Marine Reptiles

We will now discuss the giant reptiles that lived in the sea in the Mesozoic era, when the dinosaurs were living on land. They had flippers instead of feet and would probably have been pretty helpless on land. We know that they lived in the sea rather than in fresh water because the other fossils found in the same rocks include sea urchins and squid-like molluscs, members of groups whose modern members are found only in the sea.

I am going to start with the ichthyosaurs—reptiles that looked remarkably like fish (figure 9.1). Most of them were at least a meter long and some were as much as 15 meters. I have a model of one of the larger kinds, Ichthyosaurus, bought from the Natural History Museum, London. I measured its volume and calculated the mass of the living animal in the same way as for dinosaurs (chapter 2). This particular animal was 8 meters long and I calculate that its mass was 6 tonnes. Adult Killer whales have about the same length and mass.

Most ichthyosaur fossils are skeletons and nothing more, but a few have dark marks showing the outline of the body and of the fins and flippers. These marks show that the flippers were a good deal broader than you might guess from the skeletons, and that at least some ich-thyosaurs had a fin on the back. Some ichthyosaurs had straight tapering tails but the best known kinds, including the one in figure 9.1, had a sharp kink in the backbone where it entered the tail fin. Dark marks on a few fossils show that these ichthyosaurs had tails shaped like crescent moons.

There are two groups of modern fish that look very like these ich-thyosaurs: the tunnies and the porbeagle sharks (figure 9.2). As well as being shaped like ichthyosaurs they overlap the ichthyosaur size range. For example, Bluefin tuna reach lengths of 4 meters and masses of 0.8 tonnes. Great white shark grow to maximum lengths of about 11 me-

Great White Shark Skeleton
FIGURE 9.1. The skeleton of a Jurassic ichthyosaur, with an outline of the body. From Romer 1966. I have added a frogman to scale, assuming this ichthyosaur is 8 meters long.

ters. Whales also look like ichthyosaurs, but with a very obvious difference. Ichthyosaurs had vertical tail fins (like tunnies and sharks and presumably beat them from side to side when they swam. Whales have horizontal tail flukes and beat them up and down.

In one respect ichthyosaurs were even more like dolphins than like tunnies or sharks: they had long narrow jaws with a lot of simple pointed teeth. Dolphins eat fishes and squid, and ichthyosaurs seem to have eaten similar things. Many ichthyosaurs have been found with their fossilized stomach contents still in place inside them, enclosed by their ribs. Some fish scales have been found in them, and enormous numbers of hooks from the suckers of squid-like mollusks.

Tunnies, porbeagle sharks, and whales, the modern animals shaped like ichthyosaurs, swim fast. The most reliable speed measurements have been made with dolphins trained to swim as fast as possible over a marked course, or to follow a lure towed by a fast boat. The highest speed on record seems to be 11 meters per second (25 mph) for a Spotted porpoise 2 meters long, and slightly slower speeds have been recorded for other species. These are sprint speeds, maintained for only a few seconds. They are astonishingly fast for movement in water, and equal the top speeds (on land) of human sprinters.

Speeds of tunnies have been measured, both by filming them and by catching them on a rod with an instrumented reel, that recorded the rate at which the fish pulled out the line. Several records of 5 to 13 meters per second were obtained in this way, and also two of 21 meters per second for a Wahoo and a Yellowfin tuna. I find these last two records hard to believe because they are so much higher than any others. I wonder whether some error was made: for example, a mistake could conceivably have been made about the speed at which the re-

Tonno Rosso Stilizzato

FIGURE 9.2. A Porbeagle shark, a Bluefin tuna, and an optimum streamlined shape. The fishes are from drawings by Valerie du Heaume in Wheeler 1969.

FIGURE 9.2. A Porbeagle shark, a Bluefin tuna, and an optimum streamlined shape. The fishes are from drawings by Valerie du Heaume in Wheeler 1969.

cording equipment was running. Even if we reject these records (as I am inclined to do) it seems clear that tunas, like dolphins, swim very fast. The fastest shark speed I have seen recorded is 5 meters per second, but there are very few data.

The speeds of the similar-shaped modern animals make it seem likely that ichthyosaurs were also fast. My guess is that they could have sprinted at 10 meters per second.

Drag resists the movement of bodies through water, as also through air. It is much larger in water than at the same speed in air because water is so much denser. To minimize drag, a body should be designed to disturb the water as little as possible, as it passes through. Any body will leave a wake of swirling water behind it, but the narrower the wake, the less the drag. This is because energy is needed to set the water swirling: the kinetic energy of the swirling water comes from work done against drag. Streamlining is the art of designing bodies so as to disturb the water as little as possible. The best shapes are rounded in front, and taper to a fine point behind to allow the water to close in smoothly after the body has passed. Torpedoes and submarines are shaped like this, and so also are ichthyosaurs, whales, and many fish.

When airships were used, the engineers who designed them set out to discover the best shape. The carrying capacity of an airship depends on its volume because it is supported by the buoyancy of gases that are lighter than air, so the basic problem was to find the shape that gave least drag for given volume, at any particular speed. The answer turned out to be a streamlined shape with the length 4.5 times the diameter at the fattest part (figure 9.2, bottom).

The same shape seems likely to be best for swimming animals, so it is not surprizing to find that ichthyosaurs, tunnies etc. are very nearly this shape. The Ichthyosaurus model (already mentioned) has its length 5.0 times the maximum diameter, Yellowfin tuna are 4.5 diameters long, and Porbeagle sharks and Bottle-nosed dolphins are both about 5.5 diameters long. (The diameter that I have used in these calculations is the mean of the maximum height of the body and the maximum width.)

Figure 9.3 shows how tunnies swim. They beat their tails from side to side as they move forward, so the tail takes a wavy path through the water. It is held at an angle of attack so that lift acts on it as well as drag. (Lift acts on hydrofoils in water, just as on aerofoils in air.) While the tail is moving to the right, the lift acts forward and to the left. While it is moving to the left, the lift acts forward and to the right. The components to left and right cancel out, over a complete cycle of tail movements, so the net effect is a forward thrust, driving the fish through the water. The drag on the tail acts backward all the time, reducing the thrust, but if the hydrofoil is well designed (as tunny tails seem to be) the drag is relatively small. Ichthyosaurs like the one in figure 9.1 presumably swam like this but it has been suggested that some of the ones with narrow tapering tail fins may have depended more on flipper movements.

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

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