how independent species acquire similar characteristics through their evolution in similar ecosystems, but not at the same time, such as the dorsal fins of sharks, cetaceans and ichthyosaurs. Examples Colouration that serves as a warning to predators and for mating displays has evolved in many different species. In the plant kingdom, the most familiar examples of parallel evolution are the forms of leaves, where very similar patterns have appeared again and again in separate genera and families. In Arabidopsis thaliana it has been suggested that populations adapt to local climate through parallel evolution In butterflies, many close similarities are found in the patterns of wing colouration, both within and among families. Old and New World porcupines shared a common ancestor, both evolved strikingly similar quill structures; this is also an example of convergent evolution as similar structures evolved in hedgehogs, echidnas and tenrecs. Some extinct archosaurs evolved an upright posture and likely were warm-blooded. These two characteristics are also found in most mammals. Modern crocodiles have a four chambered heart and a crurotarsal, the latter being also a characteristic of therian mammals. The extinct pterosaurs and the birds both evolved wings as well as a distinct beak, but not from a recent common ancestor. Internal fertilization has evolved independently in sharks, some amphibians and amniotes. The patagium is a fleshy membrane that is found in gliding mammals such as flying lemurs, flying squirrels, sugar gliders and the extinct Volaticotherium. These mammals all acquired the patagium independently. Pyrotherians evolved a body plan similar to proboscideans. The extinct South American litoptern ungulate Thoatherium had legs that are difficult to distinguish from those of horses. The eye of the octopus has the same complicated structure as the human eye. As a result, it is often substituted in studies of the eye when using a human eye would be inappropriate. As the two species diverged at the time animals evolved into deuterostomes and protostomes this is extraordinary. Certain arboreal frog species, 'flying' frogs, in both Old World families and New World families have developed the ability of gliding flight. They have "enlarged hands and feet, full webbing between all fingers and toes, lateral skin flaps on the arms and legs, and reduced weight per snout-vent length". The tree plant habit has evolved separately in unrelated classes of plants. Parallel evolution between marsupials and placentals A number of examples of parallel evolution are provided by