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>>/47733/ > But they are very different in their lifestyle and diet. I think body composition (center of mass, legs etc) matters more. At least from common sense - running bipedal animal that looks like ostrich (or t-rex) surely needs some body part to preserve balance, especially when it turns. Even humans use arms for similar purpose: https://en.wikipedia.org/wiki/Arm_swing_in_human_locomotion (I didn't even expect that wiki has such article)