Reptiles survived too because they appeared long long time ago when gravitation was more important than during Dinosaurs' age. Birds survived too, because the augmentation of gravitation made the atmosphere more heavy, as a paradox it was easier for birds to flight. Time is imutable, Time is the same everywhere, Einstein proved it was false. Today we build the same hypothesis for gravitation. Dirac thought to it, during the year 1937.
Recent measurements prove it.
Our measurements of the gravitational constant G are not sufficiently accurate to be sure that G is a constant. G is very weak comparatively to electro-magnetic or nuclear forces, in experimental conditions, i.e. with little distances. That's why it's very difficult to know its exact value with a good accuracy. The behaviour of G may be the same than the electron's behaviour. On the contrary, we have clues indicating that G has changed.