In his work on the origin of species, Charles Darwin postulated that no species will ever develop a shape or size that is grossly out of tune with its environment. Rather, the opposite will tend to happen.
Animals of completely different species will develop similar shapes and sizes given similar environments.
This law of nature is easy to confirm. There’s no lack of examples:
The shrew looks like a mouse. The armadillo and the pangolin are similar to each other. The muskrat, beaver and coypu are also very similar.
Yet, none of these animals are very closely related. What they have in common is their environment and feeding habits.
These animals are similar in size and shape due to their similar environments.
What then are we to make of fossils in which we find a recognizable shape that is many times bigger than the same shape today?
The Meganeura was a dragonfly with a wingspan of 65 cm. It looked exactly like dragonflies look today, so its feeding habits must have been the same.
If inertia was as strong back in the time of the Meganuera as it is today, it would have constantly crashed into things. The sort of quick manoeuvring we see in dragonflies today would have been impossible for a dragonfly the size of the Meganeura.
Moreover, today’s gravity would have made it impossible for the Meganeura to get off the ground. Its wafer thin wings would have broken if it tried.
The only way to explain the Meganeura is that it must have lived in an environment where both inertia and gravity were less.