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FIGURE 3.3. Skeletons of dinosaur feet: (a) Tyrannosaurus hind foot; (b) Iguanodon hind foot; (c), (d) Apatosaurus hind and fore feet. The outline under each skeleton shows the probable shape of its footprint.

tails of most of them dragging on the ground. There is more about this in chapter 5.

Some sauropod footprints are 10-15 centimeters (4-6 inches) deep. They were probably deeper when first formed because the mud must have lost water as it compacted to form rock, but they cannot have been very much deeper. If there had been too much water in the mud, it would have been too sloppy to take a footprint. The ground must have been fairly soft, but not sloppy.

Soft ground may have been dangerous for big dinosaurs, because of their size. Imagine two dinosaurs of identical shape, one twice as long as the other. It is twice as long, twice as wide and twice as high, so it is eight times as heavy (23 = 8). The soles of its feet are twice as long and twice as wide, so they have four times the area (22 = 4). Eight times the weight has to be supported on four times the area, so the pressure under the big dinosaur's feet is twice as much as under the small one. The big dinosaur is more likely to sink in and get bogged down.

The argument seems clear, but it is a bit too simple. Suppose there were a thick layer of soft mud with firm ground below. Small animals

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