Why did the fastest dinosaurs evolve to run on two legs while the fastest mammals on four legs!?
Actually, the earliest dinosaurs (such as Eoraptor) were bipedal … moving mostly on hind legs (with a heavier tail for balance), but using front legs for resting or for grasping. It was later that some dinosaurs became more quadrupedal as they got larger, and others, like the theropods, got *more* bipedal, relying less and less on their forelimbs … with the forelimbs getting smaller as weight got redistributed either backwards towards the tail, and into large jaws. So the question is this … both dinosaurs (which went biped) and the early ancestors to mammals (the therapsids, which stayed quadruped, and evolved into the very quadruped mammals) evolved from quadruped reptiles. So why the two very different modes of basic locomotion? For an answer, look at how reptiles (and amphibians for that matter) move. They move like evolved fish, with the spine moving from side to side. That’s great for swimming, not so good for walking, terrible for running, especially with a lung-breather. W
