Brian Dredge, of Feilding, asks :-
Why don't animals today grow as big as dinosaurs?
Duncan Steel, a Visiting Researcher at the Australian Centre for Astrobiology at the University of New South Wales, responded.
Although some dinosaurs (such as Diplodocus and Apatosaurus) were huge, most were not. Look at a Tyrannosaurus Rex fossil in a museum: its body mass is less than an elephant, and it's hardly as tall as a giraffe. Then consider the blue whale, which may be the largest animal ever to live on Earth.
It is true that there were crocodilians many millions of years ago that were larger than those now, but conditions have changed. In particular the oxygen content of the atmosphere is only about 18 percent now, but aeons ago it was above 25 percent.
When you see horror movies with giant spiders or insects, be sure that those cannot occur in reality. For one thing, the exterior skeleton of an insect would not be able to support its weight: the strength of such an exoskeleton goes up only as the square of its size, whereas its mass/weight goes up as the cube. Thus there are giant deep-sea crabs possessing long spindly legs, with their weight supported mostly by water buoyancy - but they can't walk on land.
Back on the atmospheric oxygen front, this also affects insect size limits because these tiny animals don't have lungs like us, but they depend in part on the diffusion of air bringing in oxygen to their bodies and taking away carbon dioxide. If there were a larger fraction of oxygen in the atmosphere then they could grow bigger.
I was born in a coal-mining area in North-East Somerset, England. One of the high schools there has a drawing of a dragonfly on its school badge, because the largest insect fossil ever found was such a beast with a half-metre wingspan discovered in a nearby mine. Back in the carboniferous era - long before the time of the dinosaurs - there was an oxygen content in the atmosphere high enough to allow such giant flying insects to evolve.
Such variations in environmental conditions therefore favour some animal types over others. Today we live in the Age of the Mammals, made possible by the extinction of the dinosaurs 65 million years ago - else we would not live at all.