Sauropods are the biggest dinosaurs and dwarves, even the largest land mammal, Paraceratherium. There are many theories, such as dinosaur size was fueled by vegetation. During the Mesozoic, including the Triassic, Jurassic, and the Cretaceous, carbon dioxide was a lot more abundant in the atmosphere. Increased carbon dioxide increases the global temperature. The global climate was a lot warmer millions of years ago than it is today.
The high levels of carbon dioxide meant that plants had to recycle more and more carbon dioxide. Thus the world was full of many types of vegetation. From the Jurassic to the Cretaceous Periods, dinosaurs like Supersaurus, Sauroposeidon, and Argentinosaurus were shaking the Earth. How did they get to be so big? And why did we mammals never even come close to dinosaurs in size?
Why did dinosaurs get so big?
The most massive animal ever is a mammal: the blue whale. It can get up to 30 meters long and weigh as much as 145 metric tons, more than twice as heavy as the most massive known dinosaur. But, to be fair, the rules of biomechanics are different in the water. Buoyancy and fat can do amazing things, allowing sea creatures to grow to sizes that would be impossible on land. However, when it comes to the largest terrestrial animals, mammals were never competing for the non-avian dinosaurs.
There’s always some debate about what the biggest dinosaur was. But the current record holder for the largest specimen belongs to the newly-named titanosaur Patagotitan. Experts estimate that this Cretaceous herbivore stretched over 36 and a half meters and weighed upward of 64 metric tons. By contrast, the biggest mammal that ever walked on land was the hornless rhinoceros known as Paraceratherium. It weighed a mere 15 tons and stood about 5 meters high at the shoulder, roaming Eurasia from Romania to China. At the end of the Oligocene Epoch, long after the non-avian dinosaurs disappeared.
So, how can two such successful and prolific types of animals end up having such different size constraints? Part of it might have to do with how they reproduced. Paraceratherium was a placental mammal. It means that it gestated its young inside its body. If this ancient rhino were anything like the large mammals alive today, that would’ve taken a very long time. Big mammals like giraffes, rhinos, and elephants usually have only one offspring at a time. And gestation can last longer than some other kinds of animals live.
Elephants, for example, carry their babies for more than two years! Now consider the dinosaurs. They didn’t have to take their babies at all because all dinosaurs laid eggs. Even the biggest of the giant dinos hatched from an egg no bigger than a soccer ball. What does this have to do with size? Well, bigger mammal species give birth to bigger young. It requires a huge amount of time and energy to gestate. Dinosaurs bypassed that problem.
Instead of having bigger babies, the largest dinosaurs laid comparatively small eggs, which held equally small hatchlings. Reproducing this way with babies hatching and growing outside the mother’s body removed the size limits that gestation places on mammals. Dinosaurs also had another evolutionary advantage. Their skeletons had a special feature that mammals lacked: a sophisticated system of air sacs.
These sacs were pockets of soft tissue that were connected to the lungs. Think of them as biological balloons. Some of these sacs sat in the body cavity, next to bones, usually in the neck, back, and hips. But others ran inside the bones themselves. These air sacs helped shape the dinosaur’s skeleton and allowed the bones of the biggest dinosaurs to remain light without sacrificing strength.
How do we know that extinct dinosaurs had these sacs? Partly because non-extinct dinosaurs have them too! Birds have a similar system of sacs that help draw air into their tiny lungs while also making their skeletons remarkably light. The respiratory system of birds to those of the giant dinos is pretty striking. In sauropods, for example, the vertebrae of the neck and back have the same pockets where the air sacs were attached.
When paleontologists scan fossils of some dinosaur bones, like vertebrae, they often find hollow spaces inside the bones where the air sacs used to sit. Now, keep in mind that bones with spaces created by air sacs are different from hollow bones. And, not all of the extinct dinosaurs had these handy airbags. Only the type of dinos known as saurischians had them. In the traditional dinosaur family tree, this group encompasses the theropods, the two-legged dinosaurs that include today’s birds. The sauropods and the quadrupeds include the giant titanosaurs.
All of the rest of the dinosaurs, like the horned, armored, and duckbilled dinosaurs, are known as ornithischians. They didn’t have these features. So they weren’t as light on their feet. Mammals don’t have anything like them, either. We retained a skeletal system of dense, heavy bones that limit how big we can get before our bones crack under our weight. But the thing is, these adaptations are just what allowed dinosaurs to get so big. They don’t tell us why these giants all got so enormous in the first place! That’s a different evolutionary question, with lots of possible answers.
- Maybe living large was a way to stay safe from predators. Or maybe their size allowed dinosaurs to cover more ground or reach higher leafy branches in search of food. Or maybe there is something paleontologists haven’t thought of yet.
There’s another question to consider: Was the ability to grow so large an advantage? After all, the sauropods are gone. So are most of their relatives. Of the entire dinosaur family, only birds a single group of saurischian theropods survives. They range in size from the hummingbird to the ostrich. Evolutionarily speaking, it seems that bigger is not always better.
How did dinosaurs evolve to become so huge?
Dinosaurs, especially herbivores, may have evolved to massive sizes because there were many plants to eat. Bigger sauropods mean bigger prey. This means that carnivores possibly evolved to eat these massive herbivores. When they killed and ate herbivores, they ate more food. The herbivores were big with the process.
Another theory is that dinosaurs grew to massive sizes simply for protection and self-defense. Gigantism is thought by some scientists to be an evolutionary advantage and gave dinosaurs immunity to predators and protection. It also brought disadvantages such as slow speed and the need for limited population size. Massive Dinosaur size is a side effect of cold-bloodedness. Most paleontologists that study sauropods regard it as true that sauropods were cold-blooded because of two convincing arguments.
- First, based on physiological models, a warm-blooded sauropod would’ve baked itself completely.
- Second, no warm-blooded mammals such as Elephants, Giraffes, or Zebras come close to the size of sauropods.
Scientists believe that if a sauropod evolved to sizes big enough, it would have achieved the quote-unquote “homeothermy” or the capacity to preserve its interior temperature regardless of its environment’s conditions. Argentinosaurus would warm up at a slow pace in the sun during the day. And it cools down also at a slow pace at night, giving it an average body temperature. A small reptile in the same conditions would be at the mercy of the environmental temperature on an hour-by-hour basis.
During the late Triassic and early Jurassic, the Protosauropda, the forerunners, were rather small. Some were even bipedal walking on their two hind legs, much like modern birds. Like Eoraptor and Anchisaurus, these little dinosaurs were not much bigger than a fox-terrier, and some, like Plateosaurus, reached donkey sizes. However, there was natural selection working on making these dinosaurs larger overtime during the Middle Jurassic. This selection came from the fact that during the late Triassic and early Jurassic carnivorous dinosaurs were getting larger and more formidable.
While the bird-hipped ornithischian dinosaurs developed armor and horns, the Sauropod dinosaurs simply increased their size. The larger one, the less likely a predator would take, for fear of getting squashed. The faster it grew, the better it at surviving into adulthood and reproducing. This natural selection by predators put pressure on these dinosaurs gigantic from about 190 to 170 million years ago.
Dinosaurs started walking on all four legs and gave up the grasping ability to use their hands for gripping and defense. These forearms and hands also become pillar-like to support the increasing weight of a larger body. The paleontology is that their limbs become graviportal. It means that the bones became thicker and the foot becomes reinforced to support the increasing weight. The dinosaurs also adopted a digitigrade stance which means that the dinosaur supported more of the weight using its fingers rather than a plantigrade stance, in which the ankle bears the weight. This pillar-like support upon the fingers gives the weight of the body strong beam-like support for an increasingly heavy creature.
These giant dinosaurs supported their weight with their digits in the hindfoot yet retained large claws on their first digit, or big toe. These claws were likely helpful in scratching and some gripping of irregular surfaces as they walked. These gigantic sauropod dinosaurs had a secret increasing body volume yet maintaining less dense and lighter bodyweight. It has to do with their long necks.
Sauropod dinosaurs have cervical vertebrae that are very complex, with large open spaces between the bones and large processes that extend from the centrum. Even the centrum is hollowed out with spaces, called Pleurocoels. Along the sides of the vertebrae are spaces within the bone called Pneumatic foramina. In life, these spaces would be filled with air sacs. The birds also need to lighten their skeletons, and they do this by expanding out an air sac system from the lungs. This air sac system in birds is called a unidirectional respiration system. This respiration system is much more effective in providing a steady source of oxygen to the lungs.
They are increasing most of their body size through these air-filled cavities. Thus these dinosaurs were much like balloon-filled floats in a Macy’s day parade. Mostly air in these long necks, allowing these dinosaurs to get enormous. This scaffolding of bone is all that remains. The complexity of processes and apophysis give the enormous necks of sauropods their characteristic natural engineering. That amazes and confounds paleontologists trying to reconstruct these gigantic creatures today.
Why aren’t modern animals as large as dinosaurs?
The American Museum of Natural History has a cast of a titanosaur that’s so big. It doesn’t completely fit in the dinosaur wing. The 37 meters (122 foot) long behemoth’s head has to poke out towards the elevators. It’s got individual bones bigger than people. The very name of the group of dinosaurs it belongs to has the word TITAN in it.
Paleontologists have many different factors that could explain why some giant reptiles of yesteryear dwarf modern land creatures. It’s essential to keep in mind that dinosaurs lived a long time ago. Before the dinosaurs were wiped out, they lived on a completely different kind of earth than today. Dinosaurs started coming onto the scene about 230 million years before the present.
During their existence, carbon dioxide levels were all over the place. Estimates vary, but there could have been 3 to 5 times as much carbon dioxide in the atmosphere as there is today. It means the world, on the whole, was a lot hotter, even up at higher latitudes. At the beginning of the Mesozoic Era, all land was clumped together in Pangea. The ocean didn’t have as much of a cooling effect on the supercontinent.
This would make the planet a more hospitable place for giant cold-blooded reptiles that wouldn’t have to waste energy maintaining body heat. They’d benefit from being bigger because longer intestines would be better at breaking down plant matter and extracting its nutrients.
Then they could spend less time chewing and more time stuffing green down their giant gullets. Also, if the herbivores grew larger, then they were harder for predators to take down. Until the predators got bigger, then the herbivores would get bigger.
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