Paleontology, 1S, 147-163
John W. Merck, Jr.*
*This text may not be reproduced without my written consent.
The skeleton of Hesperornis )was weighted with deposits of calcium phosphate in the cavities of its bones. As a result, its dense skeleton assumed the role of ballast. This allowed Hesperornis to retain in its air sacks (between its internal organs) and lungs the large volumes of air necessary for prolonged dives while wasting little energy counteracting undesired buoyancy. The largest members of Hesperornis )reached 1.8 meters in length (Martin, 1983), and were reminiscent in body outline of their distant relatives, the modern loons and sheldrakes. Thanks to their dense downy integument (known from its impressions in fossil remains) and their advantageous correlation of body mass and surface area (which determined their small rate of heat-loss) these birds could survive over a wide latitudinal range. Champions among marine birds in dive depth and speed and economy of underwater movement were probably representatives of the family Asiahesperornithidae, recently discovered in our country in northern Kazakhistan. Their existence has also been established in southern Sweden.
Therefore pterosaurs were limited to the thin surface layer of the ocean for capturing food, where they obtained only what hesperornithiform birds could not capture beneath the water. Apparently, just the warm waters of the lower latitudes remained free of hesperornithiform birds, but these waters lacked the high-productivity zones conducive to pterosaurs. The flying reptiles of the Cretaceous could barely dive beneath the surface of the water, due to the unusually high pneumatization of the skeleton and the unquestioned presence of air spaces between the internal organs. Complete immersion could hardly have been characteristic behavior, due to the danger of destroying the thin phalanges of the flight membrane supporting finger and the terminal portion of the jaw, and the possibility of tearing the flight membrane.
The difficulties of acquiring food were particularly significant for juvenile pterosaurs of the time, in that the trophic base available to them, shores and the near-shore shallows, estuaries, lagoons, and fresh water bodies, was already intensively patrolled by small birds (a situation which appears to have been fixed by Coniacian times). Large adult pterosaurs of the Late Cretaceous could still accomplish enormous flights over water and locate a reasonable quantity of dead fish and other aquatic organisms, but for juvenile pterosaurs to do the same was more difficult. All the while, competition for the near-shore zone with birds was intense. All of this must have engendered and strengthened various forms of parental care on the part of pterosaurs. These would particularly involve the transport of food to the young, their maintenance in safe places, and the escort of fledglings to feeding sites in the ocean. The appearance of large birds related to the frigate birds, such as relatives of the Limnofregatinae and of the genus Volgavis, above the oceans of the terminal Cretaceous (Late Maastrichtian) must have played a dramatic role in the history of the last pterosaurs. These birds possessed hooked beaks, more effective for capturing food from the surface of the water than the straight beaks of azhdarchids.
Thus, for the duration of the Late Cretaceous, not just its end, flying reptiles must have experienced considerable difficulties stemming from the appearance and development of bird groups as competitors. Even more disastrous for the last pterosaurs were changes in abiotic media such as details of ocean water temperatures, oceanic circulation patterns, and even changes in the geomorphology of the shore regions in which their young were reared. Particularly important is that the flying reptiles of the Cretaceous found their ideal trophic base only in those few regions of the oceans in which upwelling, the upward movement of water enriched with biogenic elements like phosphorous (in the form of the phosphate ion), silicon, and nitrogen, occurs.
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