Earth has shrunk since the Triassic period, between 243 and 233.23 million years ago, the the gravity pull may of been 2 or 3 times stronger than now. The air was thinner, that changed gradually as the the effects of the gradual shrinking made the the planet increase its spin and as the earth shrunk the atmosphere volume of gasses of course volume count raised accordingly.
The Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaur groups except for the neornithine birds. It would be logic to think be that the gradual shrinking of the earth had a lot to play in the extinction Dinosaur extinction. Their weight was why they died from heart related problems and over weight problems and become extinct.
The shrinking was three folded, volcanoes spewing Hugh molten matter out in to space. Due to the pressure caused by a prolonged thickening of the earth crust and the solidifying of the metallic core at the center of the earth. The weather changed as the earth spin sped up gradually to suit the shrinking mass. The Ice age being part of the Earth’s adjustment. plus many other natural events.
This logic theory it based on the Hugh size of species, in the Triassic period there had to be a good reason, for their size and mass. Logically thinking it was a much strong gravitational pull and the need for very strong muscle heavy bone exoskeleton. The bones however where in the legs were cleverly constructed to be light but very strong as they had bubbles of air with in the structure of the bones.