## Mar 22 How dinosaurs walked

Bones may be indispensable in the reconstruction of an animal; but are of less use in providing evidence of what an animal was actually doing in everyday life. The traces and tracks left by dinosaurs going about their business is what brings the skeletons to life. Herds moving at different speeds, hunts, stampedes, and even limping dinosaurs have all been documented in fossil tracks.

Most dinosaurs spent much of their time walking. Trackways provide information on the speed and direction the animals were moving, as well as their posture — the angles at which they held their limbs. Unlike lizards, dinosaurs were not sprawlers, they did not hold their legs out sideways at nearly 90 degrees to their bodies. In fact bipedal dinosaurs (who walked on their two rear legs) produced nearly straight trackways with their hind limbs held nearly vertical and tucked in to their sides.

Other gaits, besides walking, have also been recognized in trackways made by bipedal dinosaurs. Using the stride length SL, the distance between two successive left or right footprints, and hip height h which is approximated as four times the footprint length, and using the ratio SL/h or relative stride length, it is possible to estimate whether the dinosaur was walking (SL/h < 2.0) or running (SL/h > 2.9). These values are based on those observed in modern terrestrial vertebrates.

It is possible to take this investigation further, and estimate the actual speed the dinosaur was moving. A number of formulas have been proposed to determine the absolute speed V of dinosaurs from their trackways. Among the most commonly used equation is that proposed by Alexander (1976) as follows (where g is due to gravity):

$$V = 0.25g^{0.5}SL^{1.67}h^{-1.17}$$

It comes as no surprise that the majority of speeds calculated from trackways show that most dinosaurs spent their days walking — perhaps feeding or migrating — at speeds of between 4 and 8 km/h (Thulborn 1990). Rarely, speeds of up to 40 km/h have been documented for bipedal theropods. The gaits of quadrupedal dinosaurs (sauropods, stegosaurs, ankylosaurs) are not so readily analysed as those of the bipeds, in part due to the structure of their shoulder girdle that may inhibit stride length of the forelimb and their frequently longer hind limb.

Footprint rotation, or the angle that the long axis of the footprint makes with the median axis of the trackway, provides further evidence of the style in which the dinosaurs moved their limbs. Examples of both positive and negative rotation are known, and reflect the varying gaits adopted by different dinosaur groups.

Finally, the record of tracks of swimming dinosaurs may at fi rst appear contradictory since a swimming animal will not leave traces in the sediment. However, animals taking off from a river bank, or occasionally hitting the bottom sediment in fairly shallow water do leave scratch marks in the sediment. These tracks may be differentiated from normal footprints by the parallel nature of the digit imprints as they were dragged through the sediment; contrasting with the more typical radiating digit imprints left by a walking or running animal. Scenarios of animals swimming in fl oods against the current have been recorded from the Middle Jurassic of the Yorkshire coast (Whyte & Romano 2001).

In short, virtually every type of locomotion may be preserved in the fossil record; except flying!

A swimming print of the hind foot of a tridactyl bipedal dinosaur, from the Middle Jurassic of the Yorkshire coast, England. Note the three slightly curved subparallel traces made as the foot digits were dragged through the sediment. The length of the print is approximately 30 cm.

References

Alexander, R McN (1976). Estimates of speeds of dinosaurs. Nature, 261, 129–130, DOI 10.1038/261129a0.

Thulborn, T (1990). Dinosaur Tracks. Chapman and Hall, London, 410 p.

Whyte, M A and M Romano (2001). A dinosaur ichnocoenosis from the Middle Jurassic of Yorkshire, UK. Ichnos, 8, 223–234, DOI 10.1080/10420940109380189.