The theory about the origin of flight
It is not easy to resolve the problem of flight evolution. Above all, a thorough knowledge of flight physics is required to illuminate various aspects and to arrive at a reliable solution. Former ideas about the evolution of flight suffered from the handicap that they were unable to explain the origin of the wing and the evolution of the wing beat. As mentioned above, Darwin was fully aware of this problem, and he presumed a modification of a former function, but he did not find such a function. Certainly, his presumption was correct, but his modern adherents apparently do not understand his reservations. Ideas offered so far were simply too amateurish, since they emanated from wings already present. Unfortunately, such models demonstrate the missing ability of palaeontologists to propagate really independent as well as reliable ideas, based on scientific research. The reason for this deficiency is probably caused by their education which is inadequate to manage such problems. As a consequence incorrect ideas persist to be proclaimed.
Again and again it was assumed that the ability to fly could only have evolved in the air. Moreover, it was assumed that a ‘primordial’ wing was already present which made active flight soon possible. These assumptions are not in agreement with Darwin’s ideas. When I abandoned such ideas completely this change in looking at problems made a new explanation easily possible. The transition from arms to wings had been the main problem so far. A convincing idea must in any case be able to explain the development of the wing, emanating from an unspecialized arm and assuming a continuous process which does not erect fences too high for any transitional form. Obviously, this transition was not possible in the air. Yet, for the evolution of the wing beat the involved complicated movement of arm and hand had to be developed already in early forms, however, not necessarily in the air, but without any complications in the water. As a matter of fact, there is no alternative explanation in harmony with physical laws.
3.1. The part played by Archaeopteryx and Rhamphorhynchus in the evolution of flight ability
Having seen a TV-documentation showing an auk in pursuit of a fish under water the idea struck me that birds as descendants of reptiles might as well have learned flying under water and extended this capability later on to the air. I have critically reflected about this possibility and have not found any arguments contradicting this idea, only confirmations. There are, on the other hand, many advantages supporting such a development, with the evolution of flight in the air only being a kind of by-product which did not happen as an original evolutionary aim. Presumably, there is no directed evolution at all, although afterwards such an impression may arise. Evolution is a continuous sequence of chances that can be interrupted by unfavourable circumstances at any point of the process.
Flying under water creates the pre-conditions for necessary modifications to flying in the air
The wing beat is not simply a flapping movement of arms and hands up and down, as it may look at first view, but it includes a complicated superimposed rotational motion. Archaeopteryx has already wing bones looking almost as those of modern birds. However, in detail there are differences. There must be a certain reason for these differences. These result from the fact that Archaeopteryx was primarily an underwater flyer, not capable of flying in the air. Many water-dwelling vertebrates such as turtles or penguins for example use almost the same course of wing motion as birds do. Moreover, some reptiles older than Archaeopteryx had developed a sternum which is believed to be lost later on, although this presumption is not beyond doubt. Indeed, water-dwelling reptiles with so-called rowing arms were widespread during the Triassic period. Apparently, there was a considerable selective pressure towards a modification of the anterior extremities as propulsive organs. Presumably the proceeding radiation of fish offered a new very attractive food source.
Fig. 10. Comparison between the arm bones of Archaeopteryx and a modern bird. Archaeopteryx still lacks the alula, but the arms are already very similar. The hands of Archaeopteryx still have claws.
If the assumption is correct that evolution of bird flight began in the water there should be recognizable features pointing to such a development. These features are present. A comparison between the arm bones of Archaeopteryx with those of a modern bird reveals conspicuous differences. The proportions are very similar already, but instead of the alula, an important feather for flight control, the first finger has still a claw. This fact indicates that the airflow around the hand area could not yet effectively be controlled. However, if it was not possible it was not yet required. In any case Archaeopteryx had to go on well with its hands as they were. On the other hand, the arms were able to perform all movements of a bird wing, however they acted under water. A second important hint at the missing flight ability consists in the shape and weight of the bony tail. In all modern birds the tail bones are reduced to the rudimentary pygostyle, that is the bones are considerably shortened and equipped with long feathers for flight control. The conspicuous stiffening of the tail in Archaeopteryx surely served an important function, but not a function such as imposing behaviour which has been supposed by certain palaeontologists. The heavy tail caused an unfavourable position of the centre of gravity, an important obstacle for flying high in the air.
Fig. 11. Skeleton of Archaeopteryx with its conspicuous long bony tail and claws of the hands, after J. Ostrom 1976.
The same overall configuration is found in the pterosaur group of Rhamphorhynchus. Indeed, the pterosaurs are even better suitable to explain the evolution of flight ability in vertebrates, since two different groups existed at the same time during the Upper Jurassic. The long-tailed rhamphorhynchoids appeared in the Late Triassic, and the derived pterodactyloids were present in the Upper Jurassic. The tail of Rhamphorhynchus is equipped with a small velum, which hardly would have been capable of practising an effect on the flow in the air, but certainly in the water with its high density.
Actually, the relatives of Rhamphorhynchus were water-dwellers. As turtles or crocodiles they had to bury their soft-shelled eggs in the ground and had them incubate by the warmth. Recently found well preserved eggs validate this context. The later following derived short-tailed pterosaurs such as Pterodactylus had to live with this heritage.
It is not a matter of chance that the most important witnesses of the flight evolution have together been found in the same environment at the northern border of the Thetis sea near Solnhofen, Bavaria. This region with lagoons and islands contained many fish species. Most abundant is the small Leptolepides sprattiformis which presumably was a suitable prey for Archaeopteryx and Rhamphorhynchus. Hunters and prey were frequently quickly covered by sediment and perfectly preserved in the lithographic limestones. From where the ancestors of Archaeopteryx came cannot be cleared at present, since the limestones of the region in the north of Solnhofen have been eroded. Whether the ancestor was a land-dweller or a water-dweller can also not be stated with certainty.
The saurischians which gave rise to the pterosaurs as well as to Archaeopteryx always lived in the boundary area between land and water, and adaptations in either direction were possible, just as required or initiated by favourable conditions. Of course, they had to retain their legs for going ashore for the deposition of eggs. Presumably, also the claws of the hands served a function in connection with the walk on land, but surely not for climbing trees.
Some authors have supposed that the preserved flyers had been drifted into the lagoons as carcasses or had gone lost during storms. However, the excellent preservation precluded a long transport. Most probably, all the well preserved fossils have been inhabitants of this area. The environmental conditions of the Solnhofen limestones region are well suitable to illustrate the course of evolution to the ability to fly in the air.
Archaeopteryx has essentially attained the same level of evolution as the contemporary Rhamphorhynchus. But the further evolution can better be illustrated using the long-tailed pterosaurs. R. Wild (1984) has described several new forms from the Upper Triassic of the southern Italian alps which he attributes to three different species.These are Eudimorphodon ranzii, Peteinosaurus zambellii, and an unnamed species which is the oldest one, but rather incomplete.The gastric content in Eudimorphodon consisted of scales of ganoid fish. The two other forms differed by increasing wing span and further modifications. Apparently, the pterosaurs underwent such a fast development during this period that the ancestor cannot be identified. Obviously, the possession of feathers was not the most important prerequisite for attaining the ability to fly in the air. But there must have been a precursor of Archaeopteryx that had already feathers, which besides the rhamphorhynchoids became a fish hunter under water. These fish hunters can be regarded in a way as the Mesozoic penguins.
For initiating the evolution to the flight ability first of all the arm propulsion had to be developed
In my opinion the evolution began in a necessarily small theropod which used its arms and hands as propulsive organs for hunting fish under water, as many close or remote relatives such as plesiosaurs, turtles, rhamphorhynchoids etc. did, and many seabirds do. This idea demonstrates a continuous path to the ability to fly in the air, since
- the flight musculature could improve simultaneously with increasing wing span and wing area,
- the most favourable movement for the generation of propulsion could develop step by step,
- only propulsion, but no lift was required,the motion was active only for a short time during pursuit of prey,
- the transition to flying in the air became possible within a rather long time,
- since the animals had to stay near the water surface there was no hazard of crash,
- under water the animal was carried by its buoyancy in the beginning and, therefore, did not fall deep in case of imperfectness.
Fig. 12. Rhamphorhynchus from the Solnhofen limestones, after O. Abel (1912). Note the long bony tail, even more conspicuous than in Archaeopteryx, in addition the short arm bones and the large wing span by enormous elongation of the fourth finger.
As you may know from your own experience, you can well feel the motion in water if you move a hand through it, since water is 800 times denser than air. Moving a hand through the air as fast as you can does not create an unambiguous impression.This means that outside the water there were no criteria available which could offer a direction for the natural selection, but in the water things were different. In the water an animal can much better than in the air feel the appropriate arm movement that leads to the highest speed. It is certainly not a matter of chance that many water-dwelling vertebrates have independently developed modified arms for the generation of propulsion, though obviously utilizing different strategies leading to different shapes.
Archaeopteryx was an underwater fish hunter, as well as the long-tailed pterosaurs, which similar to modern seabirds lived on the water or near a coast. Underwater fish hunters use to have dark feathers
In large and heavy swimmers such as plesiosaurs or turtles the arms became shorter and the hands larger. Modifications mainly affected the hands. However, the speeds generated by these animals were moderate, they were too large. Modern turtles can offer a good scale of comparison concerning beat frequency and available speeds. Archaeopteryx and pterosaurs were small enough to be able to produce sufficient speed with their enlarged hands. The small size allowed a considerable excess power.
Fig. 13. Different shapes of arms and hands in swimmers and flyers which generate propulsion with the forelimbs, Left. plesiosaur, middle. turtle, right, Archaeopteryx, after O. Abel 1912.
Archaeopteryx had to generate propulsion exclusively with her arms. This task requires efficient muscles which developed step by step. Locomotion in the water is attained by a backward acceleration of an as big as possible mass of water. For this purpose large hands developed. This development is found in Archaeopteryx as well as in pterosaurs. Pterosaurs can yield interesting details of the evolutionary process, since there were long- and short-tailed forms, whereas Archaeopteryx is the only known feathered form of this period. In contrast to Archaeopteryx the wing of pterosaurs is covered by an integument comparable to bats. Apart from this difference there are many conformities with Archaeopteryx.
Fig. 14. Schematical kinematics of the wing folding in birds
Although Archaeopteryx had already large hands, nevertheless contrary to birds flying in the air a very important feature, the alula, was still missing. The alula is very important for bringing to bear an influence on the flow along the wing in the hand area, in particular at high angles of attack. On the other hand, the kinematics for generating propulsion was completely developed. Propulsion is almost automatically produced, because the twisting effect during pronation and supination is ensured by the complicated design. The necessary distortion is given by the appropriate attachment of the musculature to the bones and its Generation of propulsion can well be observed in rays. The movement of the fins is comparable to the wing movement of underwater flyers and also to birds in the air. The principle of locomotion is always the same, namely a backward transport of water which as a reaction effects a forward motion. It is Newton’s good old mechanical law
F = m x b.
The generated force equals the product of mass and its acceleration. This force, propulsion in the water respectively propulsion plus lift in the air is attained by an acceleration of the agent’s mass adjacent to the animal’s body. Since water is considerably denser than air lower voumes of water had to be moved compared to air. Therefore, the fences for the evolution of propulsion generated by the arms were not too high. In the air an acceleration of the required capacities is remarkably more difficult. I am deeply convinced that a direct evolution of flight in the air was absolutely impossible. This applies also to the origin of flight in bats.
Fig. 15. Wing motion in rays, the forward movement is achieved by a backward acceleration of water
Video Penguin flying
The bird wing performs almost the same movement as the fins in rays. The propulsion under water was produced with the arms fully stretched. This type of propulsion nowadays can easily be studied in penguins. These have even blocked their elbow joint by sesame bones and cannot any more fold their wings. A good optical comparison is also offered by the hovering flight of a kestrel. This extremely slow flight is performed with the wing at maximum extension, with propulsion and lift generated simultaneously in order to compensate for the headwind and be able to stay above a certain point on the ground. The almost constant wing span during hovering flight is responsible for the unusual flight manifestation. Size and direction of the force generated by the wing can considerably be varied by birds. In addition, birds can also vary the wing area by folding the wings as required. Thus, there is a wide range of possibilities to modify the generated force as to size and direction which in combination leads to a perfect flight behaviour in modern birds.
Fig. 16. Wing (manus) motion in flyers and forces of different size generated by the wing
Under water propulsion was generated by the entire arms as in rays or other vertebrates utilizing the extremities for propulsion. Flying in the air makes a division of functions necessary, since the arms generate continuously lift, whereas propulsion is produced by the motile hands.
During underwater flight the occurring forces must be balanced. The balance between propulsive force and the hydrodynamic drag confines the attainable maximum speed. It is easily understandable that the evolution of underwater flight in any case began in small forms, since the ratio of mass to muscle area available is most favourable in small forms. The weight of Archaeopteryx was 250 grams approximately. Modern birds exceeding a mass of 2 kg are no more capable of a standing takeoff.
Fig. 17. Level flight under water and acting forces in balance
Large birds such as swans need a long takeoff run, and many large birds such as ostriches cannot fly at all, since they have become too heavy during evolution. Therefore, the evolution of flight capability would not have been possible in an animal considerably heavier than Archaeopteryx or in pre-cursors of Rhamphorhynchus in the Upper Trias. Physical constraints would definitely have prevented such an evolution. Apparently, new lines of specialization always have their origin in small forms which are not yet particularly specialized and adapted to a certain food source.
In the course of time the pectoral musculature had to be strengthened to be able to generate the power needed. However, lift was not necessary before Archaeopteryx proceeded to fly in the air and then had to balance her weight. At this stage all preconditions had been created by perfect adaptation to underwater flight. However, the transition to airborne flight happened much later. The idea that the evolution of flight might have started in the water appears hardly credible to palaeontologists who have not the slightest knowledge of flight physics.
Asked by Prof. Dolf Seilacher during a symposion (2001) in honour of J. Ostrom about their opinion regarding my paper a J. Rayner (2001) answered:
"Alternative paradigms for flight evolution: Debate about avian flight evolution has rarely extricated itself from the polarized cursorial or arboreal models, or from compromise hypotheses of the type raised here. An alternative biomechanical model for the origin of flight has been put forward recently by Ebel (1996); this is similar to a hypothesis I advanced -- in part in jest-- somewhile before (Rayner 1985d).
This idea, that flight evolved underwater, is consistent with the taphonomy of Archaeopteryx in the shallow lagoonal environment of Solnhofen, and also with speculation of a piscivorous diet. It is also an attractive idea for mechanical reasons, since the density of water is sufficiently high to support most of an animal's weight by buoyancy. Several other lineages of tetrapods that have secondarily become obligate swimmers have evolved similar swimming mechanisms using the forelimbs as wings, and effectively flying underwater (ichthyosaurs, plesiosaurs, marine turtles, sea lions). This adaptation is a particularly effective form of locomotion, and all of these groups are particularly efficient swimmers. It has also evolved independently in several avian groups; some have become secondarily flightless (penguins, some ducks and some alcids), while others can fly in both water and air (some ducks, diving petrels, morid gannets, some alcids, and dippers). Evidently this swimming mode presents rather few mechanical or morphological obstacles.
Unfortunately, this model cannot explain avian evolution, and the evidence against it comes from the morphology of Archaeopteryx. All wing propelled underwater fliers among birds have unusually large supracoracoideus muscles to elevate the wing during the upstroke (Rayner 1988b). Yet Archaeopteryx had no supracoracoideus (Ostrom 1976b; see above), and there is no evidence of another wing elevator muscle.
Although insubstantial, this brief digression is informative. The origin of flight may have followed a pathway appreciably distant from those normally envisaged, or discussed here. Both cursorial and arboreal models, as normally formulated, raise significant difficulties that have yet to be resolved. A combination of unusual, or possibly rapidly changing, environmental conditions may be responsible, and this may make such hypotheses resistant to physical modeling of the kinds reviewed here."
Thus, a deus ex machina must be postulated and will certainly help to resolve all problems. Unfortunately, this is basically the usual simple argumentation of these ‘experts’, who cannot produce correct ideas, and even cannot judge plausible ones. But they claim to be experts, without the slightest reservations.
Nevertheless, he has admitted that the old dreams are continuously burdened with big problems.
3.2. The function of the long bony tail in Archaeopteryx and in Rhamphorhynchus
During underwater flight the forelimbs could produce propulsion only, but not in addition the relatively small steering force. This was the function of the long bony tail which in contrast to other theropods was remarkably stiffened, surely for this purpose.
The long and stiffened bony tail was urgently needed for steering under water !
Former workers have never lost many thoughts regarding function and meaning of the long stiffened tails in Archaeopteryx and in rhamphorhynchoids as well as about its absence in pterodactyloids. I have arrived at the impression that considering the evolution is completely unimportant to them.
- all of them have seen the great differences between long-tailed and short-tailed forms,
- all of them have known that the long-tailed forms were the first to appear during the Upper Triassic,
- all of them have known that the short-tailed forms appeared later during the Upper Jurassic,
- all of them have known that the long-tailed forms died out after the appearance of the short-tailed forms,
- all of them have known that the short-tailed forms persisted until the Upper Cretaceous,
- all of them have known that the long-tailed forms remained relatively small,
- all of them have known that in the short-tailed forms there were several very large representatives
- all of them have all pterosaurs considered as flyers in the air.
- Nobody has stated the slightest doubts.
3.3 The transition to air-borne flight above the water surface
The modifications required for this transition were smaller as it would appear at first view. The size of the generated force remained almost unchanged. Whereas during underwater flight the considerable hydrodynamic drag had to be overcome, in the air the aerodynamic drag is much lower because of the lower density, but now in addition lift had to be produced to compensate for the weight. Thus, the main modification consisted in a change of the direction of the propulsive force. Instead of a forward direction the new direction was forward upward.
The long bony tail was urgently required during the transition stage from surface-skimming to air-borne flight
The long bony tail is one of the most important elements of my theory. Features that are retained during long periods in a geological sense always serve an important function which is urgently required for the survival. The tail could not be abandoned before the complete achievement of the ability to fly in the air. For the transitional phase it was an inevitable must. But for flying in the air it was an enormous obstacle, made it impossible. Thus the shortened tail of pterodactyloids is an extremely important evidence of the successful transition to aiborne flight high in the air. In all real flyers we find a short tail, because the long one had become disadvantageous and superfluous. Ability to fly in the air is not just given by the capability of producing lift, equally important is the attainment of stable flight conditions and an appropriate controllability. Flight also means the ability to gain altitude.
The long bony tail made a real air-borne flight high in the air impossible and therefore had to be shortened later
In the course of the Upper Jurassic the wings of Archaeopteryx and Rhamphorhynchus had reached a developmental stage which made the transition to flight in the air possible. Now a new problem arose in connection with the long tail. During flight under water the position of the centre of gravity had not been of great importance, since the directions of all forces passed through this point. But outside the water the lift force which now acted in front of the centre of gravity had to change its position. The forces of lift and weight resulted in a continuous moment that tilted the animal, it was tail-heavy and in a steady hazard of crash. For a stable flight the centre of gravity must be located infront of the centre of lift, never behind it.
Flying high in the air was impossible for long-tailed forms. However, they had the opportunity to escape from this problem by skimming with the tail over the water. The additional force due to friction balanced the moment of the lift force (fig. 18).
Archaeopteryx was unable to fly high in the air, maybe she was a surface-skimmer, but even this idea remains uncertain
Archaeopteryx did not reach the ability to fly high in the air. Her development on the way to real flight capability remains unclear. It is even uncertain whether this animal became a surface-skimmer. No descendants are known, and the first real bird appeared only in the Lower Cretaceous of Spain, Eoalulavis hoyasi. This bird has the important alula and, of course, the rudimentary tail.
Fig. 18. Long-tailed forms were obligatory surface-skimmers, unable to fly high in the air
3.4. The transition to air-borne flight high in the air
The transition to flight ability high in the air can be demonstrated only for the pterosaurs, which have solved this problem successfully, since long-tailed and short-tailed forms can be found side by side in the Upper Jurassic limestones. Obviously, the short-tailed pterodactyloids are derived from the older long-tailed rhamphorhynchoids. Only pterodactyloids were capable to fly high in the air. These forms clearly show which modifications were urgently required, how these were achieved and which possibilities were available to them.
Fig. 19. Pterodactylus from the Upper Jurassic limestones of Solnhofen. The most conspicuous feature is the reduced tail, after O. Abel (1912). This species is very small, according to Abel the size is similar to a skylark.
The short-tailed forms persisted for a long time almost unchanged, from the Upper Triassic until the end of the Upper Jurssic. Since flyers in the air were derived from these forms it is rather probable that certain representatives were able to fly over the water as surface-skimmers, maybe even Archaeopteryx. The pterosaurs appeared almost suddenly in the Triassic, as well as Archaeopteryx in the Upper Jurssic. Obviously, such developments take a fast course if an adaptive aim has been recognized, for example a new attractive food source.
By continuously touching the water surface the long stiffened tail could prevent a pitch-up movement and the resulting crash into the water (fig 18). The small velum at the end of the tail that was used for steering under water now achieved a new function as a glide surface. These forms had reached much progress toards the flight ability, but the stretcher bone was not yet completely developed.
Fig. 20. Skeletal modifications required in pterosaurs for the transition from flight under water to flight in the air
The long tail had not erected any problem during locomotion under water, since the action line of the propulsive force passed through the centre of gravity as well as the water drag. However, in the air not all forces could meet the centre of gravity, which resulted in trimming problems. The animals were aerodynamically unstable (fig. 20). There was an urgent need for a forward shift of the centre of gravity, in order to achieve a stable condition or at least an indifferent one. Occurring modifications are:
- reduction of the tail,
- lengthening of neck and skull,
- reduction of teeth for reduction of total weight,
- modified connection between neck and skull,
- formation of a closed thoracic cage, the so-called notarium,
- proportions of the wing,
- growing absolute size and ability to occupy new ecologic niches
Most modifications aimed at a forward shift of the centre of gravity. These modifications can only be found in pterodactylids, but not in Archaeopteryx. She was restricted to surface-skimming, flying near the water surface, if at all. Only these modifications led to the required flight stability and made possible an effective fight in the air high above the water surface.
The recognition of this relationship is not easy and requires a detailed knowledge in flight physics, which of course cannot be expected in palaeontologists. Nevertheless, for a serious propagation of a new theory about the origin of flight this knowledge is indispensible. Otherwise the old stories will be heard again and again. The application of physical laws, on the other hand, yields very satisfactory results. Up to now the required functional pre-conditions had not thoroughly been taken into account.
Since now the head had to be borne continuously above the water its connection with the neck experienced a downward shift. The simultaneous reduction of teeth served a lower total weight. There was another big problem. The locomotion in the much thinner air required an enormously increased beat frequency, at least 28 times faster than under water. This value follows from the square root of the density ratio between air and water for the achievement of the same dynamic pressure. Accordingly, the flight speed increased. If for example the animals hunted under water at a speed of 0.5 m/s the same dynamic pressure resulted in a speed of 14,14 m/s or 51 km/h
Fig. 22. The notarium of large short-tailed pterosaurs, after O. Abel (1912). By fusion of shoulderbones this area was intensively reinforced in order to be able to transmit the increased forces due to higher beat frequency.
Because of the higher beat frequency shoulder girdle and thoracic cage had to be considerably reinforced. By fusion of skeletal parts which were no longer moved the stiff notarium was formed (fig. 22). In these forms the proportions of shoulder and thoracic girdle are emphasized, obviously flying high in the air has become the most important function of the wings.
As often in such cases conspicuous modifications have their first appearance in small forms. There is even a transitional form between Rhamphorhynchus and Pterodactylus, namely the little Anurognathus ammoni with approxi- mately 30 cm wing span, again a very small form, compared to other rhamphorhynchoid forms with commonly 1 m wing span or more. Curiously, so far it has not been noticed that Anurognathus is a very important transitional form. Up to a few years ago all pterosaurs were regarded as flyers in the air.
Particularly funny appears to me that Anurognathus because of certain features of the skull was placed in the long-tailed forms, although its tail is short and the name (tailless beak) stresses this feature. In any case Anurognathus together with Rhamphorhynchus and Pterodactylus forms a fine line of evolution, with three different forms occurring for some time together and only the short-tailed surviving during the Cretaceous.
Fig. 23. Anurognathus as a likely transitional form to real flyers. However, presumably there were further so far unknown transitional forms.
Further skeletal modifications concern the wing itself (fig. 25). Aside from modified proportions of the wing bones the enlarged stretcher bone is remarkable. This hand bone served a corresponding function as the alula in birds, namely influencing and controlling the air flow in the hand region. Unequivocally, this is a wing made for flying in the air. It has developed within a short period. On the contrary, the wing of the rhamphorhynchoids remained essentially unchanged during a very long period. Obviously, it was perfectly adapted to its main function of producing propulsion under water.
Presumably, many later pterodactyloid forms because of their large size would have been sailors above the sea, maybe comparable to modern albatrosses
Fig. 24. Pteranodon, a large specialized pterosaur from the Cretaceous. Compared to head, neck and wing the trunk is very small.
Actually, a thin wing covered by a skin only cannot be aerodynamically perfect. It can only produce relatively little lift compared to its relatively great drag. This wing can be compared in a way with that of bats, which are mostly small and night-active. With respect to modern birds these wings are primitive. Nevertheless, there are many large pterosaur forms. Presumably, these have taken advantage of ascending air currents which allowed soaring without wing beats.
A further handicap accompanied the pterosaurs until their final decline at the end of the Cretaceous. They were unable to change their wing configuration, since the integument was attached to the only present finger tip. Such an extremely tapered wing makes a slow flight as in birds for landing impossible. The bird has a very favourable configuration. The induced drag which adds a large portion to the total drag is divided into several vortices at the wing tip and thereby these are diminished. The induced drag is dependent on angle of attack respectively on lift coefficient
Fig. 25. Change of wing proportions from Rhamphorhynchus to Pterodactylus. The increased beat frequency during flight in the air made a shortening of the flight finger and a reinforcement of the arm necessary. The stretcher bone cannot practice a function as shown in this sketch. In fig. 19 it is better visible. This sketch is only meant to illustrate the different size of these bones.
In order to assure an induced drag as low as possible the wing span had to be as large as possible. In any case this was an unpleasant requirement, however inevitable. The pterosaur wing exhibits features of a fast flyer, maybe comparable with the tapered wings of albatrosses or swallows. The large wing span indicates that these animals had to fly at low angles of attack, and there was a steady risk of wing fractures by air turbulences. The flight problems of pterosaurs were sharper than those of birds and make probably that the large forms were specialists in extreme niches.
It is very curious and hardly believable that the enormous differences between long-tailed and short-tailed pterosaurs so far have never initiated discussions concerning the meaning for the mode of life. Obviously there is always the same problem, that is, that the workers in this field are missing objective criteria and are restricted to their old method of telling fanciful stories.