ane

Hey, I haven't understood anything...

May be I haven't explained clearly enough.
Let us take an example:

300 millions years ago, three species of reptiles appeared

Reptilus Fredericus Primus
Reptilus Fredericus Secondus
Reptilus Fredericus Tiertus

Their characteristics are: :

Reptilus Fredericus Primus is 0.01 cubic meter, its weight is 10 kg.
Reptilus Fredericus Secondus is 0.1 cubic meter, its weight is 100 kg.
Reptilus Fredericus Tiertus is 1 cubic meter, its weight is 1000 kg.

265 millions d'années ago, gravity decreases, it became ten times lighter than before.

That means that EVERY thing and EVERY body on the earth will get a weight equal to 10% of the weight he/she/it had before!
Only weights are changing, volumes, forms, and colours of everything and everybody stay what they were before!

Our reptiles' characteristics are now:

Reptilus Fredericus Primus is 0.01 cubic meter, its weight is 1 kg.
Reptilus Fredericus Secondus is 0.1 cubic meter, its weight is 10 kg.
Reptilus Fredericus Tiertus is 1 cubic meter, its weight is 100 kg.

( Reptilus Frédéricus Primus, Secondus and Tertius are exactly what they were before, only their weight have changed, exactly like your weight would change if you'd go to live on the Moon.)

Three species of dinosaurs appear:

Dinosaurus Fredericus Primus is 0.01 cubic meter, its weight is 1 kg.
Dinosaurus Fredericus Secondus is 0.1 cubic meter, its weight is 10 kg.
Dinosaurus Fredericus Tiertus is 1 cubic meter, its weight is 100 kg.

Notice that if Dinosaurus Fredericus Primus et Reptilus Fredericus Primus have equal weights and volumes
they are RADICALY DIFFERENT ("secondus" and "tertius" too ).
Their basic compositions are different; the quantity of blood, the ability to absorb oxygen etc...etc... will be different in dinosaurs and reptiles.
Reptiles has been built to bear a strong gravity, not dinosaurs, that implies they have, inside their body, a strcture different from dinosaurs' structure.

65 millions years ago gravity increases again. It took back the value it had 300 millions years ago.

Our reptiles get back their original weight.

Our dinosaurs' characteristics are now:

Dinosaurus Fredericus Primus is 0.01 cubic meter, its weight is 10 kg.
Dinosaurus Fredericus Secondus is 0.1 cubic meter, its weight is 100 kg.
Dinosaurus Fredericus Tiertus is 1 cubic meter, its weight is 1000 kg.

( Dinosaurs' forms and colours aren't changed, only their weight are changed! )

For reptiles: no problem; they get back exactly the characteristics they had when the species appeared 300 millions years ago.
But for dinosaurs: big big problems.
Their metabolism is absolutely not adapted to that gravity.
Dinosaurus Fredericus Primus which   is 0.01 cubic meter has not been designed to be able to bear a weight equal to 10 kg.
Dinosaurus Fredericus Secondus which   is 0.1 cubic meter has not been designed to be able to bear a weight equal to 100 kg.
Dinosaurus Fredericus Secondus which   is 1 cubic meter has not been designed to be able to bear a weight equal to 1000 kg.

So, all these three dinosaurs are going to D-I-E.

What about tomorrow ?

As a way of thinking about the problem, imagine that tomorrow in 2001 the gravity becomes ten times lighter than today...

We will survive. No problems. My weight will be 8 kg; but when my wife will look at me she will say:
- He is the same man than yesterday! He is just able to run faster!
When he goes on a scale; the scale indicates only 8 kg; but he is as fat ( or as thin, it's up to my wife's mood ) as he was yesterday!

New species of animals adapted to this low gravity will appear.

And if 10 millions years after the gravity become what it was during the years 2000.

Human beings, adapted to that gravity will survive. Men like me will be 80 kg again instead of 8 kg, but their form their volumes won't change (to the great displeasure of their wife? )!

But all species which have appeared after that change of gravity will DIE plus... species which appeared before the decrease of the gravity but which had made a bad bad choice, the choice to feed on animals which appeared when the gravity became lighter!