Conclusions

In general, predatory animals are of two kinds: ambush or sit-and-wait predators, many of which are capable of a sudden dart after their prey, and pursuit predators. Both types occurred among marine Mesozoic reptiles. Ambush predators dominated the large predator communities of the Middle Triassic and Upper Cretaceous periods, whilst pursuit predators were dominant in the Lower Jurassic, as they are among the fishes and whales of today's oceans. Between Late Middle and Upper Jurassic times there was an almost even mixture of ambush and pursuit marine predators, filling nearly all of the feeding guilds (Massare 1997). (A guild is a group of species, all the members of which exploit similar resources in a similar manner.)

The largest Mesozoic marine reptiles were the pliosaur Kronosaurus (Fig. 39) and the ichthyosaur Shonisaurus (Fig. 41), both of which reached lengths of about 15 m, as we have seen. Now, this is a similar size to that reached by the largest living toothed whale, the sperm whale (Physeter macrocephalus) - a fact not without significance. Sperm whales are less than half the length of the blue whale (Balaenoptera musculus) which may grow to more than 33 m and weigh over 170 tonnes. Such an immense size could only be achieved by a suspension feeder that continuously filtered large amounts of plankton from its environment. As animals become progressively larger, their nutritional requirements increase exponentially. In very large animals they become so great that plankton is the only renewable source of nutrition that does not become exhausted.

It is thus highly probable that none of the plesiosaurs or ichthyosaurs were filter feeders. This conclusion was also reached by Collin and Janis (1997) who argued that the primitive tetrapod character of an undivided oropharyngeal cav-ity,the chance of muscular lips and cheeks, a relatively immobile tongue, and the lack of a coordinated swallowing reflex were not conducive to the evolution of suspension feeding. Nevertheless, although unlikely, a posterior secondary palate could possibly have developed among the crocodilians and become a feature of the Mesozoic marine crocodiles (Sect. 4.6.3.1). Morphological constraints rather than scarcity of suitable planktonic prey would therefore have prevented the evolution of mechanisms for filter-feeding. Without the mammalian ability to form a tight seal at the back of the mouth and to collect and swallow small food particles, Mesozoic marine reptiles would have been limited to feeding on larger individual prey items. For this they were well adapted.

At first sight, it might appear strange that the ichthyosaurs, which were so highly adapted to marine life, should have died out before the plesiosaurs. Throughout their existence, however, the ichthyosaurs occupied a more homogeneous environment than that of the plesiosaurs and, consequently, show much less diversity in the fossil record. There are about 38 extant species of porpoise (PhocaenidaeJ and dolphins (Delphinidae) - not counting the four river dolphins (Platanistidae) - and 31 species of seals, walrus and sea lions that comprise the ecological equivalents of the plesiosaurs. These differences are probably without much significance, but there is undoubtedly more morphological diversity among the Pinnipedia than among the smaller toothed whales (Odontoceti), the ecological equivalents of most of the ichthyosaurs.

The last ichthyosaurs disappeared about 25 million years before the end of the Cretaceous, while plesiosaurs have been found in the late Maastrichtian at the very end of the period, although they were much reduced in numbers and diversity by then. This suggests that gradual climatic change was taking place towards the end of the Cretaceous period. Sea levels apparently dropped comparatively rapidly and the climate became more variable. Pterosaurs and dinosaurs were also becoming less common at the time. Food chains would have been adversely affected throughout the world, and this may well have played a part in the extinction of the giant marine reptiles of the Mesozoic Era (Sect. 12.4).

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