A evolução dos maiores animais terrestres de todos os tempos

Elephants are the largest land mammals today. And they are huge, a relic of the megafauna that remains to this day, albeit endangered. But even they can feel light next to the size animals can reach in water! While the largest African elephants can weigh 7 tons, the biggest blue whales can weigh in at 180 tons! 25 times bigger, probably the biggest animal that ever lived. But being big in the water is infinitely easier than on land, for many reasons. The main thing has to do with support and structure. Stranded whales die of suffocation by their own weight, since in water, as the density is very similar to that of the body, weight means almost nothing. Huge ships float taking advantage of this principle. And this is one of many other difficulties of gigantism on land, involving energy, movement, breathing, thermoregulation, growth, feeding, etc. The largest known land mammal, the Paraceratherium, commonly called the giraffe-rhinoceros, which lived in the Oligocene, weighed 15 tons, twice as much as an elephant. But there were land animals that weighed in the region of 70 tons, weighing 10x more than the heaviest of the heaviest land animal today, 5 times the size of the largest land mammal that ever existed! As? Why? When? Where? What is the size secret? And what prevents mammals from getting so huge on land? That’s what I’m going to try to answer today. My name is Abner and welcome to ABC Terra! Recently, I visited expodinos, in Ibirapuera park, which unfortunately has already left. In it, the main attraction was the Patagotitan, a model of the skeleton of what is probably the largest dinosaur and largest land animal that ever existed. There, people lined up to lie down next to a real femur, which was taller than most people. It was possible to get under the ribs, and the dimensions of the animal’s interior were larger than my room. The neck was longer than a bus, the legs like huge columns of a sturdy construction. He rented a farm in my mind, and became a permanent inhabitant of my imagination. Wherever I went, I tried to fit him in, and when I could, even for a second, imagine him alive, it scared me. And I did not understand this animal. It didn’t make sense. It was too absurd. So I started researching. “I was seeing that it was something fascinating! It was a group that was only known because of their size, you know… they are the biggest animals on earth… oh they are. Type: They are heavy animals! Yes, cool, and? They are very big animals! Uaaargh! Ah, very cool, but hey? How is their metabolism? How is the breath? I realized that some things were very open and I was enchanted by that. Yes, because to reach these sizes, something very different was happening in the bodies of these animals, right? Many things… Many things!” This is Professor Kamila Bandeira, doctor in zoology, specialist in South American titanosaurs. She described the Austroposeidon, the only Brazilian dinosaur that appeared in the first season of Prehistoric Planet. “It was ecstasy! It was a surprise, an ecstasy, I pinched myself a lot to find out that it was real, that it wasn’t a dream, a delusion or something!” Our complete conversation you can listen here on the channel too, it’s really worth it. But who were the titanosaurs anyway? They are part of the group of sauropod dinosaurs, the well-known long-necked and giant herbivores , such as Diplodocus and Brachiosaurus, being closer evolutionarily to Brachiosaurus. They are the most diverse group of sauropods, having emerged at the end of the Jurassic, diversifying and dominating the landscape during the Cretaceous, mainly in the southern hemisphere. Having been the only group of sauropods that still existed on the day the meteor hit. Thus, they were the most diverse and successful sauropods, but also the largest. We usually have no idea how big these animals are, but here’s a comparison. A human, an African elephant and a Brachiosaurus to scale. It would already be a huge and intimidating animal, but the largest titanosaurs such as Patagotitan were almost twice the size of Brachiosaurus! Before them, there had never been a land animal that pushed gigantism to that extreme, and there probably never will be again. Despite the success of titanosaurs throughout the Cretaceous world, it was in the southern hemisphere that much of their diversity developed, especially here in South America, where the largest land animals of all time lived, such as Patagotitan and Argentinosaurus. These super titanosaurs all seem to be tied to a time span of approximately 20 million years, from the Albian to the Turonian age of 110 to 90 million years. THE CHALLENGES Getting giant isn’t easy, evolutionarily speaking. It is a path that many animal groups have followed, and on earth it is even more difficult, because all the challenges of life in the atmosphere are raised to the tenth power when you are a giant. And most of these problems, if not all, stem from a simple fact: surface and volume increase at very different speeds. If you have two two-liter water bottles and you want the first one to dry as quickly as possible and the second one to remain full, you can spread the contents of the first one over a court, increasing the contact surface, accelerating gas exchanges. The second can remain in the bottle, with a smaller contact surface with the atmosphere, minimizing exchanges and delaying dryness. All animals depend on these gas exchanges, primarily to maintain their water content, breathe and exchange heat with the environment. But let’s think about the difference between an elephant and a mouse, for example. The mouse has a very large surface area in relation to its volume, which means that it cools down very quickly, but it also heats up very quickly. This low efficiency in maintaining its own temperature is one of the reasons for its rapid metabolism. The elephant, on the other hand, has a very large volume in relation to its surface, which makes all exchanges difficult. If an elephant gets too hot, it can take time to lose heat. That is why African elephants, adapted to hot environments, have large ears, which increase the contact surface and accelerate exchanges with the environment. It is also more difficult to breathe, since the oxygen that reaches every cell in our body comes from contact with atmospheric oxygen in our lung alveoli. Moving also becomes more and more energetically costly, as each step moves tons of weight, whose constant impact can easily injure joints and bones. On land, the difference in density between animals and the atmosphere is so great that gravity governs all aspects of our lives, unlike water, where problems of buoyancy and hydrodynamics are more influential. Another major challenge is obtaining energy, since the larger the body, the greater the caloric intake needed per day and the more sensitive one becomes to environmental and ecological changes, which is why giant animals are especially sensitive to extinctions, even the slightest ones. Therefore, saving energy for giant animals, and making the most of everything ingested is a priority. Which makes it even more incredible that sauropods have existed so large for so long! So how did sauropod dinosaurs mitigate these challenges to break unbelievable and almost mind-boggling records for size and weight? BONES It all starts with the bones of these animals, which were nothing like mammalian bones. Looking at the bones of birds, which are the only living dinosaurs, we realize that they are hollow, very light and strong, full of cavities, which makes sense for flying animals. But the amazing part is that dinosaur bones looked a lot like this long before they took flight. Therefore, they don’t have hollow bones because they fly or to fly, and yes, they could only fly because they had light bones. The bones of sauropod dinosaurs are full of small and large cavities, very active, capable of remodeling bone very quickly, which played a large role in regenerating impact bone stress, something that any animal weighing tens of tons has to face. This remodeling and regenerative capacity could be very useful in cases of fights between males disputing territories or females, for example, which could be very common. FOOD ENVELOPE Another part of the secret that allowed sauropods to grow so large is the most obvious and striking feature of these animals: the ridiculously long neck. It is possible that the neck had a number of functions in life, attracting mates, using it as a weapon, as a thermoregulation surface, but it is certain that they were a powerful energy-saving strategy. This is because the long neck allows the animal to feed on plants in a very large area without having to move from one place, thus expanding its food envelope. It may seem like a small bill, but when you spend most of your waking time eating, that savings makes all the difference. With this reach, they explored the most diverse eating habits that the herbivory niche could harbor, becoming the most competitive primary consumers during most of the Mesozoic. This is a strategy adopted by many herbivores to date, such as giraffes, gazelles, camels, and even the elephant, which even without much mobility in its short neck, has a large food envelope due to the reach of its trunk. GIGANTOTHERMY But this was not by far the only energy conservation strategy for these animals. They were everywhere. One of the things that consume the most energy and calories in the human body, for example, is the constant maintenance of our temperature at 36 degrees. This is true of all warm-blooded animals. heat is expensive. Of course, the constant hot temperature allows us to be more active, but it also gives us an insatiable hunger. That’s why animals whose temperature is regulated by the environment and who don’t produce as much heat of their own need to eat much less and less often, like crocodiles and snakes. Titanosaurs, like birds, living dinosaurs, were warm-blooded animals that needed to keep their bodies at a high temperature. But that was less of a challenge for titanosaurs than it is for today’s mammals for two reasons: First, planet Earth was almost 10 degrees hotter on average, so perhaps there was even more of a need to cool down than to warm up. And second, because the very size of the animals was capable of storing so much heat and it cooled so slowly that very little new heat must have been generated by the cells. They were gigantothermic. The combination of these factors made warm-blooded living much more viable and energy efficient. COLUMNAL LIMBS Sauropod feet are pretty weird, but there’s good reason for that. Forget the round feet of elephants they often appear in the media. They bet everything on the “less is more” strategy! On the front legs, they lost all the fingers, forming a concave structure with the bones of the hand, while on the hind legs, a wide foot with 3 claws guaranteed stability and ease of digging. These anatomical simplifications are much less vulnerable to injury from impact and weight than the complex meshwork of bones in the hands and fingers. “In the case of Titanosaurs, one way out was the progressive loss of digits, as ironic as that sounds. The loss of digits allowed them to have increasingly columnar limbs, remembering that they are always supported on four legs, the more you have a structure similar to a pillar, the more it is possible to grow without causing major losses that would cause great impacts for the animal . So they progressively lose all the digits of the hands, they are walking on the metacarpals which are these bones, the feet also lose some digits, not all as in the hand, but there is also a change in posture. If you compare, for example with more primitive sauropods, their walking still had their hands very open, and progressively you make that more in the form of a column, more erect, straighter, and making the limbs themselves when they still have digits, quite columnar.” AERIAL SACKS But these factors alone still do not explain our question, as this alone would not allow them to reach the maximum sizes of titanosaurs. Quite possibly the most important factor of all is a feature they share with birds: the air sacs. These structures attached to the respiratory tract and embedded in the skeleton are capable of both flight and ultra gigantism. Birds don’t breathe like people, their breathing is much more efficient than ours. We are never able to breathe in completely new air, because the air goes in and out through the same place, preventing us from exchanging 100% of our lung’s air content with each breath. In fact, with each breath we only exchange a little air from the lung. Birds, on the other hand, have air sacs spread throughout the body, which exchange temperature and oxygen, but which also store air in a way that allows the lung to receive new air all the time. Sauropod dinosaurs shared this trait with birds and all theropods. We know that they were housed in the vertebrae of the neck, but also inside the bones and scattered throughout the ribcage. This changes the game completely, because it means that a sauropod would not be as dense and heavy as a mammal of the same volume. This increases the contact surface of exchanges, facilitating thermoregulation, breathing and support for these animals, making them lighter and absorbing impacts. With their bodies filled with air sacs, they were able to lead surprisingly energetic and active lives for animals of their size. REPRODUCTION When you are so huge, almost everything is a challenge, including reproduction. It is possible that they were able to lift themselves up enough to mount each other, but without the help of a very long and mobile male organ, fertilization would be very difficult. Titanosaurs nested in large groups, in regions of volcanic soil and geothermally active, which certainly accelerated and facilitated the development of eggs. “It is very interesting in Patagonia they also find grooves in the ground, filled with sediment and with eggs, these hind claws opened, they dug these grooves and there they deposited the eggs, they were their nests, dug with their hind legs, in the walls of these grooves there are the marks of his fingernails” But it was leaking, because they didn’t stay around to take care of their little ones. They were supposed to spawn in groups to increase their chances of survival. “Despite the enormous size of an adult sauropod, they were born at 15, 20 centimeters, it’s very small! There should be a hatching moment, the eggs and it should be like sea turtles, which hatch and then a bunch of predators come waiting for the turtle to appear because they are going to get a baby. They should all wait anxiously for the hatching season to fill their buchinho.” 44:54 They were born small, but they were many, the opposite of the elephant’s strategy, which has only one calf, but it is large and well developed. And in about 1 year they reached half the size of an adult. These impressive growth rates were not sustained throughout life, slowing down rapidly at the onset of reproductive age. PREDATORS Very young sauropods could even be easy prey for predators, but an adult would hardly find a worthy opponent. Its size may have been a response to predation pressure from some of the largest land carnivores that ever lived: the Carcharodontosaurids. Being giant enough to discourage predators may have been a very effective ecological and evolutionary strategy for sauropods for a long time. This, combined with the fact that the adults roamed in packs, made them extremely difficult targets for even the biggest predators of their time. To bring down a titanosaur, these predators would first need to separate it from the pack, then attack it as a group, in a concerted, cooperative effort to take down impossible prey alone. So it could be that the gigantism of titanosaurs and carcharodontosaurids is the consequence of an evolutionary arms war, in which size definitely mattered. ENVIRONMENT Many of the South American titanosaur fossils are found associated with sediments interpreted as semi-arid environments. Which is at least counterintuitive, right? How would these gluttonous herbivores be able to sustain themselves in this environment? We know that the plants that these animals fed on, especially in these environments, had a rich caloric and nutritional value, compared to other plants, which means that a smaller mass of vegetation would be necessary to satiate these flocks. This can also be explained by fossilization bias. These environments were most likely simply some of the most predisposed to fossilization, or are the only ones that have survived erosion to this day. But they were just passing through. Being able to clear a huge area of ​​vegetation in a short time, they were always on the move, an important strategy for gigantism. These herds constantly migrated in search of greener regions throughout the year, allowing time for the vegetation to recover between the passages of the herds. In this way, the balance between plant growth and flock feeding was maintained. Titanosaurs were the last group of sauropods to go extinct. They were there when the meteor fell, resisting until the last day of the Cretaceous. In North America, one last species of titanosaur, Alamosaurus, witnessed the planetary-scale explosion that ended 180 million years of dinosaur ecological dominance. It is possible that some of these animals in the Southern Hemisphere and Asia may have survived earthquakes, fires, tsunamis and heat waves, only to see the land dark, with reddish skies, and in gray, in the first days, weeks or months of the Cenozoic. If true, they died of starvation, with photosynthesis extremely limited, and the ecosystems more than impoverished, devastated. Perhaps, a last brood of sauropods was born during the beginning of the mammalian age, never having had the opportunity to grow up like their parents, and being the last prey of hungry and desperate predators, who would not survive either. And that was the end. Never again, in over 66 million years, have the continents seen such large animals, and most likely, never will again. It is for these reasons that elephants and mammals in general are unlikely to ever reach the size reached by titanosaurs. His biology was too unique. To understand more about paleontology and natural history, get to know my course “evolution, earth and time”, a didactic introduction for those who want to start studying these things. The first class is public and you can watch it here, but the course continues for channel members for only 7.99 reais per month, with a new class every month until the end of 2023. This is the best way to help the channel to keep existing and posting. A special thanks to the channel’s art director, Lucas Mateus, who made these amazing Patagotitan models. If you liked this model of his, take a look at these others, printed in 3D and shipped all over Brazil! Tyrannosaurus, Giganotosaurus, Mammoth and Borealopelta available at abcterra.com. I’m sure they’ll look amazing in your home too! And if you liked this video, you might like the most recent documentary on the channel even more , in which we explain how we know the colors of 5 dinosaurs, just click to see! Thanks to Kamila Bandeira for the interview on which this essay was based! And as always, thank you so much for following along to the end and have a great life!