Did a Change in Inertia Kill the Meganeura?

In my post on the head of the Quetzalcoatlus, I noted that this giant flying dinosaur would have had two problems with today's environment on our planet. The first and most obvious problem is that of flight. Gravity is much too great today to allow an animal the size of a giraffe to fly. The second problem, which is often overlooked, is that of inertia. Its enormous head would have been impossible to control.

From this, I concluded that it is not only gravity that has increased. Inertia has increased too. Whatever has made our planet more massive, has made all animals, trees and rocks more massive too.

The Quetzalcoatlus went extinct together with all other large dinosaurs about 60 million years ago, and a change in gravity is seen by many as the cause of this extinction. However, judging by the impossible size of the Quetzalcoatlus' head, a change in inertia may have been a contributing factor to its demise.

But what about the much larger extinction event that took place 240 million years prior to the extinction of the dinosaurs? Was that due to gravity too, and to what extent would inertia have played a role?

To answer these questions, the Meganeura can provide important clues. This was a dragonfly with a wing span of up to 70 cm. It went extinct together with a large number of other animals some 300 million years ago. The Meganeura was about 4 times bigger than the largest dragonfly in existence today.

If gravity killed off the Meganeura, then it is interesting to note that it was not particularly large relative to contemporary living dragonflies when compared to the difference between a Quetzalcoatlus and a large modern day bird, such as the swan.

The Trumpeter swan has a wingspan of 3 meters. The wingspan of the Quetzalcoatlus was about 16 meters. That is more than 5 times bigger than the swan. The difference between a Quetzalcoatlus and a Trumpeter swan is a good deal bigger than that of a Meganeura and a large modern day dragonfly.

It appears then that gravity did not change all that much over the 240 million years that separated the two extinction events.

Why didn't the Meganeura survive up until the death of the dinosaurs? If it was wiped out by some event unrelated to gravity, then why didn't super large flying insects reappear? The Meganeura is directly related to modern day dragonflies. The small ones survived. Some of them could easily have grown larger again after the extinction event. But this did not happen.

The reason for this may well be that inertia had changed so much that flying insects the size of the Meganeura were no longer possible. They would constantly bump into things. They would be unable to hunt.

What prevented Meganeura sized flying insects from reappearing was not gravity but inertia, and the physics behind this is quite simple.

Inertia is what gives things momentum, and momentum changes linearly with mass:

p = mv

Gravity on the other hand changes exponentially with mass:

F = Gm1m2/r^2

Note: If m1 and m2 are changing in equal measures, we get an exponential change in F.

This means that momentum may have changed more dramatically than gravity to start with, when the exponential change in gravity was in its initial flat phase.

Comparing a straight line with an exponential curve in maths, we see that the straight line outperforms the exponential curve for some time before the exponential curve takes off. This means that changes in momentum were more dramatic than changes in gravity during the early phases of mass condensation.

Now that we have reached the later stages of mass condensation, it is gravity that changes the most. As matter continues to increase in mass, inertia continues to increase, but gravity changes much more dramatically. It is not an increase in inertia that will eventually kill off the African Elephant. Gravity will get to it long before inertia becomes a problem.

However, for large flying insects like the Meganeura, inertia was the deciding factor.

Meganeura, lifesize model (from Land of the dead blog)