There are good reasons why invertebrates are as small as they are – ecology and environment keep them in check. But there was a time when in
The Permian was a period bursting with life, and much of it would have been familiar to us. There were amphibians, reptiles, fish and insects, while the dinosaurs were yet to walk the Earth. Sharks were the apex predators of the Permian seas, rather than the prehistoric marine reptiles like ichthyosaur and liopleurodon that later appear.
Oxygen were as high as 30% in the Permian, compared to today's 21%. The once Giant Insects breathe through spiracles – openings in the sides of their bodies – which lead to a network of tubes containing fluid, into which oxygen diffuses and then is taken up by the muscles.
Vertebrates (like mammals and dinosaurs): are also limited in how large they can grow being large makes moving harder, and takes more energy to generate the blood pressure needed to circulate blood through their systems.
Larger Animals retain body heat far more effectively than smaller ones, which is one reason why mammals with a lot of volume, like elephants, rhinos and hippos, are relatively hairless and must cool themselves by other means, like mud baths.
Land Invertebrate: grow too big, and the new shell will deform under the effects of gravity. In water, however, the soft body is supported, allowing invertebrates to grow larger before their bodies become too unwieldy.
Animal's: muscle gets bigger it gets, relatively speaking, less powerful. So, the larger an animal, the less powerful its muscles for its body size.
Animals are the size they are has a lot to do with the environment, the food available to them, and the predators that surround them.
