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much larger. The largest of these is
Quetzalcoatlus northropi
, which had an
estimated wingspan of between 11 and 12 meters. Such immense size would
make it difficult for such a creature to undertake powered flight. Other than
taking off or landing, it probably depended largely on soaring. Apparently this
specimen lived nowhere near either the ocean or large bodies of water, so,
unlike the earlier pterosaurs, it probably depended on something other than
fish for its diet.
The one thing that all pterosaurs had in common was the manner in which
the wings of stretched skin were held between their arms and legs to form the
smooth surface of a wing.
There is a fair degree of uncertainty as to exactly how and when the
pterosaurs took to flying, but today's flying lizard of Borneo may offer us some
indication. These small lizards climb trees, as do many lizards, but these use
flaps of skin connected between the front and hind legs that, when stretched,
form a wing that enables them to glide great distances from tree to tree.
The first bird flew around 145 million years ago. Then, by around 60 million
years ago, all the modern species of birds had established themselves. By that
stage they had become, much as they are now, the modern masters of the air.
Evolution has seen birds develop into a huge variety of species that has allowed
them to colonize the entire earth, from the tropics to the poles, and in doing so
exploit all possible environments. It is as a direct result of the development of
the wing and powered flight that has allowed this colonization and made birds
capable of undertaking the most remarkable of journeys.
Just as flight is an evolution response to conditions, most probably to avoid
predation, becoming flightless is also an evolutionary response to conditions.
If birds couldn't fly, they wouldn't last long against their natural predators.
Desire for safety from these predators was probably the reason they took to
the air in the first place. Without such ground predators, one group of birds
has found much less need to fly; as a result they have lost the power of flight,
some of them having no feathers that are even suitable for flight. Birds that
have gone down this evolutionary route of flightlessness find themselves
either having to find other ways of avoiding predation or suffering the
consequence of potential extinction. Such was the fate of the dodo. Only a
small minority of birds have become flightless; they are generally ones that
live in island communities with no ground predators or they occupy the more
isolated parts of the world.
Penguins are one of the best-known examples of the flightless bird. These
birds are not much better at walking than they are at flying. They no longer
have any need to fly because they have no ground predator to escape from.
Coupled with other environmental factors, this fact has resulted in penguins
developing a highly specialized and very distinctive physiognomy. They may
no longer be able to fly, but they have become one of the world's greatest
swimmers, capable of achieving very high speeds with great maneuverability.
