches as rodents do today. How did this diverse collection of seemingly helpless creatures, including our ancestors, survive the apocalypse?
The Rise and Reign of the Mammals author Steve Brusatte and his colleagues at the University of Edinburgh have been attempting to answer this question.
The day the asteroid struck was a really horrible day to be alive for anything, including mammals, birds (the avian dinosaurs), and reptiles, Brusatte emphasizes. According to Brusatte, “This was no ordinary asteroid; it was the largest asteroid to strike the Earth in at least the last half a billion years.” “Mammals nearly became extinct like the dinosaurs.”
There was plenty at stake. According to Sarah Shelley, a postdoctoral researcher in mammal palaeontology at Edinburgh, there was a surprisingly high diversity of mammals present as early as the Late Cretaceous. Numerous them were these insectivorous little things That were up in the trees of burrowing.
According to Brusatte, about nine out of ten mammal species perished when the asteroid impacted, providing the survivors with a previously unheard-of opportunity.
Just picture yourself as one of our ancestors, a mouse-sized creature hiding in the shadows, suffering during this period in Earth’s history, advises Brusatte. When you emerge from the other side, the world is suddenly open and the T. rex and long-necked dinosaurs are gone.
This cataclysmic extinction paved the way for an enormous diversification that gave rise to blue whales, cheetahs, dormice, platypus, and, of course, humankind.
But first, a slight problem: the world’s woods had been destroyed by wildfire, and the sky was choked with ash, blocking sunlight and impeding photosynthesis in plants. Ecosystems were disintegrating “like houses of cards,” according to Brusatte. The Earth’s surface was expected to experience a series of heat pulses that would make it hotter than an oven, followed by a nuclear winter during which the average temperature would fall by 20C (36F) for more than 30 years. Although many of the most hazardous predators for mammals had vanished, the environment had changed to an unimaginably harsh state.
What then did mammals do?
The small size of mammals, which had been restricted by dinosaur competition and predation, became a benefit for the “disaster fauna,” as the survivors of the asteroid are known.
According to Brusatte, these creatures most likely behaved and looked like mice or rats. They would typically be quite nondescript, but in this strange new world they were spreading because they were so well suited to the dreadful conditions immediately following the impact.
Animals’ ability to reproduce may have benefited from their small size. In modern mammals, “the bigger the animal is, the longer the gestation time is likely to be”, explains Ornella Bertrand, a postdoctoral researcher in mammal palaeontology at the University of Edinburgh. For instance, whereas an African elephant takes 22 months to gestate, whereas a mouses last about 20 days. Faced with the Apocalypse, the mouse have the better odd for keeping it population up.
A larger body often takes longer to mature sexually, which is another reason dinosaurs, especially the larger ones, didn’t survive. “They took a long time to develop into adults. It took around 20 years to create something like a T. rex “Brusatte says. It’s not that they didn’t grow quickly; rather, a large number of them were so massive that it took a long time for them to develop from a small hatchling into an adult.
The “extremely strange” body shapes found in the Paleocene and later provide another clue as to how mammals escaped the effects of the asteroid. Shelley examined ankle bones, which are compact, durable, and well-preserved tiny bones, to determine how similar early Paleocene mammals were to one another and to mammals in existence today.
“The Paleocene mammals are strange, as we discovered. They differ from contemporary mammals, “states Shelley. And what links them together is the fact that they have such substantial, strong morphologies.
These mammals resemble ground-dwelling and burrowing species the most among living creatures because they have massive muscle attachments and generally substantial bones, claims Shelley. The theory that emerged from this was that the creatures that survived the extinction did so mostly because they could dig to go underground, endure the fires, the nuclear winter, and simply hunker down for a while.
There is no other term to describe the survivors as hench, therefore their grandchildren also received their strong body types. During the Paleocene, “you can see it for that 10-million-year span,” adds Shelley. “They’re incredibly substantial, even if you’re an animal that lives in trees.”
Bertrand speculates that if mammals did adapt to living underground—either by digging their own tunnels or using others’—this may have also affected their level of agility. Because to the forest’s collapse, she explains, “all those animals dwelling in trees no longer had a habitat.” So, one of the theories is that there were fewer animals that could act in a highly nimble manner.
Bertrand wants to examine if the inner ear bones of mammals from this time period support the theory that the asteroid took an underground detour. Since the inner ear is essential for balance, an animal’s ability to move with precision and agility may sometimes be mirrored in the fragile bones that make up the inner ear. They wouldn’t have needed to be as agile if they had been big diggers. It might provide us with more hints, she said. The disadvantages of depending too heavily on bones to infer how an animal moved around, however, are pointed out by her, something that struck her while viewing the current.
Most live plants, the foundation of numerous food chains on earth, were wiped out by the asteroid. Mammals that could adapt their palates to anything did better than those with more specialized diets.
According to Shelley, “the species that survived the extinction basically just survived by not being too specialized.” For instance, the Didelphodon, a carnivorous marsupial relative that was the size of a cat, preyed on creatures that had become scarce due to extinction. It lost its niche because it specialized too much, according to Shelley. “A little animal, however, may more quickly adjust its food and way of life. That is a sensible strategy for avoiding extinction.”
According to Brusatte, there were a few specializations that would have performed well in addition to those who could generalize. Seed-eaters in particular were lucky. He claims that seeds served as a food reserve that was simply there for any animal with the necessary digestive capacity. Therefore, if you were a dinosaur like a T. rex, you were out of luck because evolution did not give you the capacity to consume seeds. But whoa, what kind of wonderful fortune is that for animals with beaks and some birds that were specialized seed eaters?
When the nuclear winter ended, seeds not only helped replenish the disaster fauna but also helped reestablish forests and other plants. When the sun came out, those seeds survived in the soil.
Ecosystems started to recover as the Paleocene progressed, and mammals started to fill the voids left by the non-avian dinosaurs. Following the extinction of the dinosaurs, Bertrand claims that mammals immediately began to diversify and were extremely diversified.
One factor was that bodies grew swiftly. The Edinburgh researchers discovered that for a while, the size of mammals’ brains did not keep up.
According to Bertrand, “I think that’s incredibly essential, because we might think intelligence is what makes us live and be so able to dominate the earth.” But based on the findings, it doesn’t appear that animals’ enormous brains helped them survive the asteroid.
In fact, mammals with enormous brains compared to their body size may have suffered in the early Paleocene. “Why would you grow a huge brain?” is the query. Bertrand queries. “It is actually expensive to keep a huge brain. If you have a large brain, you must feed it in order to keep it alive; else, you will perish from lack of food.”
The more advantageous adaptation wasdf to grow big and strong. Within a few hundred thousand years of its demise, the herbivore Ectoconus—a member of the Periptychidae, which may be connected to contemporary hoofed mammals, the ungulates—reached a height of roughly 100 kg (220 lb). That is a moment in geological time. It’s absolutely crazy, in Shelley’s opinion, that they are growing and specializing so quickly. And as you can see, larger carnivores emerge very quickly once herbivores of a certain size do.
Numerous other enigmatic mammals experienced dramatic growth in size as well. Taeniodonts, for example, grew enormously quickly, according to Shelley. Taeniodonts don’t have entire skeletons, but their skulls are about the size of a large butternut squash, and they seem to be one of the species that developed chunky skulls as an adaptation for digging. It has enormous front teeth that resemble rodents and teeny-tiny openings for beady little eyes, but that’s about it, according to Shelley. They are quite mysterious.
Shelley claims that this quick menagerie of mammalian life that developed after the catastrophe fauna had been disregarded for far too long. They’ve been labeled as old, primitive, and generalized, but in reality, she claims, they’re just different. “The second-largest mass extinction in the history of life was survived by their forebears. They weren’t just undifferentiated idiots who limped through existence. They were flourishing, surviving, and performing exceptionally well.”
These animals filled numerous ecological gaps left by the majestic, highly specialized dinosaurs that were so well suited to the Late Cretaceous but utterly unprepared to survive in an asteroid-hit world.
According to Brusatte, “it’s astounding to think that you had a group like dinosaurs that had been around for so many tens of millions of years, that had done such sublime things as evolve into giants the size of airplanes, and meat-eaters the size of buses and all these things – and then it all came crashing down in an instant when the Earth changed so quickly.” They simply couldn’t adjust since they were so unprepared for the new reality.
The Edinburgh crew seems to be in tune with the event’s arbitrary nature.
We primarily ended here here by accident, explains Bertrand. “The species that evolved could have been different if the asteroid had not hit Earth or if it had landed in the ocean of another part of the world. When I consider the entire situation, it’s absurd.”
Brusatte concurs. “It might have just flown right by, stirred the atmosphere’s upper layers, or disintegrated as it drew closer to Earth. It had the option to do anything, but by sheer dumb luck, it headed straight towards the Earth.”
Maybe it’s a good thing it did for the mammals that are alive now.