Evolution of Flight

Every day, approximately 200,000 people pass through the Frankfurt Airport. Most are there to travel, but others visit the airport to work, shop, eat at one of its many restaurants, or to watch the airplanes take off and land. Since January 26, 2017, another attraction awaits all travelers and airport visitors: In Terminal 2, the Fraport AG and the Senckenberg Gesellschaft für Naturforschung (Senckenberg Society for Nature Research) put on their joint exhibition “Evolution of Flight.” Underneath several gigantic replicas of pterosaurs, two “science cubes” present texts, images and films with fascinating facts about the topic of flight.

The basic principles of flight

A body suspended in the air is subject to two opposing forces: Gravity pulls the body down, while the uplift pushes it upward. Therefore, the primary issue of flight is to overcome the earth’s gravity. In order to move forward, some type of propulsion is required. This force counteracts the friction.
In general, there is a distinction between passive and active flight. In active flight, uplift and propulsion are generated through muscle power, e.g., by the wingbeat, while passive flight utilizes air currents and updrafts with the aid of wings.

Wings

The more aerodynamic a body, the less energy is required for it to fly. Bats possess a patagium (flying membrane) that connects the fingers, arms, legs and tail. A similar principle is found in the pterosaurs; however, here only a single finger is integrated in the patagium. These types of wings are flexible and show an extremely lightweight construction.
The wings of airplanes have a characteristic cross-section that corresponds to the wings of birds. This design aids in the optimal guidance of air currents. However, these wings are rigid; steering and propulsion are achieved by different elements.

The feather

More than 140 million years ago, some dinosaurs developed feathers. The “prehistoric bird” Archaeopteryx already had a differentiated plumage, similar to that of modern birds: Down feathers provided insulation, while the wings carried asymmetrical flight feathers. Feathers are masterpieces of lightweight construction. They sport a hollow shaft, and the feather’s barbs are connected by extremely fine barbules with tiny hooklets that act like a Velcro fastener. With 3.60 meters, the albatross has the largest wingspan among all birds, and it can reach a weight of up to 12 kg.

Water and air

Air is a mixture of different gases. The individual molecules move freely and do not touch each other. Water is a liquid, and its molecules form chains. Contrary to the air, water cannot be compressed. Air and water also show significant differences in their densities: One cubic meter of air weighs a mere 1.3 kg, while a cubic meter of water weighs 1,000 kg. Some organisms “fly” in the water. Here, they are subject to similar forces as in the air, but in other dimensions, due to the difference in density – therefore, the propulsion mechanisms differ as well.

Grey’s fish experiment

In the 1930s, the English zoologist Sir James Grey explained by means of an experiment how animals move in the water and on land. For this purpose, he took a trout out of the water and placed it on a wet board. The fish merely bent its body back and forth in a convulsive manner, but barely moved from the spot. For comparison, Grey let the fish move through an obstacle course of nails that had been hammered into the board. This immediately allowed the fish to move forward: Using the nails as an abutment from which to push off its body, it was able to achieve forward motion. In a similar manner, a fish will push itself off of the water. The same principle also applies in the air.

Swimming flies?

Very small insects have developed their own way of flying: With up to 1,000 wingbeats per minute, they literally paddle through the air, which – converted to the human scale – is as viscid as honey. The wings themselves are rigid. Only when viewed in slow motion does it become apparent that the common fruit fly uses a rowing technique, very similar to that of an athletic rower.

Suspended in the water

It is practically impossible for an animal to be suspended in the air like a balloon – the difference between the density of the air and the body is simply too great. In the water, this difference between the densities is significantly less dramatic. In several groups of animals, such as jellyfish, for example, the densities are practically identical. Animals that tend to sink due to their higher density use buoyancy organs such as lungs (e.g., turtles), a system of air sacs (birds), swim bladders (fish) or grease or oil (sharks). The shell of Nautilus even contains gas-filled chambers that can be flooded or pumped empty, as needed. The same principle is used in submarines.

Tupandactylus imperator (pterosaur)

Group: Pterodactyloids (short-tailed pterosaurs)

Age: Early Cretaceous, approx. 120 million years ago; Santana Formation

Distribution: Brazil (Ceará)

About 120 million years ago, the earth was inhabited by numerous species of pterosaurs. Many specialized in catching fish; among them, Tupandactylus. Like its smaller cousin, Tapejara, this short-tailed pterosaur was also distinguished by a giant crest, which could achieve a surface area of more than one square meter. Its function is not clear; it may have served as a rudder or a means of thermoregulation, or perhaps it was merely used in the animal’s display.

Liopleurodon rossicus (plesiosaur)

Group: Plesiosaurs, Pliosauridae

Age: Middle Jurassic, approx. 160 million years ago; e.g., Oxford Clay Formation

Distribution: Europe

Liopleurodon was the largest plesiosaur of all times. Some individuals reached a length of 15 m and were equipped with teeth over 20 cm long. During the Middle Jurassic, about 160 million years ago, Liopleurodon was the undisputed top predator in the tropical shallow oceans of Europe. This pliosaur moved by means of a virtual under-water flight: All four of its extremities had developed into gigantic paddle-shaped fins. Nevertheless, its skeletal design reveals that Liopleurodon descended from land-dwelling ancestors.

Pteranodon (pterosaur)

Group: Pterodactyloids (short-tailed pterosaurs)

Age: Late Cretaceous, approx. 70 million years ago

Distribution: North America

With a wingspan of seven meters, Pteranodon was one of the largest flying reptiles of all times. As in all pterosaurs, its patagium extended from one significantly elongated finger to its hind leg. This made Pteranodon an efficient glider that hunted fish in the ocean with its toothless beak. With the curved lower mandible, it grabbed fish from the water, which were immediately swallowed whole. To date, it is unknown whether the crest on the back of its head served as a rudder, a counter-weight for the heavy skull or whether it was used in display behavior.