THE PENTADACTYL LIMB
The pentadactyl limb is the typical limb of the mammals, birds, reptiles, and amphibians. These vertebrates are all descended from primitive amphibians whose immediate ancestors were fleshy-finned fish. The limb which evolved in those amphibians had three parts: a hand/foot with five fingers/thumbs/toes etc, a lower limb containing two bones, and an upper limb containing one bone.
SPECIES 1: Bat (Chiroptera)
Its wing is the forelimb version of the pentadactyl limb. In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from trees. Bat wings are modified tetrapod forelimbs. Because bats are mammals, the skeletal structures in their wings are homologous to the skeletal components found in other tetrapod forelimbs. Through adaptive evolution these structures in bats have undergone dramatic changes. This evolution involved many changes such as webbed fingers, elongation of the forelimb, and reduction in bone thickness as bat wing bones are very light and thin.
SPECIES 2: Horse (Equus ferus caballus)
The horse's pentadactyl limb has changed in a number of ways, usually most tetrapods have four digits (fingers) but the early horses adapted towards the three-toed, spring foot. The foot slowly adapted to the single toed hoof with internal frog to pump blood in the horses we have today. The earliest ancestors of modern horses (dated about 50 million years ago) were small and the feet had 3 toes (as well as a smaller fourth toe on the front legs). This is a development from the standard pentadactyl (5-toed) limb which most mammals possess. The development of larger limbs and hooves allowed horses to spread out and become more successful in grassland habitats as they developed.