Dolphin Downstroke Flukes Lift Drag

FIGURE 9.3. How tunnies swim. Ichthyosaurs presumably swam in the same way.

The aspect ratios of tails can be measured in the same way as for wings, by dividing the span of the tail by its mean chord (figure 8.4). High aspect ratio tails give the same lift for less drag, like high aspect ratio wings, and make for efficient swimming. Tunny tails have high aspect ratios, for example 7 for Yellowfin. Whale flukes have lower aspect ratios, for example 5 for White-sided dolphin, possibly because they have no skeleton to stiffen them and might be too flexible if their spans were increased. Tail outlines preserved in ichthyosaur fossil show still lower aspect ratios, for example 3.7 for Ichthyosaurus. In this respect ichthyosaurs seem markedly inferior to tunnies.

As well as being similar in shape, tunnies, porbeagle sharks, and whales have a remarkable thing in common. All of them are endo-therms. You would expect whales to be endotherms, because they are mammals, but tunnies and porbeagle sharks seem to be the only en-dothermic fishes. They do not keep themselves as warm as mammals, and they do not heat the whole body to the same temperature: the warmest parts deep inside the body are seldom above 35°C and often much cooler, whereas mammals keep their bodies at 36-40°C. However, these warmest parts are often 10 to 15 degrees warmer than the water in Bluefin tuna, and 10 degrees in Porbeagle sharks. It seems quite likely that ichthyosaurs were also endotherms.

Dolphins often leap repeatedly from the water, as they swim: this is called "porpoising." It looks as if they are playing or exercising, but it has been suggested that they may actually save energy by this apparently strenuous behavior. The main point is that drag is very much less in air than in water, so if the dolphins can make part of their journey out of the water they may save energy.

I will try to explain the idea in a bit more detail, because it seems possible that ichthyosaurs also porpoised. Obviously, it costs energy to leap: the amount of energy is proportional to the height of the leap. The length of the leap depends on the dolphin's speed and on the angle at which it leaves the water, but faster take-off (at any particular angle) makes the leap longer and also higher. If leaps at the same angle are compared, the height of the leap (and so the energy needed) is proportional to the length of the leap, so the energy needed per meter leaped is the same for all swimming speeds.

A leap saves energy if the work needed for it is less than would be needed to swim the same distance. The work needed for swimming is done against drag, and can be calculated by multiplying the drag by the distance. (The work done against a force is the force multiplied by the distance moved against it.) Drag is roughly proportional to the square of speed, so the work needed to swim a meter increases with increasing speed, while the work needed to leap a meter remains constant. As a dolphin swims faster and faster it must eventually reach a speed at which leaping saves energy.

People have tried to calculate the critical speed, and one estimate for dolphins is 5 meters per second, which is well within their range of speeds. However, there are all sorts of doubts about the calculation, both about the amount of work needed for leaping and about the amount needed for swimming. All we can feel sure of is that there must be some speed, above which leaping would save energy if dolphins can swim that fast. If dolphins leap for this reason, perhaps ichthyosaurs leapt too. I like to imagine that they did, but tunnies give no support to the idea. They do not leap although they are much the same size and shape as dolphins and seem able to swim as fast.

Most bony fish have a gas-filled float (the swimbladder) in the body cavity, making their densities about the same as the density of the water they swim in. Sharks and most tunnies have no swimbladder, so they are denser than water and would sink if they stopped swimming. Porbeagle sharks and tunnies live near the surface of the oceans, often with the bottom thousands of meters below, so there is no question of stopping for a quick rest on the bottom. These fish swim all the time, and some apparently have to keep swimming in any case, to get the oxygen they need. (Instead of making breathing movements like other fish they simply swim around with their mouths open, letting the water flow through their gills.) They prevent themselves from sinking mainly by keeping their pectoral fins spread like airplane wings, at a suitable angle of attack to give the necessary lift. (The pectoral fins are the large pair close behind the head.)

Whales are less dense, partly because they have a lot of blubber (fat is slightly less dense than water) but largely because they have air-filled lungs. Whales can often be seen floating at the surface of the sea with their backs protruding slightly above the surface, showing that they are actually less dense than the water. Their buoyancy enables them to stop swimming and rest without sinking. Ichthyosaurs presumably also had lungs (it would be surprising to find a reptile without them) and probably also had densities close to the density of water. Similarly crocodiles have about the same density as (fresh) water, as I showed in chapter 2. Ichthyosaurs probably had no need to keep swimming like tunnies and Porbeagle sharks, but could stop and rest like whales.

Ichthyosaurs must have come to the surface regularly to breathe, but they may also have dived to substantial depths, like whales. If they had the same density as water while at the surface, they would be denser while diving, because the air in their lungs would be compressed to a smaller volume. The pressure at a depth of 10 meters is twice the pressure at the surface and would halve the volume of the air, at 20 meters the pressure is three times as much as at the surface and the volume of the air would be reduced to one third; and so on. During a deep dive the lungs would give very little buoyancy and the animal would sink if it stopped swimming. It might get the necessary lift, as it swam, by spreading its flippers.

Ichthyosaurs may have had to dive quite deep, to get their food. Their skulls show that they had large eyes, which suggests that they depended on sight to find prey. Further, it suggests to me that they fed by day (but I admit that big eyes could be an adaptation for feeding in dim light, at dusk). Many fish and squids spend the night near the surface but dive quite deep by day: for example, herring that spend the night near the surface often dive to 100 meters or more. If ichthyosaurs fed by day on prey that behaved like that, they would have had to dive.

The ichthyosaurs seem splendidly adapted for swimming, but they would probably have been as helpless on land as a stranded whale. They could hardly have crawled up beaches to lay eggs above the high tide mark, as the sea turtles do. They could not have laid their eggs in water, because the embryos would have suffocated, as the embryos of birds and modern reptiles do if their eggs are submerged. The reason is that oxygen diffuses much more slowly through water than through air. If the pores of the eggshell get filled with water, oxygen cannot diffuse in fast enough.

Ichthyosaurs seem to have got round this problem by giving birth instead of laying eggs. Adults have been found with up to 12 young ones fossilized inside them, enclosed by their ribs. It is possible that they had eaten the little ones, but it is pleasanter to think that they were pregnant. Though most modern reptiles lay eggs, some sea snakes and others give birth.

The mosasaurs were another group of marine reptiles, but they looked much less fish-like than the ichthyosaurs, more like crocodiles with flippers instead of legs. The details of their skulls show that they were actually lizards, closely related to the Komodo dragon and other monitor lizards. The biggest of them were at least 9 meters long, about the same as the biggest modern crocodiles. (Though the Komodo dragon is the largest modern lizard it grows little more than 3 meters long.) The mosasaurs lived in the Cretaceous period, at the end of the Mesozo-ic era.

I speculated that ichthyosaurs may have dived, and there is some evidence that mosasaurs did. X-ray pictures of many of their vertebrae show the kind of damage that would have occurred if the blood supply to parts of the bone had got cut off, while the animal was still alive.

It has been suggested that the damage may have been caused by the bends, a serious hazard of diving.

Here is how the bends happens to human divers. The air they breathe has to be compressed to match the pressure of the water where they are working. The extra pressure makes extra gas dissolve in the blood. When the diver returns to the surface the extra gas comes out of solution, forming bubbles that may block blood vessels, causing damage, pain and even death. Human divers avoid the bends by returning slowly to the surface but a diving mosasaur would have to get to the surface reasonably soon, for its next breath. Whales avoid the bends largely by having small lungs and a big windpipe: the high pressures that they meet when they dive collapse their lungs, forcing the air back into the windpipe whether there is less danger of too much gas being absorbed into the blood.

The plesiosaurs lived in the Mesozoic seas, like ichthyosaurs and mo-sasaurs. Also like ichthyosaurs and mosasaurs they had two pairs of flippers instead of the fore and hind legs of most other reptiles. The shapes of their bodies, however, were quite different from those of the other marine reptiles. The trunk was broad and relatively low, a less extreme version of the body shape seen in modern turtles. Attached to it was either a long neck with a small head, or a short neck with a relatively large head (figures 9.4, 9.5). Some of them were very large, as the picture shows, but even Elasmosauius was only half as long as

FIGURE 9.4. A short-necked plesiosaur [Kronosaurus] and a long-necked one (Elasmosaurus) from Romer 1968, with a frogman.

Kronosaurus Size Comparison

FIGURE 9.4. A short-necked plesiosaur [Kronosaurus] and a long-necked one (Elasmosaurus) from Romer 1968, with a frogman.

Elasmosaurus Skeleton
FIGURE 9.5. Skeletons of two long-necked plesiosaurs. Cryptoclidus (length 4 meters) is shown in side view and Thaumatosaurus (length 3.4 meters) is shown from below. From Brown 1981, by courtesy of the British Museum (Natural History), and Romer 1966, respectively.

the dinosaur Diplodocus. The Natural History Museum, London, sells models of a very large (14 meter) long-necked plesiosaur. One of my students, Debbie O'Hare, measured its volume and calculated that the living animal had a mass of 7.5 tonnes (or perhaps a little less: the model is possibly a little too deep in the body). In comparison with that, the Leatherback, the biggest of the modern sea turtles, grows to shell lengths of only 2 meters and masses of about 0.6 tonnes.

Plesiosaurs seem to have had small tails that would not be very effective for swimming. They presumably propelled themselves mainly by flipper movements, either by rowing (like freshwater turtles) or by "underwater flight" (like marine ones).

Figures 9.6a and 9.7a show how plesiosaurs may have rowed. They show the flippers moving backward and forward, like the oars of a boat. In the backward stroke the blades of the flippers are held vertical, so as to push as hard as possible against the water. In the forward stroke they are held horizontal so as to strike the water edge-on and meet as little resistance as possible. (A human rower would lift the oars out of the water for the forward stroke, but an animal swimming below the surface cannot do that.) Notice that in the power stroke the flipper blade is moving backward through the water so the drag on it (the force opposite to the direction of movement) acts forward. Rowing boats and rowing animals are propelled by forward-acting drag on backward-moving oars.

Figures 9.6b and 9.7b show a different method of swimming, which is called underwater flight because the movements are like those of flying birds. Penguins, which cannot fly in air, use their wings in this way to swim underwater. Sea turtles also swim this way, using their flippers. The flippers beat up and down. On the downstroke they are held at an angle of attack so that lift acts on them, forward and upward (figure 9.7b). For the upstroke the angle is adjusted so that the lift acts forward and downward. The upward and downward components cancel out over the complete cycle of movements so that the net effect is a forward thrust (which is reduced a bit by the drag on the flipper). Notice how similar figure 9.7b is to the diagram of a tunny swimming in figure 9.3. Tunnies and presumably ichthyosaurs swim by means of their tails, beating them from side to side, and turtles and possibly plesiosaurs swim by means of their flippers, beating them up and down, but the basic principle is the same in both cases.

I would like to emphasize that rowing and underwater flight are utterly different techniques. In rowing, the flippers or oars are moved backward and forward and the propulsive thrust comes from the drag on them in their backward strokes. In underwater flight the movement is up and down and the thrust comes from lift: in this case, the drag is simply a hindrance.

I have been assuming that plesiosaurs had about the same density as water, so that an upward force at one stage of the cycle of flipper movements must be balanced by a downward force at another. In figure 9.7b this balance comes from an upward-sloping force in the down-stroke and a downward-sloping force in the upstroke. Figure 9.7c shows another possible way of avoiding unbalanced vertical forces. The flipper is moved almost vertically downward, held at an angle of attack so as to give forward lift. It is then raised on a sloping path, moving edge-on, with no angle of attack, so that the forces on it are very small. The downstroke gives horizontal thrust and the upstroke very little force. Figure 9.6c shows on the left how the flippers would have to move relative to the animal's trunk, to follow the appropriate path through the water: they would have to beat down and back, then forward and up. The thrust, in this swimming technique, comes from lift, so it is a form of underwater flight. Sea lions seem to swim rather like this.

Plesiosaurs would have had to drive water backward, to propel them

Mosasaur Swim Cycle

FIGURE 9.6. Three possible swimming techniques for plesiosaurs: (a) rowing; and (b) and (c) underwater flight. The diagrams on the left show how the flippers would have been moved relative to the body and those on the right show successive positions of the animal moving through the water. Only the fore flippers are shown.

FIGURE 9.6. Three possible swimming techniques for plesiosaurs: (a) rowing; and (b) and (c) underwater flight. The diagrams on the left show how the flippers would have been moved relative to the body and those on the right show successive positions of the animal moving through the water. Only the fore flippers are shown.

selves forward. In rowing, the flippers would push fairly small lumps of water backward (figure 9.8a). In underwater flight the flippers, beating up and down through a large angle, would affect much more water (figure 9.8b). When I wrote about the aspect ratios of wings (chapter 8) I explained that less energy is needed to get a force by accelerating a large mass of fluid to a low velocity, than by accelerating a small mass to a high one. This argument says that underwater flight should be more economical than rowing. It should need less energy for swimming at the same speed.

That is one reason for thinking underwater flight more likely than rowing. Animals tend to evolve efficient ways of doing things. There are animals that row, but at least some of them (freshwater turtles, and ducks) use their legs for walking as well as for rowing. It would

FIGURE 9.7. Diagrams showing a section through a flipper, and the forces acting on it, at different stages of the three swimming techniques of figure 9.6.

be very difficult to design a foot which was both effective for walking on land and suitably streamlined for use in underwater flight.

Another reason for thinking underwater flight more likely is that plesiosaur flippers taper at the tips. There is no advantage in tapering the tip of an oar blade, and the oars used in rowing races are made with square ends. However, there is an advantage in giving aerofoils and hydrofoils tapered, rounded ends: such shaping can spread the lift out over the span of the hydrofoil in the best possible way, so as to get lift with as little drag as possible. Bird and airplane wings, and propeller blades, generally taper toward their tips. The shape of plesiosaur flippers suggests that their function was to provide lift, not drag.

For underwater flight, plesiosaurs would have to have been able to flap their flippers up and down. The shapes of the joints seem to show that they could have done this, though they probably could not have raised the flippers very high above their backs. They would also have

FIGURE 9.8. A plesiosaur (a) rowing and (b) "flying" under water. Broken outlines show the water driven backward by the swimming movements.

needed appropriate muscles. Scientists have tried to work out how the flipper muscles were arranged, by looking at plesiosaur skeletons. Figure 9.5 shows big plates of bone to which muscles could have attached in the chest (between the fore flippers) and on the underside of the abdomen (between the hind flippers). These are the ventral parts of the pectoral and pelvic girdles. They seem excellent areas of attachment for muscles that would pull the flippers down, in a powerful down-stroke, but there does not seem to be much to attach upstroke muscles to. The big plates of bone are all below the shoulder and hip joints. The upward extensions of the pectoral and pelvic girdles (seen in the side view) seem to be attached rather weakly to the ribs and backbone. Strong upstroke muscles could have been attached to the backbone, but there is little to prevent them pulling the girdles bodily upward instead of flapping the flippers. The symmetrical style of underwater flight, shown in figures 9.6b and 9.7b, needs equally strong upstroke and downstroke muscles. The style shown in figure 9.6c and 9.7c needs big forces for the downstroke but only small forces for the upstroke, and seems the more likely swimming technique for plesiosaurs.

Plesiosaurs probably could not swim very fast. One reason for thinking this is that the volume of flipper muscle that it seems possible to fit into their bodies is relatively small, compared to the volume of tail muscle that would be found in similar-sized fishes or whales. The swimming muscles of a 43-kilogram porpoise had a mass of 9 kilograms. (This includes the tail muscle and most, but not all, of the back muscle.) That is 21 percent of body mass. I do not see how the flipper muscles of plesiosaurs could have been as much as 21 percent of body mass.

Another reason for thinking that plesiosaurs were probably rather slow is that the swimming technique we suspect they used (figure 9.7c) requires the flipper to be moved almost vertically down through the water. To do this while the animal was moving forward, the flippers would have to move backward (relative to the body) as fast as the body was moving forward (relative to the water). Imagine the plesiosaur shown in figure 9.7c swimming at 10 meters per second, about the maximum sprinting speed of tunnies and dolphins. During the power stroke, a point at the center of the flipper would have to move backward through a distance x at about 10 meters per second. If the animal was a moderate-sized short-necked plesiosaur, three meters long, x would be a meter or less and the movement would have to be made in one-tenth of a second. If the upstroke was made at the same speed, the cycle of flipper movements would be completed in 0.2 seconds and the flapping frequency would be 1/0.2 = 5 cycles per second. I doubt whether so large an animal could have managed so high a frequency of movement. Small penguins beat their wings when they swim at up to 4 cycles per second, but they are very much smaller. There is a general rule that large animals cannot move their limbs at as high frequencies as small ones: a horse cannot make as many strides per second as a mouse, and a swan cannot make as many wingbeats per second as a sparrow. Figure 9.9 shows some data. Penguin wing beat frequencies are about the same as the wing beat frequencies of similar-sized flying birds (although penguins move their wings in water) but considerably less than the stride frequencies of similar-sized mammals, which is not surprizing: the penguins were moving their wings in water. The points are widely scattered around the lines but the general trend is clear: bigger animals use lower frequencies. By extending the penguin line, I estimate that an underwater flier 3 meters long (the size of the plesiosaur I have been discussing) would have beaten its flippers at a maximum frequency of about 1 cycle per second. This is only one fifth as much as I estimated would be needed for swimming at 10 meters per second and suggests that the plesiosaur could only manage about 2 meters per second.

It is not just chance that makes big animals move their limbs at lower frequencies than small ones: there is a good mechanical reason. Imagine two animals of the same shape, one twice as long as the other (and twice as wide and twice as high). It is eight times as heavy as the

Length (meters)

Length (meters)

FIGURE 9.9. Bigger animals move their limbs at lower frequencies. The graph shows •, mean wing beat frequencies of flying birds; O, maximum wing beat frequencies of swimming penguins and mean galloping stride frequencies of hoofed mammals, all plotted against the overall length of the body. The penguin data come from Clark and Bemis 1979, the mammal data from my own observations in Kenya, and the flying bird data from various sources.

small one and has eight times as much limb muscle, able to do eight times as much work to accelerate the limbs at the beginning of each stroke. This work is used to give the limbs kinetic energy (half mass times speed squared) but its limbs, plus any water moved by them, have eight times the mass of the smaller animal's ones, so can only be accelerated to the same speed. The big animal's limbs have to travel twice as far as the small ones to make each stroke, so their cycle of movements takes twice as long. This argument says that doubling the length of an animal should halve its frequency of limb movements, and figure 9.9 shows that this is roughly true for flying birds and swimming penguins.

I have argued that the style of underwater flight shown in figure 9.6c is unlikely to be very fast, because the flippers have to move back, relative to the body, as fast as the body advances through the water. Penguins and sea turtles use more symmetrical styles, like the one shown in figure 9.6b which is not limited in this way, but even so they are not very fast. The highest speed shown in films of penguins swimming in Detroit Zoo was 3.4 meters per second (for a King penguin, about 90 centimeters long) and adult Green turtles seem to swim no faster than 2.0 meters per second.

Penguins and sea turtles move their left and right wings or fore flippers in unison. If they did not they would waste energy by swimming a slightly zigzag route. Plesiosaurs probably also moved the left and right flippers of a pair together. Sea turtles swim mainly with their big fore flippers, beating their small hind flippers only occasionally. Ple-siosaurs had big hind flippers as well as big big fore ones and probably used both pairs about equally.

I have argued that the downstroke was the power stroke. If so, there might be an advantage in beating the fore and hind flippers out of phase with each other as shown in figure 9.8b. Fore-power strokes would alternate with hind-power strokes, keeping the animal moving at a steady speed. This might save energy; drag is about proportional to speed squared, so the average drag is greater when swimming at a fluctuating speed than when swimming steadily at the same average speed. However, there might be a very serious disadvantage in beating the fore and hind flippers alternately. The hind flippers might find themselves moving in water that had already been accelerated by the fore flippers. They might accelerate this water further, but it would be more efficient for them to work on different water. This is another application of the principle we have already met several times: it is more efficient to get thrust by accelerating a lot of water to a low velocity, than less water to a higher velocity.

The long-necked plesiosaurs had extraordinarily long necks, some of them longer than the whole of the rest of the body. If they swam under water with the long neck stretched out in front it would have been quite tricky for them to steer a straight course: if the animal accidentally veered slightly to one side, the water, striking the neck obliquely, would tend to make it veer more. This is the opposite to the effect of flights on an arrow, which tend to correct any deviation, pulling the arrow back to a straight course. The difference is that the neck has a big surface area in front of the animal's center of gravity and the flights have a big area behind the arrow's one. It seems possible that plesio-saurs often avoided this problem by swimming at the surface with their necks out of the water. More energy is needed to swim at the surface than to swim well submerged, because an animal at the surface pushes a bow wave in front of it like a boat, but this might not be a big disadvantage if the plesiosaur swam slowly, and in any case it would have to visit the surface frequently to breathe.

If plesiosaurs had eaten worms or clams, we might suppose that they used their necks to reach down to the bottom, dabbling like ducks or swans, but their spiky teeth seem more suitable for catching fishes and squid-like animals which would probably have been too active to be caught easily that way. It seems likely that they darted at prey, extending their long necks to catch things as they swam by. The movement could have been fast, if the neck was held out of water. Herons use their long necks to dart at fish, though they stand in the water instead of floating as plesiosaurs presumably did.

The fossil record seems to show that the plesiosaurs became extinct at the same time as the dinosaurs, 65 million years ago, but some people believe that rather similar animals are still living in Loch Ness, Scotland (figure 9.10). The picture is not very like a plesiosaur (notice the humped shoulders, and the central ribs in the flippers) but there is some resemblance.

There have been reports of the Loch Ness monster since the Middle Ages, and tremendous efforts have been made in modern times to get good evidence of its existence. The surface of the lake has been kept under observation, sonar (echo sounding) has been used, and thousands of underwater flash photographs have been taken at random in the hope that the monster will swim into view. Some surface photographs have

FIGURE 9.10. An impression of the Loch accounts and (unclear) photographs. From permission. Copyright © 1975 Macmillan

Ness monster, based Scott and Rines 1975 Magazines Ltd.

on eyewitness Reprinted by

FIGURE 9.10. An impression of the Loch accounts and (unclear) photographs. From permission. Copyright © 1975 Macmillan

Ness monster, based Scott and Rines 1975 Magazines Ltd.

on eyewitness Reprinted by been taken that show monster-shaped images, but it never seems certain that the image is not a floating log or an odd pattern of ripples. Some sonar traces have detected unexplained objects about 15 meters long and there are a few hazy underwater photographs (hazy even after computer enhancement) that show shapes like the neck and flippers in figure 9.10. If the monster is 15 meters long and has the shape shown in the picture, it must weigh well over 10 tonnes. If there is one monster there must be several, or at least there must have been several until quite recently: no animal is immortal, and very small populations are in danger of dying out. If there really are monsters there, it seems odd that we have not got better evidence of their existence.

This chapter has been about three groups of fossil reptiles. The ichth-yosaurs were beautifully streamlined, like tunnies and dolphins, and probably swam fast. They may have been endotherms (like tunnies), they may have dived deep (like dolphins), and they may have porpoised when swimming at the surface.

The mosasaurs were giant lizards with flippers instead of legs. Some have damaged vertebrae that look like symptoms of the bends, a hazard of diving.

The plesiosaurs used flippers to swim, probably by underwater flying rather than rowing. They probably swam rather differently from turtles and penguins, getting thrust only from the downstroke. If so, they must have been rather slow. Some had remarkably long necks, which may have been held out of the water and used for darting at prey.

As for the Loch Ness monster, I am not convinced that it exists. Are you?

Principal Sources

Much of my information on fish swimming and on endothermic fishes comes from Hoar and Randall (1978). The hypothesis about porpoising comes from Au and Weihs (1980) and the top speed for dolphins from Lang and Prior (1966). Clark and Bemis (1979) and Davenport and others (1984) describe how penguins and turtles swim. Robinson (1975) and Godfrey (1984) discuss plesiosaur swimming. Rothschild and Martin (1987) found the evidence of mosasaurs getting the bends. Scott and Rines (1975) and Witchell (1975) present the evidence for the Loch Ness monster. The other items in the list are sources for illustrations.

Au, D. and D. Weihs. 1980. At high speeds dolphins save energy by leaping. Nature 294:548-550.

Brown, D. S. 1981. The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria. Bulletin of the British Museum [Natural History) Geology 35:253-347.

Clark, B. D. and W. Bcmis. 1979. Kinematics of swimming of penguins at the Detroit Zoo. journal of Zoology 1 8 8:41 1 -428.

Davenport, )., S. A. Munks, and P. J. Oxford. 1984. A comparison of the swimming of marine and freshwater turtles. Proceedings of the Royal Society B 220:447475.

Godfrey, S. |. 1984. Plesiosaur subaqueous locomotion: a re-appraisal. Neues jahrbuch fur Ceologie und Paldontologie. Monatshefte. 1984:661-672.

Hoar, W. S. and D. J. Randall. 1978. Fish Physiology. VII: Locomotion. New York: Academic Press.

Lang, T. G. and K. Prior. 1966. Hydrodynamic performance of porpoises [Stenella attenuata). Science 152:531—533.

Robinson, |. A. 1975. The locomotion of plesiosaurs. Neues jahrbuch fur Geologie und Paldontologie, Abhandlungen 149:286-332.

Romer, A. S. 1966. Vertebrate Palaeontology. 3d ed. Chicago: University of Chicago Press.

Romer, A. S. 1968. The Procession of Life. London: Weidenfeld and Nicholson.

Rothschild, B. and L. D. Martin. 1987. Avascular necrosis: occurrence in diving Cretaceous mosasaurs. Science 236:75-77.

Scott, P. and R. Rines. 1975. Naming the Loch Ness monster. Nature 258:466-467.

Wheeler, A. 1969. The Fish es of the British Isles an d North- West Europe. London: Macmillan.

Witchell, N. 1975. The Loch Ness Story. Harmondsworth: Penguin Books.

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  • sayid
    Why is a whales upstroke more powerful than its down stroke?
    9 years ago
  • adiam saare
    Where did thaumatosaurus live?
    8 years ago

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