When vertebrates moved onto land, long ago, some of them eventually became airborne, way after the insects did. There are three groups of vertebrates that have used powered, muscle-driven flight: pterosaurs, bats, and birds. On separate evolutionary lineages, their wings are constructed differently.
Some 200 million year ago or a bit earlier, pterosaurs evolved as a special group of reptiles not related to dinosaurs. They were significant predators on fish and squid, or insects, or small reptiles, depending on their size and habitat. They were warm-blooded, walked on two legs, and flew on membranous wings that were supported by arm and finger bones — especially by a long fourth finger, with the back edge of the membrane attached to the ankle. Their wing spans ranged from a few inches to over 30 feet. There was also a membrane between the legs that contributed to flight and some had a long thin tail had a stiff paddle-like vane at the end that may have contributed to stabilization and steering. They flourished until about 66 million years ago, when a huge asteroid, several miles wide, crashed into planet Earth near the Yucatan Peninsula in Mexico, causing massive global extinctions. Pterosaurs and almost everything else perished, but some mammals and birds survived.
Mammals evolved from therapsid reptiles, over 200 million years ago. The only flying mammals are bats, which evolved over 50 million years ago — maybe well before that. Bat wing spans range from about six inches to over five feet. The wing membrane is stretched between the body and legs and the arm; it is supported by five fingers, four elongated fingers and a short thumb. In some cases, the wing also includes the tail. Wing shape varies: long and narrow in species that forage in the open, broader and shorter in those that maneuver through vegetation.
Around a 150 million to 160 million years ago, birds appeared on the scene, probably evolving from theropod dinosaurs, although that has been debated. Bird wings typically have a flight surface provided by feathers, supported by arm, hand, and finger bones. Wing spans range from about three inches (hummingbirds) to almost 12 feet (an albatross). Wing shapes vary enormously, from broad expanses, long or short, to narrow and curved, and are adapted to different uses. For example, a short, wide wing is useful for quick upward movement and for navigating in and out of trees and bushes (grouse), while a long, wide wing can be great for soaring on rising air currents (eagle). The narrow, tapered wing of albatrosses is good for dynamic soaring, turning and climbing over ocean waves.
Although several other vertebrates are sometimes said to “fly,” they are incapable of powered flight and really just glide. Many of these are rodents. American and Eurasian flying squirrels glide on a membrane stretched out from the body by four legs. They have a special cartilage on the wrist that adjusts the wingtip during a glide, possibly serving as an airfoil to reduce drag. The tail is a rudder, a stabilizer, and an airbrake when landing. Africa has its own kinds of flying squirrel, in a different taxonomic family, and they too have a special cartilage that helps extend the gliding membrane, although in this group the cartilage attaches to the elbow, not the wrist.
The so-called flying lemurs (a.k.a. colugos; distantly related to primates) of Southeast Asia glide on a membrane supported by four legs and the tail, assisted a bit by webbed feet. Australia has several species of marsupial mammals called gliders; they too sail on skin flaps expanded on outstretched legs.
Flying lizards of Southeast Asia glide on expanded membranes supported by elongated ribs and attached to the arm, which helps control direction of the glide. Certain geckos glide by using their webbed feet and flaps of skin along the body that spread out during a glide.
Addendum: If not aloft, then underwater! There are animals that use muscle power to “fly” underwater: the forelimbs of penguins have short, fused arm bones that support a paddle-like ‘wing’ with which the birds ‘fly’ underwater. And the “wings” of skates and rays are highly modified pectoral fins that undulate to propel the animal forward.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
