Life Styles

The pterosaurs were the first vertebrates fully to conquer the air. Although many fossilised examples have been collected, much of what is known about their modes of life is based on inference - as we have seen in the case of their locomotion on the ground (Sect. 6.4).

6.7.1 Feeding

Apart from a few small insectivorous forms such as Peteinosaurus, Batracho-gnathus and Anurognathus, most of the pterosaurs so far discovered have been specialised fish-eaters. Their long jaws and forwardly directed front teeth would have enabled them rapidly to seize slippery fish and to prevent them from escaping. In several cases, the remains of partly digested fishes have been observed in the fossils of pterosaurs. As already mentioned, many pterosaurs probably fished on the wing with lower jaws hanging in the water, as do skimmers (Rhynchops spp.) today. The tips of their jaws had horny beaks which have not been preserved in their fossils. Had they landed on the water surface whilst fishing, they could easily have taken off again by facing the wind, spreading their wings (Sect. 6.3) and paddling their feet, as do albatrosses (Diomedea spp.). The jaw articulation of the pterosaurs shows that they could have opened their mouths very widely. When they did this, their jaws would have splayed out, thereby increasing the opening to the throat pouch in which food was temporarily stored - as it is by pelicans (Pelicanus spp.). Perhaps food was pre-digested in the pouch before being swallowed or brought back to feed the nestling young (Sect. 6.7.2; Wellnhofer 1991).

To conclude, the smaller pterosaurs were mainly insectivorous while the larger species were largely piscivorous. Among the latter, larger forms usually fed upon larger fishes, but there were many genera of roughly the same size in which the teeth, paradoxically, were markedly different. However, there are more ways than one of solving a problem, and one type of piscivorous dentition may be quite as efficient as another. When two genera of similarly sized pterosaurs but with different dentition existed contemporaneously it could well reflect different prey or different ancestry or both. And, if the two had evolved at different times from different ancestors, the apparent paradox would be simply explained.

In the African savannah, most of the terrestrial predators are also scavengers. Nevertheless, they do not eat anything like the amount of flesh that is consumed by vultures. Soaring at great heights, vultures have a much better chance of spotting the corpses of animals that have died from wounds or disease than terrestrial scavengers do. When one vulture drops to the ground it is seen by others from far away. These follow it to its destination attracting, in turn, other vultures even more distant. The same could have applied in the case of pterosaurs, but there is no evidence that scavenging provided much of their diet - even in the case of Quetzalcoatlus which, it has been suggested, might possibly have fed on the carcasses of dinosaurs, using its slender beak to probe into their bodies. Nor are there any records of fruit-eating pterodactyls although, when the first higher flowering plants evolved, some species would almost certainly have been responsible for distributing their seeds. Birds were relatively scarce during the Late Cretaceous period (Sect. 6.5) but frugivorous pterosaurs probably lived inland in higher regions subject to weathering and erosion. Here, the chances of fossilisation would have been small (Fleming and Lips 1991; Wellnhofer 1991). Fleming and Lips (1991) based their argument on three premises: (1) pterosaurs had a metabolism similar to that of birds; (2) their adaptive radiation was more extensive than is indicated by the fossil record; and (3) birds and bats evolved a frugivorous diet, and pterosaurs probably did so too (see, however, Sect. 6.8).

Reproduction

It is not yet known for certain whether the pterosaurs gave birth to living young or laid eggs, but it seems more probable that theywere oviparous. Fossils of eggs, which could easily have been those of pterosaurs, have been discovered on several occasions. In the case of Quetzalcoatlus, fossils of eggs have been found at the same site in Texas as have fossils of the adults. As yet, however, there is no positive proof that the two were associated. So far, no fossils of adult pterosaurs have been found with either eggs or embryos in their bodies. Indeed, there is no indication of the gender of most individuals although in the case of certain species of Rhamphorhynchus there are sufficient fossils for statistical confirmation of the fact that two forms are present. One, probably representing the males, has larger skulls and wings than the other that may consist of females (Wellnhofer 1991). Quetzalcoatlus northropi (wingspan ca. 12 m) is known only from wing fragments, but bones of another form with a wingspan ca. 7 m have also been found. These bones could represent another species, the opposite sex, or may perhaps have belonged to immature individuals (Langston 1981). Even larger than Q. northropi, Hatzogopteryx had a wingspan in excess of 12 m and a skull ca. 1.5 m in length.

Among birds, the males are usually larger than the females. There are exceptions, however, as in the case of birds of prey (Falconiformes). In these, the females are markedly larger than the males, and the two sexes may even hunt dif ferent quarry. Not only size differences, but also sexual dimorphism occurs in birds. For instance, the bill and casque of the male black-casqued hornbill (Caratogymna atrata) are very much larger than those of the female. Differences in size and dimorphism may well have been present among pterosaurs. Indeed, both have been assumed in the case of Pteranodon in some species of which it is possible to distinguish a larger form with a big cranial crest from a smaller form. This had a smaller cranial crest and a wider pelvic canal so, presumably, it is the female (Bennett 1987).

Thepelvisof pterosaurswas quitedifferent from that ofbirds, so theeggsmust have been rather small and, if the young were born alive, they would have been very immature at birth. Either way, parental care must almost certainly have taken place. With all the emphasis on the reduction of weight in pterosaurs, it would scarcely have been possible for the females to have carried more than one egg or fully formed young at a time. Nests must therefore have been built and the offspring reared in them. Masses of fossilised conifer needles associated with the bones of Quetzalcoatlus may be the remains of such nests.

Since they were endothermal and hairy, it is quite possible that the pterosaurs incubated their eggs as birds do. Like birds, they would have paired for at least a season, if not for life, and the males might have taken their turn at incubating or have fed the females from their pelican-like pouches. Alternatively, pterosaurs might have placed their nests in secure places where they were

■ Fig. 64. Ornithocheirus (Pterodactyloidea; Lower Cretaceous; wingspan ca. 2.5 m) feeding its young. (After Wellnhofer 1991)

warmed by the sun and have taken no part in incubating the eggs. The hatch-lings would, however, have been extremely small and the parents undoubtedly had to feed them. Figure 64 shows what Ornithocheirus might have looked like whilst feeding a fairly large youngster.

Pteranodon, like many other pterosaurs, probably lived in colonies where suitable roosting places were available. Each individual would have needed sufficient space to spread its wings in order to be able to take off into the air. Mating took place in the hanging position, as it does in bats, and the young would have been much safer in a colonial nesting site than they would have been were the animals solitary in habit (Bramwell and Whitfield 1974). Similar considerations may well have applied to many other pterosaurs, but little evidence is available on the subject.

Wellnhofer (1970, in Wellnhofer 1991) was able to reconstruct the growth pattern of Pterodactylus on the basis of statistical investigations. In the case of P.kochi, the smallest individual had a wingspan of only 19 cm, while the largest had one of 67.5 cm. A continuous growth series was assembled between these two extremes and it was shown that the skull grew more quickly in young than in older individuals. The eye socket was relatively larger in younger individuals, as it is in crocodiles and birds. Many other parts of the skeleton also showed differential growth rates.

6.7.3 Enemies

Pterodactyls were skinny creatures and would not have been very attractive to predators. Their enemies would doubtless have included carnivorous dinosaurs on land, and marine reptiles especially mosasaurs in the sea. On land they would have launched themselves from their roosting places to escape. On the water, if they had time, they would also have taken flight. If flight was impossible, they might have used their relatively large wings and jaws in deimatic threat. (Intimidating or startling behaviour is called 'deimatic' after the Greek word meaning 'to frighten'.) Nevertheless, like birds and bats, pterosaurs would have had few defences whilst on the ground, and their natural response to enemies must inevitably have been to take to flight.

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