Running for life

All theropods were obligate bipeds, unable to walk or run on anything but their hind legs. The body was balanced directly over the pelvis, with the vertebral column held nearly horizontally (see Figure 9.5). Evidence from the skeleton and trackways indicates that the hind legs were held close to the body, feet so close to the midline that it appears that one foot was placed ahead of the other, rather than along its side. The trackways, as well as skeletal material, also indicate that the foot was held in a digitigrade stance (Figure 9.7).

Many small- to medium-sized theropods, especially ornithomimosaurs, must have been fast runners. Their thigh bones were short compared to the length of the rest of the hindlimb; a condition typical of fast-running bipeds (Figure 9.8). Calculations of running speeds on the basis of hindlimb proportions indicate that the fastest theropods clocked 40-60 km/h. Some

Figure 9.3. Global distribution of non-avian Coelurosauria.

Dinosauria

Figure 9.4. Cladogram of Dinosauria emphasizing the monophyly of Theropoda. Derived characters include: at 1, extreme hollowing of vertebrae and long bones, enlarged hand, vestigial fourth and fifth digits, remaining digits capable of extreme extension due to large pits on the upper surfaces of the ends of the metacarpals.

Dinosauria

Figure 9.4. Cladogram of Dinosauria emphasizing the monophyly of Theropoda. Derived characters include: at 1, extreme hollowing of vertebrae and long bones, enlarged hand, vestigial fourth and fifth digits, remaining digits capable of extreme extension due to large pits on the upper surfaces of the ends of the metacarpals.

footprint evidence bears these numbers out; for example, a trackway in Texas was made by a theropod that thundered away at upward of 45 km/h (see Box 12.3).

The running speeds of large theropods, however, are less clear. Some investigators have calculated, using limb proportions and models of leg motion, that therapods were limited to walking at no more than 4 km/h. Using similar approaches, however, others have deduced much faster running speeds.

Another approach to speed estimations involves reconstructing the muscle mass and volume. In the case of large theropods running at high speeds, so much leg musculature

Figure 9.6. Left lateral view of the skull and skeleton of Demonychus.

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Figure 9.7. A theropod trackway from the Middle Jurassic Entrada Formation, Utah, USA. Note how closely the right and left prints are placed, suggesting a fully erect stance. When spectacular trackways like this are found, it is not hard to imagine the ghostly image of the trackmaker leaving a row of footprints in the soft mud.

would have been required that the animal would have been grotesquely overmuscled. The balance of dispassionate evidence, therefore, suggests that the largest theropods likely were not the fleetest runners of their time.

For all that running, it is now known that some theropods, at least, could also swim. In 2007 a trackway was discovered in Spain clearly demonstrating the imprints of a medium-sized theropod swimming.

Figure 9.8. Left lateral view of the skull and skeleton of Struthiosaurus.

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Figure 9.8. Left lateral view of the skull and skeleton of Struthiosaurus.

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