Many aquatic reptiles use their limbs as paddles and swim relatively slowly. Turtles almost fly through the water with their front flippers but, even so, they are not as fast as most fishes and dolphins of comparative size. Some reptiles, however, especially the pliosaurs and ichthyosaurs, were streamlined and extremely fast (Chap. 5). They swam with the aid of flattened bodies and fish-like tails. When an eel swims, the inner sides of the curves on its sinuous body press against the water. The resistance is increased by the expansion of the dorsal, caudal and ventral fins which are combined into one.

The machinery necessary for typical swimming has been analysed by Sir James Gray and Hans W. Lissman (Gray 1968) and consists of three parts - the vertebral column, the axial musculature and the lateral surfaces of the body, including the caudal fins. The vertebral column can be regarded as a series of rigid units, hinged so that the body can bend only in a transverse lateral plane. The whole tail is essentially a shortened continuation of such units. Eel-like or anguilliform movement is similar to the serpentine locomotion of a snake. Its dependence upon the thrust made by the inner side of each curve of the body can be illustrated by the inability of an eel or snake to crawl across a polished surface. Both, however, can pass through a series of smooth pegs projecting from the surface of a polished board (Fig. 16). The sinusoidal curves of a snake's body when it is thus gliding by serpentine motion are stationary relative to the ground. The same is true of the swimming eel. It is the animal's body that moves, each part closely following the path of the part in front.

In most fishes, and other animals whose bodies are relatively much shorter than those of eels and snakes, the lateral surfaces and caudal fins represent

■ Fig. 16. Snake gliding past a series of smooth pegs projecting from the surface of a polished board. (Cloudsley-Thompson 1999). Further explanation in the text only sections of the sinuous curve. Furthermore, the separated dorsal, ventral and caudal fins then often have more than one function (Gray 1968; McGowan 1991).When snakes swim, much of the energy is wasted because their rounded bodies present little resistance to the water. Even in sea snakes, the tails are only slightly flattened (see Cloudsley-Thompson 1999). The mechanics of swimming have been analysed very thoroughly by McGowan (1991).

According to Massare (1997), marine reptiles past and present can be divided into four groups according to their body forms or Baupläne (Fig. 17). Each of these was characterised by a different mode of swimming. The first group (Bauplan I) consisted of post-Triassic and some Triassic ichthyosaurs (Chap. 5).

Bauplan III Bauplan IV

■ Fig. 17. The four body forms or Baupläne displayed by Mesozoic reptiles. (After Massare 1997)

Bauplan III Bauplan IV

■ Fig. 17. The four body forms or Baupläne displayed by Mesozoic reptiles. (After Massare 1997)

These had deep, streamlined bodies, deepest in the pectoral region and tapering posteriorly to the caudal fin. They propelled themselves by oscillating the tail, while the fin-like limbs were used mainly for steering. The general shape of the body was within the optimal range to minimise drag and for efficient swimming. Probably the fastest marine reptiles capable of sustained swimming, they were predators that hunted fishes and other prey over large areas.

Bauplan II contained the mosasaurs, thalattosaurs, marine crocodilians, pachypleurosaurs, and the remaining Triassic ichthyosaurs. These animals had narrow, elongated bodies and long,broad tails. Propulsion was achieved by undulation of part or all of the body along with the tail. Consequently, the tail could be used to produce a sudden thrust, and the prey must have been caught by rapid bursts of speed.

Bauplan III is exemplified by nothosaurs, pliosaurs and plesiosaurs; these had stiff, ellipsoidal bodies with two pairs of elongated limbs, wing-shaped in the plesiosaurs. Swimming consisted of a modified underwater 'flight' somewhat similar to that found among extant sea lions (Fig. 32; Chap. 5). The nothosaurs and plesiosaurs had long necks and small heads, while the plio-saurs had shorter necks, larger heads and more compact bodies.

Although not so speedy as the ichthyosaurs,but like them, the pliosaurs were probably predators that pursued their prey over long distances. In contrast,the other taxa would have been much slower. Movement of their necks and heads must have tended to cause them to veer off course if they moved too fast; so they ambushed their prey by stealth and seized it with a sudden movement of the neck.

Finally, Bauplan IV, found in placodonts and sea turtles, was a body compressed dorsoventrally and covered by bony armour. Propulsion was achieved by paddling with the limbs, although in some turtles the forelimbs were modified into 'wings', and propulsion was by subaqueous flight, thus achieving greater speed The placodonts, which lacked such adaptations, were incapable of speed or acceleration and preyed on immobile shellfish. Locomotion was therefore not an important factor to them in the acquisition of food.

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