Hollow bones and in some cases spaces within their bodies that were just filled up with air. The end result being that dinosaurs were a lot lighter than their frame would suggest, which is what allowed them to get so big in volume.
Being big is advantageous as long as the animal is able to find enough food to sustain itself. Food was plentiful at the time, so dinosaurs grew quite large.
In modern times, most mega fauna is gone because Humans hunted them to extinction.
First, not all dinosaurs were huge. It’s not a trait of dinosaurs in general. Rather, the environmental factors in the past and some factors that are true for reptiles, allowed being huge as an acceptable evolutionary niche, more than today!
Why would some of them grow so big:
In evolution, it’s always a bit of a hen and egg problem. And there is between species competition called “Red Queen Hypothesis”.
So a likely explanation is that, due to high CO2 atmosphere, plants grew larger which lead to having a long neck or being tall being an advantage. And for carnivorous species bigger herbivores meant that being bigger is an advantage. That, again, meant their prey had pressure to grow bigger (and/or faster), and so on and on.
How could some of them grow that much:
Dinosaurs are reptiles, so they were poikilothermic. Since temperatures have been higher and more stable at their time, a bigger body allowed to keep body temperature stable as well. It doesn’t cool off as fast which allowed more activity which allowed eating more which allowed a bigger body.
There was also significantly more oxygen in the atmosphere which is associated with bigger growth in all species since our metabolism depends on it.
This is especially true for Arthropoda btw, some of them were huge in the time of dinosaurs because they breath through their exosceleton. The biggest centipede found (yet) was 2.5 m long! The difference in size between insects in the past and insects today is much bigger than between reptiles today and in the past. All due to bigger plants and more oxygen and the interaction spiral between prey and predator.
At that point in Earth’s history, the atmosphere was a lot more oxygen rich than it is now! This allowed all sorts of creatures to grow to immense sizes, like trees, insects and dinosaurs. Dinos like Brontosaurus probably grew large for the same reasons Giraffes did too. The best greenery is the one no one else can get to!
grew large for the same reasons Giraffes did too. The best greenery is the one no one else can get to!
Recent evidence in the fossil record regarding giraffids suggests their necks did not evolve to be long for feeding purposes, but rather sexual selection / fighting for dominance with their necks and heads.
Not sure you got the oxygen part right. But I can say that since trees and animal breath each others exhaust, they won’t both thrive due to atmospheric oxygen concentration.
They weren’t all big, but anyway, they (probably) evolved like giraffes to reach for food and as protection against physical damage from predators. The climate was also different and they had plenty of food.
Anyway, evolution does not select. It’s not survival of the coolest features… it’s only reproduction of those that manage to reproduce.
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