The Mystery of Dinosaur’s Success and Gigantism
Why did dinosaurs thrive?
When they first appeared in the Late Triassic, dinosaurs were nothing more than small animals living in the shadows. So what’s the secret behind their explosive growth and success?
In the early days of dinosaurs, the land was dominated by large animals related to crocodiles and mammals. Some of these reptiles, closely related to crocodiles, even had upright limbs and high mobility, comparable to dinosaurs. The exact reason for their rapid decline in the Late Triassic is unknown, but it is believed that their circulatory systems required them to drink a large amount of water. As the Earth became drier in the Late Triassic, these large animals were forced to leave the land.
The Earth during the Late Triassic period
Meanwhile, “birds,” the modern-day dinosaurs, don’t need much water to survive and excrete white, solid urine. It is believed that dinosaurs were similarly water-efficient animals. It’s highly likely that the main reason dinosaurs thrived was because they were efficient at conserving water.
Furthermore, during the Jurassic period, when dinosaurs were most successful, active volcanic activity caused the concentration of carbon dioxide in the atmosphere to reach 20 times the current level. As a result, this excess carbon dioxide trapped heat, and the Earth’s average temperature is thought to have been more than 10 degrees Celsius higher than it is today. (It is said that the average temperature near the Arctic Circle was about 15 degrees Celsius.)
Active volcanic activity led to higher average global temperatures.
This warm climate helped plants flourish, which in turn allowed plant-eating dinosaurs to thrive, contributing to their overall success.
Why Did Dinosaurs Become So Big?
Putting their success aside, what were the reasons for the gigantism of dinosaurs? The following theories have been proposed:
1. They had a skeleton suited for gigantism.
From the beginning, dinosaurs had long, sturdy hind limbs that extended directly under their bodies. While reptiles’ sprawling skeletons put a heavy load on their hip joints, the upright posture of dinosaurs was well-suited to supporting a heavy body.
They had a skeleton suited for gigantism.
The earliest dinosaurs, such as the small carnivore Eoraptor, had already developed the ability to run at high speeds. This also meant they could withstand heavy loads. Therefore, from the moment they appeared on Earth, dinosaurs had the conditions to evolve into the massive herbivores they later became.
2. They had a respiratory system suited for gigantism.
Dinosaurs had a system of “air sacs” throughout their bodies that helped their lungs, allowing them to take in oxygen more efficiently. While crocodiles and monitor lizards also have air sacs, the development of these sacs in dinosaurs was exceptional. This efficient oxygen delivery is what allowed for their massive bodies to form.
The air sacs had other benefits as well. They helped keep their bodies light, and having these spaces between bones actually increased the strength of their bodies. They also allowed for smooth gas exchange throughout the body. This is why migratory birds like cranes can fly over the Himalayas where oxygen is thin.
Thus, air sacs not only supported the gigantism of dinosaurs but also contributed to their move into the air, which was perfected from the mid-to-late Jurassic period—in other words, the evolution of birds.
3. They grew as long as they lived.
This is based on evidence from fossils. It’s a tendency also seen in reptiles, their close relatives. Unlike mammals, dinosaurs continued to grow as long as they were alive.
Dinosaurs continued to grow as long as they lived.
4. Food was plentiful at the time.
After the Triassic period, the Earth’s climate became warmer, and a large number of plants existed on the planet. This allowed dinosaurs to consume a large amount of food.
Food was plentiful at the time.
However, while plant growth was stimulated, the nutritional value per volume became extremely low (like bean sprouts). Many herbivorous dinosaurs needed to eat massive quantities, which led to their large size.
5. To protect themselves from predators.
While it becomes difficult to run away from enemies once a creature reaches a certain size, growing larger has the advantage of making it harder for predators to take them down. From a defensive standpoint, this may have encouraged them to grow even larger.
They grew larger to protect themselves from predators.
Ironically, this also led to the gigantism of the very creatures that preyed on them—the carnivorous dinosaurs.
6. They failed to miniaturize.
While many dinosaurs were smaller than humans, only one or two species are known to have been as small as a mouse in adulthood. It is believed that mammals and their ancestors had already adapted to the environment as small, active animals. Therefore, dinosaurs “failed to miniaturize,” and some sought to find a niche by becoming giants to fill a space that mammals had not occupied.
