[Reading Science] The Reason Why Dinosaurs Had to Be Gigantic

There is a reason why the gigantic dinosaurs of the sauropod group had to have massive bodies.
[Photo by movie 'Dinosaur' still cut]

[Asia Economy Reporter Kim Jong-hwa] Until their extinction caused by a comet impact 65 million years ago, the largest creatures on Earth were dinosaurs. Not all dinosaurs were gigantic, but generally, the dinosaurs we call 'sauropods' were large and massive.

Dinosaurs known as sauropods typically had long necks and tails and walked on four legs. Argentinosaurus, Brachiosaurus, and Apatosaurus are representative examples. These sauropod dinosaurs usually exceeded 20 meters in length.

In the case of Argentinosaurus, its weight reached about 100 tons (t), equivalent to the weight of 20 Indian elephants. Comparing a human to an Argentinosaurus, a single vertebra of the dinosaur was as large as a human, demonstrating its overwhelming size.

Scientists who studied their fossils speculated until the mid-20th century that such massive sauropods could not have moved on land and must have lived in water.

Why were they so large and massive? Scientists analyze that the first reason is the 'air sacs' that dinosaurs had inside their bodies. Just before dinosaurs appeared on Earth, during the late Permian period of the Paleozoic era, the Earth's oxygen levels were at an all-time low. To survive in an oxygen-deficient environment, dinosaurs evolved to develop air sacs inside their bodies.

Air sacs are air-filled pockets connected to the lungs, and by examining dinosaur fossils, scientists can infer where these air sacs were located. Air sacs also helped reduce the force exerted by gravity. For large-bodied dinosaurs, air sacs made their body weight over 30% lighter, enabling them to move and hunt.

The second reason dinosaurs grew so large was 'oxygen.' In the early Triassic, Earth's oxygen was at low levels, but it increased during the Jurassic and Cretaceous periods. Small organisms can supply enough oxygen internally even in low-oxygen environments, but large organisms face size limitations if oxygen is insufficient.

Organisms receive energy through oxygen, and large organisms cannot get enough energy if oxygen supply is inadequate. However, from the early to late Jurassic, oxygen levels slightly increased, and from the late Jurassic to late Cretaceous, oxygen levels rose significantly, creating an environment where gigantic dinosaurs could thrive and their populations exploded.

Oxygen was not the only factor that made dinosaurs huge. 'Plants' also had a significant impact. Scientists speculate that as herbivorous dinosaurs that ate plants developed longer intestines, their body sizes also increased accordingly.

During the Mesozoic Jurassic period, carbon dioxide levels were up to eight times higher than today. Therefore, the amount of photosynthesis by plants was incomparable to today, plants were enormous, and their growth rates were likely very rapid.

Conifers, ancestors of modern pine trees discovered from that time, had circumferences exceeding 3 meters and heights well over 60 meters. However, the nutritional content of these plants was poor, and protein synthesis was low, so herbivorous dinosaurs of that era ate only large but nutritionally deficient plants.

A scene from the movie 'Jurassic World: Fallen Kingdom.' [Photo from the movie 'Jurassic World: Fallen Kingdom' still cut]

To compensate for the lack of nutrients, herbivorous dinosaurs had to consume enormous amounts of plants. From the late Triassic, insects also flourished, and plants evolved to protect themselves, becoming tougher and more fibrous.

Ultimately, herbivorous dinosaurs had no choice but to eat large quantities of tough, fibrous plants and digest them. To aid digestion, their intestines had to lengthen, and some dinosaurs grew larger as their intestines extended. This was a natural selection favorable for survival.

The reason dinosaurs had to be gigantic was ultimately a choice for survival. They had to develop air sacs inside their bodies to survive in oxygen-poor environments, their populations increased as oxygen levels rose from low to normal, and their bodies evolved to digest tough, nutrient-poor plants, which led to their large size.