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Dimensionless speed

FIGURE 3.10. A graph of relative stride length against dimensionless speed for ostrich: |9), humans (O), dogs (■!, elephant (□), rhinoceros (O), sheep (A), and camels (▲). Data from Alexander 1976 and Alexander and Jayes 1983.

the part of the foot that would rest on the ground and make a footprint, and also the height of the hip joint from the ground. (There is a danger of error here, if the skeleton has been put together with the knee unnaturally bent or extended.) I found that leg length was about four times foot length in a wide variety of dinosaurs, both bipeds and quadrupeds, and decided to assume that as a general rule.

At last we are ready to estimate a speed. Take the case of the large theropod in figure 3.1. Its foot length (measured from the most complete prints) was 0.64 meter, so its leg length can be estimated as 2.56 meters. Its stride length was 3.31 meters so its relative stride length was 3.31/2.56 = 1.3. The graph (figure 3.10) shows that we can expect this relative stride length when the dimensionless speed is 0.4. For an animal of leg length 2.56 meters, that means a speed of 2.0 meters per second:

That estimate of speed is inevitably a rough one. The scatter of points around the line in figure 3.10 shows that we cannot expect accurate answers. Nevertheless it seems clear that the big theropod was traveling quite slowly. For humans, with much shorter legs, 2.0 meters per second (4.5 miles per hour) is only a brisk walking speed.

Some other speed estimates are shown in table 3.2. They have been obtained in the same way, except that the leg length estimates for the small Winton dinosaurs are not exactly four times footprint length: the ratio has been estimated for each group of dinosaurs. Also, some of the speeds have been calculated by a different formula that I published before I had collected the data for high speeds in figure 3.10. This makes little difference to the results.

Table 3.2 also shows whether each dinosaur seems to have been walking or running. People and many birds walk to go slowly and run to go faster. Similarly, horses, dogs, and other quadrupedal mammals walk at low speeds and use various running gaits (trotting, galloping, etc.) at higher speeds. There are various differences between walking and running but the most obvious is that when we walk we have each foot on the ground for more than half the time so there are stages in each stride when both feet are on the ground. In running, each foot is on the ground for less than half the time so there are stages when neither foot is on the ground. Similarly for quadrupeds walking, each foot is on the ground more than half the time so there are stages when both fore feet, or both hind feet, are on the ground. For quadrupeds running, each foot is on the ground for less than half the time so there are stages when neither fore foot, or neither hind foot, is on the ground.

Watch an animal speeding up, changing from a walk to a run. In most cases, the change is abrupt. A person or a horse, for example, is obviously walking at one moment and obviously running at the next. Some animals (for example, sheep) make the change more gradually, but whether the change is abrupt or gradual, it occurs at a highly predictable speed. People, and mammals in general, and birds walk when their dimensionless speed is less than about 0.7 and run when it is more than about 0.7. That rule has been used in table 3.2 to distinguish between walking and running. If the dimensionless speed estimated from the stride length is more than 0.7, the dinosaur is marked as running. It would be hard to be sure which the gait had been, if the estimated dimensionless speed were close to 0.7, but all the cases in the table seem clear cut.

Very few footprints of running dinosaurs are known, apart from the ones at Winton. This is not surprising. People and other fairly large mammals walk a lot, but run only occasionally. They are particularly unlikely to run on ground soft enough to take deep footprints, except in emergencies.

Though running dinosaur footprints are unusual, there are some that seem to show faster running. The fastest recorded are some theropod tracks in Texas: footprints of two dinosaurs, one with 29 centimeter feet and the other with 38 centimeter feet, indicate speeds of 12 meters

TABLE 3.2. Estimated speeds of dinosaurs.

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