Complex sentence: Footprints from azhdarchids show that at least some pterosaurs walked with an erect posture with their four limbs held almost vertically beneath the body, an energy-efficient stance used by most modern birds and mammals, rather than the sprawled limbs of modern reptiles.
Split into simpler sentences: Footprints from azhdarchids show that at least some pterosaurs walked with an erect, rather than sprawling, posture. Their four limbs were held almost vertically beneath the body, an energy-efficient stance used by most modern birds and mammals, rather than the sprawled limbs of modern reptiles.